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Regulations (Preambles to Final Rules) - Table of Contents
• Record Type: Occupational Exposure to Methylene Chloride
• Section: 6
• Title: Section 6 - VI. Quantitative Risk Assessment

VI. Quantitative Risk Assessment


After examining all the available data, both animal and human, and both quantitative and qualitative, OSHA has concluded that MC is a multi-species, multi-site carcinogen in various rodent species, and is likely to be so in humans, and that it most probably acts via one or more genotoxic metabolite(s). The evidence for this conclusion is quite strong: there exist several positive bioassays with low background incidence and dose-related increases; there is an unusually large amount of mechanistic information; and there are several positive epidemiological studies and no negative epidemiological studies of sufficient power to rule out the animal-based potency estimates.

Furthermore, OSHA has conducted a quantitative risk assessment based on the highest-quality animal tumor data, constructing a state-of-the-art physiologically-based pharmacokinetic (PBPK) model incorporating rodent and human metabolic information. That analysis shows a final estimate of risk of 3.62 deaths per 1000 workers occupationally exposed to 25 ppm MC for a working lifetime. [An alternative analysis, which incorporated all of the data used in the main analysis plus the assumption that human enzymes are even less active to MC (as compared to mice) than that predicted by the main analysis, gave a risk estimate of 1.23 deaths per 1000]. Both estimates are clearly well above any plausible upper boundary of the "significant risk" range defined by the Supreme Court, used by OSHA in its prior rulemakings, and reported in the scientific/economic literature on risk. The estimated risk at the current PEL of 500 ppm is 126 excess cancers per 1000 workers; clearly, the 25 ppm standard will effect a substantial reduction in a very high risk. The Final Economic Analysis shows that the average risk at current exposure levels is approximately 7.6 deaths per 1000 and ranges up to 126 per 1000; at post-regulatory exposure levels (which account for the fact that the action level will encourage some employers, where feasible, to lower exposures below 25 ppm), average risk is estimated to be 1.7 deaths per 1000 (and nowhere higher than 3.62 per 1000 risk at the new PEL of 25 ppm) -- also a substantial reduction of a highly significant risk.

Prior to the October 1995 record reopening, there was strong evidence to support the determination that MC is a human carcinogen, using well-established risk assessment models based on substantial biologically-based evidence and theories: there were two multi-site positive bioassays with dose-response trends and low background, and suggestive epidemiology with no clearly conflicting epidemiology. The only question was whether to use an administered-dose scaling or a PBPK model.

Data submitted in the reopening of the record in late 1995 shed light both on the hazard identification and the quantitative risk assessment. Studies of isoenzyme activity and intracellular distribution across species were interpreted by the Halogenated Solvents Industry Alliance (HSIA) to suggest that MC is not a human carcinogen. OSHA has concluded that the HSIA interpretation of the studies is not supported by the evidence. There are numerous methodological problems with the studies: for example, in the experiment in which Graves et al. examined MC-induced mutations [Ex. 123], OSHA agrees with Dr. Douglas Bell [Ex. 126-26] that insufficient numbers of doses and mutants were examined to reach any conclusions whatsoever regarding differences in mutation spectra between chemicals.

More importantly, OSHA and most commenters agreed that the data showed a quantitative -- and quantifiable -- difference between mice and humans, not an infinite, qualitative one. In other words, there is substantial evidence that humans and mice metabolize MC similarly, only at different rates. HSIA's qualitative argument rests on two questionable assumptions, both of which are contradicted by other data: first, that the DNA single strand break assay is infinitely sensitive -- but the investigators do not even know if it is sensitive enough to show the 7-fold difference in enzyme activity between mice and humans that OSHA's main PBPK analysis uses; and second, that the human isoenzyme most active against MC, although clearly present in human cells, is located in a different part of the cell. This interpretation: 1) contradicts some basic beliefs of comparative physiology (Why would the cell structures of humans and mice be so fundamentally different?); 2) would require OSHA to do a "subcellular PBPK analysis" to predict risk -- no one has ever developed, let alone parameterized and validated, such a model; and 3) contradicts other data on activation by mouse cytosolic preparations -- MC has been shown to have enhanced mutagenicity in bacterial and mammalian cell preparations when mouse cytosolic preparations were used to metabolize the MC. This requires metabolism by cytoplasmic (not nuclear) GST and for the metabolites to be stable enough to cross membranes and interact with DNA.

Therefore, the new studies do not cast doubt on the MC hazard identification -- in fact, they should probably increase the level of concern because it is now more clear that MC is likely to act by a genotoxic mechanism [animal tests are most relevant to humans when clear genotoxic agents are involved] and that that pathway exists in humans, and may be concentrated in cells of concern in human cancers, such as the bile duct epithelium. OSHA notes that an epidemiologic study of cellulose triacetate fiber workers has shown a statistically significant increase in biliary duct tumors [Ex. 7-260].

On the other hand, the new data did reinforce OSHA's decision to proceed with a PBPK-based risk assessment and helped OSHA to incorporate the best available scientific data into a PBPK model. Here OSHA presents two PBPK-based risk analyses, both of which represent substantial refinements over the applied-dose risk assessment and over previous PBPK analyses. OSHA's final risk assessment incorporates all reliable data -- OSHA's alternative analysis, in addition to the data in the final risk assessment, also incorporates some suggestive/sparse data found in new studies. As stated above, both analyses estimate risks at 25 ppm well in excess of any possible boundary line between significant and insignificant risk.

Both of OSHA's PBPK analyses made two major advances: 1) the use of non-independent Monte Carlo simulation -- Monte Carlo simulation is a well-developed computational technique that allows the modeler to take estimates of uncertainty in each of the many variables in a complex model and generate a quantitative estimate of the total uncertainty in the result. Others have used Monte Carlo simulation in PBPK modeling, but OSHA added information on the covariance structure of all the parameters, so that the uncertainty estimate would not be biased (exaggerated, probably) by incorrectly assuming that all the variables could simultaneously be at their lowest or highest values; and 2) the use of Bayesian analysis -- this allows uncertainty distributions to be better estimated (narrowed) by cross-checking them against other independently-collected data from laboratory experiments, rather than simply guessing how big the uncertainties are and not refining the estimates as the model runs.

Both these advances enabled OSHA to strike a balance between two unsatisfactory extremes -- a) the extreme overconfidence of using estimates for each variable that did not allow for any uncertainty -- or b) the extreme "underconfidence" of assuming that all uncertainties are independent of each other and of other laboratory data. The result is an analysis that tells what science knows and does not know about the relationship between ambient concentrations and the putative relevant dose measure (concentration of GST metabolites in the target organ) in mice and humans.

Again, OSHA's final risk assessment regards the very limited human data base on GST-0 activity [a total of 39 liver samples and 5 lung samples] as useful, but insufficient to discard the traditional "allometric" assumption (the well-validated assumption that, as a general rule, metabolic parameters scale proportional to a function of the animal's body weight). OSHA's alternative analysis accepts the limited human data at face value to extrapolate without using allometry. OSHA has concluded that the main analysis is better supported by available evidence than is the alternative analysis, but both yield significant risks. An important caveat is that both models are strictly applicable to humans who are physiologically similar to the six subjects analyzed by Dow (see the discussion later in this document for a fuller explanation). Since the population of 200,000 workers will be much more heterogeneous than those six subjects, we regard these estimates as "overconfident" -- some workers exposed at 25 ppm will have higher risks than 3.6 per 1000 (although some may have lower risks as well).


OSHA performs quantitative risk assessment, when information permits, to help determine the Permissible Exposure Limit (PEL) for toxic substances (contingent on the feasibility determination). The first step of assessing risks to human health is hazard identification. This step results in the determination that an exposure to a toxic substance causes, is likely to cause, or is unlikely or unable to cause, one or more specific adverse health effect(s) in workers. This identification also shows which studies have data that would allow a quantitative estimation of risk.

If studies are available that contain information regarding the amount of exposure and disease, mathematical modeling allows extrapolation of the information in the study to conditions of concern in the workplace. OSHA uses these risk estimates to determine whether exposure results in significant risk, and whether the standards considered by OSHA substantially reduce the risk.

This section describes the record evidence received during the public rulemaking concerning OSHA's quantitative risk assessment and the reasons OSHA has maintained or modified its opinion from the proposal. In the following sections, the evidence supporting and casting doubt on the hypothesis that MC is a probable carcinogen (the "Hazard Identification" issues) is discussed first. Then the results of OSHA's quantitative risk assessments, conducted to estimate the carcinogenic potency of MC, are discussed.

A. Methylene Chloride Hazard Identification

Animal and human evidence, summarized in the health effects section, indicates that MC can cause cancer, cardiac effects, central nervous system damage and other health effects. As described in the NPRM, OSHA's preliminary quantitative risk assessment was based on cancer and relied on rodent bioassay data for quantitation of risks. In 1986, the National Toxicology Program (NTP) concluded that the mouse bioassay data provided "clear evidence" of carcinogenesis in male and female mice, based on the liver and lung tumors. The NTP also determined that the rat mammary tumors observed in the bioassay provided clear evidence of carcinogenesis in female rats and some evidence of carcinogenesis in male rats. This evidence of cancer in multiple species and in both sexes underlies the concern for MC as a potential human carcinogen. On the basis of these studies, IARC has classified MC as a 2B carcinogen, the EPA has classified MC as a B2 carcinogen and NIOSH has classified MC as a potential occupational carcinogen. OSHA concurred with these assessments.

Animal bioassays are a critical tool in determining the potential hazard of a substance for humans. Virtually all of the toxic substances that have been demonstrated to be carcinogenic in humans are also carcinogenic in laboratory animals. Although it is possible that a substance may be carcinogenic in a laboratory species, but not in humans, it is reasonable to suspect that substances that cause cancer in multiple animal species and at multiple target organ sites would be carcinogenic in humans. Therefore, in the absence of sufficiently powerful negative epidemiological studies or mechanistic studies demonstrating that the purported carcinogenic mechanism of action of the substance is irrelevant to humans, OSHA and other federal agencies rely on well-conducted, high-quality bioassays as the primary basis for their hazard identification and risk assessment. This is the case with MC.

During this rulemaking, some commenters have supported and others have questioned the hazard identification of MC as a potential human carcinogen. Most recently, some commenters contested the relevance of the mouse bioassay data for extrapolating to human cancer risks. Although these issues were raised by some rulemaking participants earlier in the rulemaking process, they were most thoroughly explored in connection with the information received by the Agency in late 1995. On October 24, 1995, OSHA reopened the MC record to receive comments on several studies submitted to the Agency by the Halogenated Solvents Industry Alliance (HSIA) pertaining to the mechanism of action of MC carcinogenesis in mice, and the implications of these studies for estimating human risks. The record closed on November 29, 1995, but was reopened in order to give the public additional opportunity to comment on the submitted studies. The record then closed on December 29, 1995. Thirty-seven comments were received on this topic and reviewed as part of this rulemaking.

The papers submitted by the HSIA consisted of a cover letter [Ex. 117], an overview of the sponsored research [Ex. 118] and seven research papers on the mechanism of MC carcinogenesis [Ex. 119-124A]. The hypothesis under investigation in these seven studies was that the pathways of MC metabolism and the mechanism of carcinogenesis in the mouse represented a unique situation that would not take place in humans, making the mouse unsuitable as the basis for extrapolating risks of cancer to humans. The specific studies are described briefly here and the comments received during the reopening of the rulemaking record are discussed in detail below.

1. Summary of Studies Submitted by HSIA

Exhibit 119 "Methylene Chloride: an inhalation study to investigate toxicity in the mouse lung using morphological, biochemical and Clara cell culture techniques," J.R. Foster, T. Green, L.L. Smith, S. Tittensor, and I. Wyatt, Toxicology 91 (1994) 221-234.

This study investigated the potential role of MC as a mouse lung carcinogen via non-genotoxic mechanisms and the Clara cell as the cell of origin in mouse lung cancer. The hypothesis was that MC acts specifically to produce toxicity (vacuolation) in Clara cells which leads to cell proliferation and production of mouse lung tumors. The authors investigated the toxicity of MC in bronchiolar Clara cells by measuring the production of vacuoles after exposure to MC. The investigators also measured DNA synthesis in Clara cells isolated from mice exposed to MC as a measure of cell proliferation.

The authors observed a transient vacuolation of bronchiolar Clara cells in mice exposed to 2000 and 4000 ppm MC, but not in mice exposed to 0, 125, 250, 500 or 1000 ppm MC. When the mixed function oxidase (MFO) pathway was inhibited, the bronchiolar cell vacuolation observed after exposure to 2000 and 4000 ppm MC was reduced. Inhibition of the glutathione S-transferase pathway (GST) had no effect on Clara cell vacuolation. The researchers also found that exposure of mice to 1000 ppm MC or greater for 6 hours induced an increase in DNA synthesis in Clara cells cultured in vitro from exposed animals.

Clara cells are present in mice, rats and humans, but appear to be more abundant in mice than other species. Clara cells contain enzymes for both the MFO and glutathione S-transferase (GST) pathways of MC metabolism. According to the authors, the results of this study suggest that metabolism of MC via the MFO pathway induces a transient toxicity in Clara cells and a transient increase in DNA synthesis.

Exhibit 120 "Methylene chloride-induced DNA damage: an interspecies comparison," R.J. Graves, C. Coutts and T. Green, Carcinogenesis, vol. 16 no. 8 pp. 1919-1926, 1995.

This study investigated the role of MC as a mouse carcinogen via a genotoxic mechanism of action. The hypothesis under investigation was that MC is metabolized to a genotoxic carcinogen via the GST pathway to different extents in different species and that expression of this genotoxicity correlates with risk of developing cancer across species. The authors used production of single strand (ss) DNA breaks as a measure of genotoxicity. The researchers measured DNA ss breaks in lung and liver cells from mouse, rat, hamster and humans. They observed increased DNA ss breaks in mouse liver cells, after in vivo exposure to 4000-8000 ppm MC for 6 hr and in mouse lung cells after exposure to 2000-6000 ppm MC. Depletion of glutathione in the liver (after administration of buthionine sulfoximine) reduced the amount of ss breaks observed. No increase in ss breaks was observed in Clara cells isolated from mice exposed to MC in vivo. However, in experiments on isolated mouse Clara cells, the authors observed increased DNA ss breaks in cells exposed to concentrations of MC of 5 mM and above.

No increases in ss breaks above control levels were detected in rat livers after exposure to 4000 ppm for 6 hr or in rat lungs after exposure to 4000 ppm for 3 hr. Increases in ss breaks were also not detected in hamster and human liver cells after exposure to MC in vitro at concentrations up to 90 and 120 mM.

In Chinese hamster ovary (CHO) cells, MC plus mouse liver cytosol (which contains the GST enzymes) also induced ss breaks, while incubation of CHO cells with MC in the presence of mouse liver microsomes (which contain the MFO enzymes) did not increase ss breaks.

The results suggest that mouse liver and lung cells are more susceptible to MC-induced ss breaks than cells from rats, hamsters or humans. Assuming that ss breaks are a relevant surrogate for carcinogenicity, the authors infer from this study that humans, rats and hamsters are insensitive to MC-induced liver cancer, because those species lack the high level of GST metabolic activity to MC found in the mouse liver cell and lung Clara cell.

Exhibit 121 "Isolation of two mouse theta glutathione S-transferases active with methylene chloride," G.W. Mainwaring, J. Nash and T. Green, Zeneca Central Toxicology Laboratory, 1995.

This study was conducted in order to characterize the mouse GST isozyme(s) responsible for MC metabolism. The results of this work could be used to explore the hypothesis that a particular GST isozyme was responsible for metabolizing MC to the carcinogenic metabolite and that there may be different concentrations of this enzyme across species.

The researchers used a variety of chromatography methods to isolate two mouse glutathione S-transferases (MT-1 and MT-2, also known as T1-1* and T2-2*, respectively) which metabolize MC, comparing the observed enzyme activity with that described in rats. Rats were found previously to have two GST isomers in the theta class (GST 5-5 and GST 12-12) which metabolized MC. The mouse MT-1 and MT-2 enzymes were found to be closely related to rat GST 5-5 and 12-12, respectively, and the specific activity of mouse MT-1 was found to be similar to rat GST 5-5. GST 12-12 and MT-2 were found to be extremely labile during purification, and so the specific activities of those isozymes have not been measured.

The results of this study suggest that the mouse and rat contain GST theta enzymes similar in amino acid sequence and in specific activity (GST 5-5 and MT-1). The authors postulate that the greater conjugating activity seen in mice in other studies is "probably due to a difference in expression of the enzyme or to a significant contribution from MT-2" [Ex. 121].

Exhibit 122 "Mouse Liver glutathione S-Transferase Mediated Metabolism of Methylene Chloride to a Mutagen in the CHO/HPRT Assay," R.J. Graves and T. Green, Zeneca Central Toxicology Laboratory, 1995.

This study investigated the mutagenicity of MC as a potential carcinogenic mechanism of action. The purposes of this study were to clarify the ability of MC to act as a mutagen, because studies in mammalian systems have yielded mixed results regarding the mutagenicity of MC, and to more fully characterize the metabolite purportedly responsible for MC mutagenicity by comparing the results to formaldehyde (one metabolite of MC by the GST pathway). Mutagenicity was measured by assaying CHO cells in vitro for mutations at the HPRT locus of DNA. Ss DNA breaks were also monitored. Cells were exposed in culture to MC mouse liver cytosol metabolites (which include metabolic enzymes for the GST but not the MFO pathway), formaldehyde (one of the MC GST metabolites) or 1,2-dibromoethane (1,2-DBE) (a reference genotoxin).

Using standard techniques, MC GST metabolites were shown to be weakly mutagenic using the CHO/HPRT assay. Formaldehyde was also determined to be weakly mutagenic in this assay, but the effect was not as great as with MC GST metabolites. 1,2-DBE, as expected, showed a potent mutagenic response. The mutagenicity of MC GST metabolites and formaldehyde was increased when cell density was increased, cells were exposed in suspension rather than as attached cultures and cytosol concentration was optimized.

MC mouse liver cytosol metabolites were observed to increase ss DNA breaks in CHO cells exposed in suspension, but caused only marginal increases in DNA-protein cross-links. In contrast, the researchers found that formaldehyde induced both DNA ss breaks and DNA-protein cross-links. Slight increases in ss DNA breaks were also seen with exposure to either MC alone or the cytosol fraction alone.

Based on a comparison of the mutagenic effects of the three compounds, particularly on the lack of MC-induced DNA-protein cross-linking in this experimental system, the authors concluded that formaldehyde does not play a major role in MC mutagenicity. Accordingly, the researchers viewed the results of this study as supporting the hypothesis that the DNA ss breaks induced by MC, and the resultant DNA mutations, are caused by interaction of S-chloromethyl-glutathione (formed by the GST pathway) with DNA.

Exhibit 123 "DNA Sequence Analysis of Methylene Chloride-Induced HPRT Mutations in CHO Cells: Comparison with the Mutation Spectrum Obtained for 1,2-Dibromethane and Formaldehyde," R.J. Graves, P. Trueman, S. Jones and T. Green, Zeneca Central Toxicology Laboratory, 1995.

The purpose of this study was to describe the types of mutations induced by MC in order to further characterize the GST metabolite likely to cause MC mutations and therefore perhaps be responsible for the carcinogenicity of MC in the mouse. The spectrum of mutations in the HPRT locus of CHO DNA induced by MC plus mouse liver cytosol was compared to mutations induced by formaldehyde (a GST metabolite of MC) or 1,2-dibromoethane (1,2-DBE, a reference genotoxin).

The results were expressed as a sequence analysis of 11 MC-induced mutations, 6 formaldehyde-induced mutations and 13 1,2-DBE-induced mutations. In comparing the distribution of types of mutations, the results suggested to the researchers that formaldehyde-induced DNA damage can contribute to MC mutagenicity, but that the majority of the mutations were derived from other types of DNA damage, probably via an interaction of S-chloromethylglutathione with DNA. The researchers noted that a glutathione conjugate also plays a role in the mutagenicity of 1,2-DBE. The increases above background mutation frequency detected through this study were 24.7-fold for 1,2-DBE, 4.7-fold for formaldehyde, and 8-fold for MC.

Exhibit 124 "The distribution of glutathione S-transferase 5-5 in the lungs and livers of mice, rats and humans" [Preliminary communication, T. Green, 1995].

Exhibit 124A "The distribution of theta class glutathione S-transferases in the liver and lung of mouse, rat and human." G.W. Mainwaring, S.M. Williams, J.R. Foster and T. Green,1995.

The preliminary communication [Ex. 124] and the unpublished report which followed [Ex. 124A] summarized the results of a study comparing the inter- and intra-cellular distribution of the messenger RNA (mRNA) for a glutathione S-transferase (GST) isoenzyme which metabolizes MC in the lungs and livers of mice, rats and humans. The purpose of the experiments summarized in these reports was to describe the distribution of the mRNA for the GST theta isozyme believed to be responsible for metabolism of MC to a carcinogenic metabolite in different species. The researchers believed that differences in distribution of the mRNA for this isozyme would correlate with differences in distribution (and activity) of the isozyme itself, and might explain differences in sensitivities of the species to the carcinogenicity of MC.

The distribution of GST theta mRNA was visualized using DNA oligonucleotide anti-sense probes complementary to the nucleotide sequences for the GST theta isozymes. This technique is used to visualize the mRNA coding for a specific protein (such as the GST theta isozymes) within cells in tissues. The mRNA is a nucleotide sequence transcribed from the DNA containing the gene for the specific protein. After transcription, mRNA is transported to the cytoplasm, where it is translated into the amino acid sequence which becomes the specific protein (in this case, the GST theta isozyme). The finished protein then migrates to its final site of activity within the cell. Localization of the mRNA does not necessarily correspond to localization of the specific protein.

The results of the study showed that the GST-specific mRNA could be found in lungs and livers of all three species. Mouse liver cells (particularly the nuclei) and mouse lung cells appeared (from the photomicrographs shown in the article) to stain more heavily for the GST mRNA than the lung or liver cells from rats or humans. Although the amount of GST-specific mRNA was not quantified in this study, the authors interpreted the photographs to suggest that, "* * * mouse tissues are stained much more heavily than sections from either rat or human." Based on the intracellular and intercellular distribution of the GST mRNA, the authors stated,

The most significant findings are the presence of very high concentrations of GST 5-5 mRNA in specific cells and nuclei of mouse liver and lung. Metabolism of methylene chloride at high rates and within nuclei to a reactive but highly unstable glutathione conjugate is believed to facilitate alkylation of DNA by this metabolite. The lack of high or nuclear GST 5-5 concentrations in rat and human tissue, provides an explanation for the lack of genotoxicity in these species. [Ex. 124]

In the letter submitting the studies summarized above to OSHA, HSIA characterized the studies as follows:

This research, which is now complete, shows that B6C3F1 mice * * * are uniquely sensitive at high exposure levels to methylene chloride-induced lung and liver cancer, and that other species, including humans, are not at similar risk. [Ex. 117]

They went on to conclude:

As a result of this research program, it appears that there are no foreseeable conditions of human exposure in which the carcinogenic effects seen in mice would be expected to occur in man. * * * The risk assessment that is the basis for the methylene chloride standard, which is in turn based on the increased liver and lung tumor incidence observed in the mouse bioassay, must be discarded in favor of scientific data that are relevant to human risk.

In response to the request by HSIA, OSHA has reviewed the cancer hazard identification of MC based on all of the evidence in the MC record, with particular emphasis on the validity of the conclusion stated immediately above. This review is presented below.

2. Carcinogenesis of Methylene Chloride

a. Animal evidence. Several long-term MC bioassays have been conducted and are summarized in the Health Effects section. These included studies in which the route of exposure was inhalation [Burek et al., Ex. 4-25, Nitschke et al., Ex. 7-29, and NTP, Ex. 4-35] and two studies in which the route of exposure was drinking water [National Coffee Association, Exs. 7-30, 7-31]. In order to ensure full consideration of the data, OSHA analyzed in its preliminary assessment all data sets which showed an elevated incidence of tumors in a MC-exposed group, compared to controls, whether or not the elevation of tumor response was statistically significant. This analysis and the individual datasets used were described in detail in the NPRM.

In the NTP bioassay [Ex. 4-35], groups of 50 nine-week old B6C3F(1) mice of each sex were exposed by inhalation to 0, 2000 or 4000 ppm MC. Groups of 50 eight-week old F344/N rats of each sex were exposed to MC at concentrations of 0, 1000, 2000, or 4000 ppm. The inhalation exposures were administered 6 hours a day, 5 days a week for 102 weeks. Food was provided to the animals ad libitum except during the exposure periods, while water was available at all times via an automatic watering system. All animals were observed twice a day for mortality and moribund animals were sacrificed. Clinical examinations were performed once a week for 3.5 months, then twice a month for 4.5 months, and once a month thereafter. Each animal was also weighed weekly for 12 weeks, then monthly until the conclusion of the study at 102 weeks. All animals were necropsied and histologically examined. Three different neoplastic lesions were observed to have significantly increased incidence over the controls: adenomas and carcinomas of the lung in male and female mice, adenomas and carcinomas of the liver in male and female mice, and mammary gland fibroadenomas and fibromas in male and female rats.

HSIA and others argued that benign tumors, especially the mammary tumors in the rats, should not be counted as a carcinogenic response. The NTP has addressed that issue in its Technical Report [Ex. 4-35] and has concluded that the benign mammary tumors observed in the F344 female rats are "clear evidence" of carcinogenicity and noted that such tumors may proceed to malignancy. OSHA agrees with this determination and has considered the rat mammary tumors as part of its cancer hazard identification for MC. However, OSHA's quantitative risk assessment does not consider rat mammary tumor responses.

OSHA believes that the NTP studies provide the strongest evidence of carcinogenicity of MC in animals. Many commenters and hearing participants [Exs. 19-46, 7-128, 7-126, 25-E, 126-11,126-12, 126-16 and others] supported the use of the NTP mouse study as the basis for quantitative risk assessment. There are several reasons for this described in the proposal and earlier in this document. In brief, the NTP study used well established standard operating procedures that are generally considered a predictor of a potential carcinogenic response in humans. This study was also replicated by a second partial bioassay, conducted by NTP, in which groups of female mice were exposed to 2000 ppm MC for 2 years. Statistically significant increases in alveolar/bronchiolar and hepatocellular tumors were observed [Ex. 27].

Before the 1995 record reopening, some commenters had raised specific arguments why a mouse study might not predict human carcinogenic response to MC. Mr. Krenson of Besway Systems [Tr. 397, 9/17/92] objected to OSHA using the NTP mouse study as the basis for setting the PELs for MC. He believed that the mouse was irrelevant to human risk because the doses used were "extremely high" and that he believed that tests conducted on rats, hamsters and human epidemiological investigations showed "no conclusive proof of cancer in human beings." OSHA disagrees with Mr. Krenson's conclusion. In general, high doses in rodent bioassay studies are appropriate to elicit a response due to the practical limitations on the number of animals that can be used in a study. In MC, there was no observed acute toxicity at the levels used in the study, which is an indication that the doses were not too high. Use of high doses in bioassay studies is common and its practical necessity has been affirmed by numerous expert bodies, including several committees of the National Academy of Sciences. In addition, for every known human carcinogen, positive results were obtained at high rodent doses. Also, quantitative comparisons, as conducted by Allen and Crump in 1988, demonstrate that, in general, observations of cancer potency from epidemiology studies agree with estimates of potency derived from rodent bioassay data. In the case of MC, statistically significant excess tumors were observed in mice after exposure to only 2000 ppm, or only four times the former PEL of 500 ppm (8-hour TWA), and excess tumors were seen in rats at 4000 ppm. This level is within the range of human exposures experienced in occupational settings. Certainly the lower exposure showing substantial effect was not "extremely high" in relation to the exposure limit, as Mr. Krenson claimed.

The HSIA and several others [Exs. 117, 126-1, 126-3, 126-5, 126-6, 126-8, 126-10, 126-13, 126-20, 126-21, 126-29] also objected to using the mouse data as the basis of human risk assessment, based on the mechanism of action studies submitted to the Agency by HSIA on December 6, 1995. OSHA's analysis of the individual studies follows, but overall, the Agency has determined that the mouse cancer data are appropriate for assessment of the cancer risks to humans (although, as discussed later in this section, OSHA has made extensive use of the submitted data to modify the quantitative estimates of risk derived from the mouse model).

b. Evidence pertaining to the mechanism of action of methylene chloride. Several lines of evidence relate to the mechanism of carcinogenesis of MC. The issues discussed in the papers submitted by the HSIA and subsequent comments can be divided into those pertaining to genotoxicity, those discussing potential non-genotoxic modes of action, and those related to the enzymatic metabolism of MC. Although some comments overlap these divisions, this organization is used in this discussion to simplify consideration of the issues.

(1) Genotoxicity. It has not been conclusively demonstrated that MC or its metabolites act by a genotoxic mechanism in mice and rats. Substance-specific DNA adducts, which are among the strongest evidence of direct genotoxicity, have not been identified from MC exposure. However, evidence has been accumulating that MC is likely to be carcinogenic through a genotoxic mechanism of action. For example, DNA-protein cross-links have been demonstrated in mouse liver [Ex. 21-16], increases in unscheduled DNA synthesis have been demonstrated in mouse lung [Ex. 126-25] and other evidence of MC metabolite interaction with mammalian DNA (such as increases in ss DNA breaks) has been observed. It is not necessary for a substance to bind covalently with DNA in order to act via a genotoxic mechanism, although evidence of covalent binding is a strong indication of genotoxicity. In the case of MC, although the reactive metabolites are presumed to exert a genotoxic effect by binding to DNA, no MC metabolite-DNA adducts have yet been identified. However, RNA adducts have been identified after MC exposure, which supports the hypothesis that MC acts by a genotoxic mechanism. Substance-specific DNA adducts have also not been identified for some other carcinogens which are presumed to act via a genotoxic mechanism.

In addition, as discussed in the Health Effects section, MC has been found to be mutagenic in bacterial, yeast, Drosophila and mammalian systems; associated with chromosomal aberrations in CHO cells; and associated with sister chromatid exchanges in mammalian cell culture systems, such as CHO and V79 cells.

Investigations of the role of metabolites of the GST pathway in the bacterial mutagenicity of MC found that in glutathione-deficient strains of Salmonella typhimurium MC-induced mutations were reduced [Ex. L107]. Mutation rates returned to normal when bacteria were supplemented with exogenous glutathione. This study supports the hypothesis that MC may act as a genotoxic carcinogen via its GST metabolites, although a study of similar design by Dillon et al. [Ex. 21-89] did not replicate these results.

(i) MC induced mutuations. Studies on the MC mechanism of carcinogenesis included two studies on the mutations induced by MC in the CHO/hypoxanthine phosphoribosyl transferase (HPRT) assay. In the 1995 study by Graves et al. [Ex. 122], the investigators compared mutations induced by MC with those induced by formaldehyde and 1,2-dibromoethane. The authors characterized the results of the studies as follows:

Using the CHO/HPRT assay we have shown that MC is metabolized to a mutagen by mouse liver cytosol in a reaction which is dependent upon GST and GSH. Mutagenicity was enhanced by exposing the cells at high density in suspension rather than as attached cultures, which is consistent with the critical metabolites being extremely short-lived.

The authors also observed that the MC-induced mutations were associated with an increase in DNA ss breaks. They remarked, "The results suggest that MC-induced DNA ss breaks seen in other cell types are associated with DNA damage which can lead to mutation."

In a follow-on to the CHO/HPRT study, Graves et al. [Ex. 123] conducted a sequence analysis of HPRT mutations in CHO cells, comparing the spectrum of MC-induced mutations with those induced by 1,2-dibromoethane or formaldehyde. The investigators analyzed 28 HPRT mutations: 13 from 1,2-dibromoethane experiments, 6 from formaldehyde experiments, and 11 from MC experiments. The authors characterized their results as follows,

All three compounds induced primarily point mutations, with a small number of insertions and deletions. * * * The mutation sequence results for MC suggest that formaldehyde may also play a role in MC mutagenesis, although the majority of mutations arise from other types of DNA damage, probably DNA adducts formed by reaction of S-chloromethyl glutathione with DNA.

Dr. Douglas A. Bell of NIEHS [Ex. 126-26] had specific comments regarding the study on the mutation spectra [Ex. 123]. He stated,

This experiment is extremely weak scientifically and not of publication quality. It is unlikely that such a naive experiment could detect differences in spectra between the different chemicals tested. To test the hypothesis that there are chemical specific mutation spectra requires analysis of hundreds of mutants at several different doses. This exhibit contains no useful information for risk assessment.

OSHA agrees with Dr. Bell that there are serious methodological problems with the paper. The Agency also agrees with Dr. Bell that the important information in these two studies is that MC increases the mutation frequency, showing a clear genotoxic effect.

(ii) Single strand DNA breaks. In a 1995 study, Graves et al. [Ex. 120] investigated the role of MC exposure in development of single strand (ss) DNA breaks in the lung and liver of mice and rats and in hamsters and human cell cultures. The authors observed a transient, dose-related increase in DNA ss breaks in mouse hepatocytes after inhalation exposure to MC. No increased amount of ss breaks was observed in rat liver cells exposed to MC as compared to control cells. The authors also reported a decrease in the amount of ss DNA breaks in liver and lung when a glutathione depletor was administered to mice immediately before MC exposure.

In mouse and rat hepatocytes incubated with MC, the authors found increases in ss breaks, but no increases in ss breaks in hamster or human hepatocytes exposed in vitro were observed. No increase in DNA damage was observed in CHO cells exposed to MC plus mouse liver microsomes, while MC plus mouse liver cytosol induced detectable ss DNA breaks.

The authors characterized their findings in the lung as follows:

Here we show that Clara cells are also sensitive to MC-induced DNA ss breaks and that the DNA-damaging metabolites are derived from the GST pathway. * * * Overall, these findings support the proposal that Clara cells are the cell of origin of MC-induced mouse lung tumors.

For liver cancer, the investigators concluded:

These studies suggest that humans (and rats and hamsters) are insensitive to MC-induced liver cancer.

Commenters raised issues about the relevance and utility of ss DNA breaks in assessing the genotoxicity of MC. Dr. Karl T. Kelsey [Ex. 126-34] and Dr. Miriam Poirier [Ex. 126-37] raised concerns about the sensitivity of the DNA ss break assay for detecting genotoxic effects.

Specifically, Dr. Kelsey stated,

Reviewing the literature, considerable weight seems to fall upon the measure of DNA single strand breaks. I have serious concerns about this assay. It is well known that the assay is extraordinarily difficult to standardize and is sensitive only to very high doses of genotoxic compounds. This data, therefore, is certainly not compelling; persuading any competent independent scientist of its relevance to humans would be difficult.

Dr. Poirier was concerned with the sensitivity of the DNA single strand break assay and the relevance of DNA ss breaks to carcinogenesis. She remarked that ss DNA breaks and mutagenicity are secondary indicators of DNA damage. She indicated that a better measure of genotoxicity would be formation of DNA adducts. Dr. Errol Zeiger [Ex. 126-28] of NIEHS agreed, stating,

If the mechanism of carcinogenicity is through an alkylating S-chloromethyl GSH complex, there should be evidence of DNA adducts in vitro and in vivo.

OSHA agrees that DNA adducts are strong evidence of genotoxicity and that ss DNA breaks and mutagenicity are not as specific or relevant as indications of a genotoxic mechanism of action. However, the Agency has determined that, even in the absence of identified MC-specific DNA adducts, the accumulated evidence suggests that MC interacts with DNA via a genotoxic mechanism of action and that the GST pathway is a plausible carcinogenic pathway.

Dr. Melnick [Ex. 126-33] stated, "* * * it has not been demonstrated that the carcinogenicity of MC in mice is dependent solely on the induction of DNA single strand breaks." Dr. Andrew G. Salmon concurred with this analysis and also raised a serious concern about the ability of the assay even to detect increases in ss breaks, regardless of their relevance:

Green's account states that "mouse hepatocytes were * * * 20-fold * * * more sensitive to the effects of methylene chloride [i.e., DNA strand breaks] than rat hepatocytes * * * " and no breaks were detected in hamster or human liver cells. This is translated in the discussion to an assertion that not only humans and hamsters but also rats are completely immune to the carcinogenic effect of methylene chloride. However, the data simply do not support the assertion of a categorical difference as proposed by the HSIA. This particular work also raises a number of other issues, such as whether the liver is an appropriate model tissue, and whether single-strand breaks are an appropriate indicator of the type of genetic damage produced by the putative genotoxic metabolites of methylene chloride.

OSHA agrees that the ss DNA break assay is not as sensitive as other methodologies for assessing the genotoxic potential of MC in different systems and therefore data from the ss DNA break study must be interpreted in a quantitative, not qualitative context, with allowance for uncertainty in assay sensitivity. It is also unclear whether ss DNA breaks are the appropriate surrogate measure for carcinogenic potential. In light of the issues raised by commenters, the Agency believes that the ss DNA break data should be interpreted with caution.

(iii) DNA-protein cross-linking. Casanova and Heck [Ex. 21-16] observed DNA-protein crosslinks in mouse liver, but not mouse lung, after exposure to 500, 1500 and 4000 ppm. This study indicated that metabolites of MC have the ability to interact with DNA. However, the quantity of DNA-protein crosslinks did not show a strong correlation with tumor incidence, and so the DNA-protein crosslinks were not used as a dose-surrogate for MC exposure in OSHA's risk assessment.

The Chemical Industry Institute of Toxicology (CIIT) [Ex. 126-25] submitted further evidence that MC exposure causes DNA-protein cross-links in mouse liver but not mouse lung, hamster liver or hamster lung. These investigators also observed RNA adducts in mouse, rat and human cells after incubation with MC, but DNA-protein cross links were only observed in the mice. In addition, they submitted a pharmacokinetic model which modeled the DNA-protein cross-links as the dose surrogate for MC exposure. Finally, they made extensive comparisons of their model with the PBPK model submitted by Clewell [Ex. 96] and EPA's risk assessment for MC. Dr. Roger McClellan summarized the conclusions they reached as follows,

The pharmacokinetic results suggest that at very low concentrations of DCM [methylene chloride], the yield of DPX [DNA-protein cross-links] is almost linearly proportional to DCM concentration * * * DPX cannot be used directly as a surrogate for the internal dose in humans, however, because human hepatocytes, unlike mouse hepatocytes, do not appear to form DPX in measurable amounts in vitro. * * * These results suggest that the mouse may not be an appropriate animal model for human risk assessment due to its unusual susceptibility to DPX formation and to the fact that cell proliferation is a uniquely high-dose phenomenon that may occur only in this species.

OSHA believes that this work provides more evidence for the formation of genotoxic metabolites in mouse liver after MC exposure. However, OSHA is not convinced that the DNA-protein cross-linking is the appropriate dose-surrogate for pharmacokinetic modeling. One of the strengths of Reitz's and subsequent PBPK models was that the dose surrogate used in the modeling was linearly related to tumor incidence. That is one reason that many investigators have focused on the GST pathway, instead of the MFO pathway of metabolism as the carcinogenic pathway. As explained by Dr. Lorenz Rhomberg [Ex. 126-16],

* * * if this proportionality in the case of GST is broken by a deeper analysis, the rationale for focusing only on GST must be reevaluated.

Dr. Rhomberg was referring to results presented by HSIA on the distribution of GST theta isozymes within and among cells, but the same sentiment applies here; if OSHA were to abandon PBPK modeling using GST metabolites, all of the HSIA and other studies would have to be re-evaluated and considerable more research might need to be done. Finally, in the CIIT study, RNA adducts, a more direct measure of genotoxicity than DNA ss breaks, were observed in human hepatocytes after incubation with MC. The amount of RNA adducts in human cells was only about 3-fold lower than the amount in mouse hepatocytes. It is therefore clear that human hepatocytes in this system are forming genotoxic metabolites after exposure to MC.

OSHA notes that, in mouse lung, the DNA-protein cross-links were not observed, even though a clear dose-response relationship for tumors has been established at this site. OSHA is not convinced that the explanation for carcinogenesis in mice is DNA-protein cross-links in liver. Overall, it is unclear whether the interspecies difference in DNA-protein cross-linking is related in any way to the carcinogenic mechanism of action.

OSHA concludes that there continue to be strong reasons for using the mouse data as the basis for its quantitative risk assessment because there is a clear dose-response relationship in the mouse liver and lung tumor incidence data; the mouse metabolizes MC by the same pathways as humans; PBPK models have been developed which account for inter-species differences in MC metabolism; statistical techniques have been developed to quantify the uncertainty and variability in the parameters used in the PBPK models; and there are no data that demonstrate that the mouse is an inappropriate model for assessing human cancer risks. In fact, OSHA finds further evidence in the studies described above which suggest that MC acts via a genotoxic mechanism in human cells as well as in mice and rats, which further supports OSHA's use of the mouse tumor incidence as the basis for quantitative risk.

(iv) Interpreting the genotoxicity studies. Several other issues were raised regarding interpretation of the results of these studies on the genotoxic mechanism of action of MC. NIOSH and others [Exs. 126-30, 126-11, 126-32] commented that, in general, the data presented by HSIA supported the hypothesis that the carcinogenic metabolite(s) of MC were derived from the GST pathway. They agreed with HSIA's interpretation of the data that the studies presented here helped to confirm that the mechanism of MC carcinogenesis is through one or more genotoxic metabolites of the GST pathway.

Interpretation of short-term effects in explaining chronic mechanisms of action.

Concerns were raised about the generalizability of the results of short-term genotoxicity assays to tumor incidence, especially when the observed effect is transient, as in the vacuolation of Clara cells, the appearance of ss DNA breaks in mouse liver and lung cells, etc. Dr. Mirer of the UAW [Ex. 126-31] commented,

1. The evidence cited concerns acute effects which appear after a few hours of high level exposure of the animal to methylene chloride vapor, or the glassware (in vitro) mixing of homogenized animal or human tissue with the solvent. In a number of studies the effect in the whole animal is transient.

2. There is no evidence to connect the acute toxic effect, or single strand breaks of DNA after acute exposure, to the chronic effect of lung or liver injury, or cancer. * * *

Dr. Maronpot [Ex. 126-22] was concerned that the vacuolation observed in Clara cells was not reproduced in the NIEHS mechanistic studies. HSIA responded to this concern by remarking that the vacuolation could only be found after single exposures to MC, and that the vacuolation of Clara cells was also associated with increased DNA synthesis in these cells. The fact that this response was only observed after single exposures to MC again raises the issue of the transience of this response and its relevance to MC carcinogenesis.

Increased cell turnover. In these studies on genotoxicity, the authors remarked that increased cell turnover was observed in the lung (transient increase in DNA synthesis after single exposures to MC). Dr. Daniel Byrd [Ex. 126-32] also commented on the DNA synthesis issue. Citing an HSIA study, he contended that there appeared to be a common mechanism of action between the lung and the liver since increased DNA synthesis was observed in both tissues. Dr. Maronpot of the NIEHS [Ex. 126-22] disagreed, stating,

The purported "liver growth" in methylene chloride-exposed mice is actually an increase in liver weight attributable to accumulation of glycogen within hepatocytes. There is no evidence of replicative DNA synthesis (cell proliferation) in the liver of methylene chloride-treated mice, and, hence, actual increases in the numbers of hepatocytes did not occur. * * * It is noteworthy that recovery to normal liver weight occurs within two weeks after cessation of exposure to methylene chloride.

OSHA agrees with Dr. Maronpot that no data in the rulemaking record show increases in liver cell proliferation as the result of MC exposure, although increased DNA synthesis was actively searched for in the NIEHS mechanistic and other studies. The increased DNA synthesis observed in mouse Clara cells is a transient phenomenon that has not been clearly linked to carcinogenesis in the mouse. In any event, cell proliferation is not necessarily related in any way to carcinogenesis and is often uncorrelated with the doses used in bioassays and the tumor rates themselves. Many substances that cause prolonged cell proliferation do not cause tumor formation and vice versa [Ex. 126-22], and many experts believe that transient increases in cell proliferation, such as seen with MC, cannot account for the carcinogenic effect. Further discussion of cell turnover as a mechanism of carcinogenicity is discussed below under "Non-genotoxic mechanisms."

Clara cell as the mouse lung tumor cell of origin.

Another issue raised by commenters concerned the cell of origin of the mouse lung tumors. The mouse lung has a higher proportion of Clara cells than the human lung. The investigators hypothesized that if the Clara cell were the mouse lung tumor cell of origin, the risk estimated from the mouse lung tumor data may overstate human risk because humans have fewer Clara cells, and therefore fewer potential target cells.

Green et al. have focused much of their research efforts into determining the mechanism of action of MC in mouse lung and liver. In lung tissue, as described above, they concentrated on experiments addressing the hypothesis that the mouse Clara cell is the cell of origin of the mouse lung tumors observed in the NTP bioassay. Dr. Daniel Byrd [Ex. 126-32] indicated that he believed that the data presented supported this conclusion. He stated, "Mouse lung tumors most likely arise from damaged Clara cells, although a few pathologists continue to speculate that mouse lung tumors arise from other lung cells, such as Type II pneumocytes."

In contrast, Dr. Maronpot of the NIEHS [Ex. 126-22] disagreed with that statement, indicating that "* * * current belief among researchers is that mouse lung tumors arise from Type II pneumocytes rather than Clara cells." Dr. Melnick [Ex. 126-33] suggested that the HSIA data are not consistent with the hypothesis that the Clara cell is the tumor cell of origin. He stated,

DNA damage was detected in lungs of mice exposed to 2000 ppm methylene chloride; however, no significant increase in DNA single strand breaks was observed in Clara cells isolated from mice exposed to 4000 ppm methylene chloride. This observation does not support the conclusion that Clara cells were the cells of origin of methylene chloride-induced mouse lung tumors.

In their paper, Graves et al. [Ex. 120] explain their results as follows,

Attempts to measure DNA damage in Clara cells isolated from mice which had been exposed to MC in vivo were unsuccessful. * * * [I]t is possible that cells extensively damaged by MC do not survive the isolation procedure. The observation that the in vivo vacuolation of Clara cells observed after MC treatment is not seen in vitro when the cells are isolated from the damaged lungs supports this proposal.

This means that the authors could induce ss breaks in the DNA of Clara cells in vitro, but in mice exposed to MC in vivo, it is not clear that the DNA ss breaks observed in lung tissue were concentrated in the Clara cells. In fact, the authors state,

Since Clara cells represent only 5% of the total lung cell population, the DNA ss breaks observed in vivo may not exclusively result from damage to this cell population.

OSHA believes that these issues raise serious doubts as to whether current evidence supports the determination that the Clara cell is the cell of origin of the mouse lung tumors. Although the absence of increased ss breaks is not necessarily an indication of lack of genotoxicity, the presence of ss breaks in lung tissue (and apparently not concentrated in Clara cells) reveals an inconsistency in HSIA's argument: either the ss breaks are irrelevant or Clara cells are not the cells of origin, or both. Further discussion of the issues surrounding identification of the Clara cell as cell of origin for mouse lung tumors is contained below under "Non-genotoxic mechanisms of carcinogenesis."

Ability of MC reactive metabolites to cross membranes. Although no data were presented by the HSIA to address this issue directly, several of the HSIA papers and the accompanying letters postulate that the reactive metabolites of the GST pathway are too short-lived to cross membranes. This argument is used in combination with the claim of high concentrations of the mRNA for the GST T1-1* in the nuclei of mouse cells (but not those of rats and humans) to support the contention that humans are not at risk of developing cancer after exposure to MC. The reasoning is as follows: (1) Mice are the only species to have high concentrations of GST T1-1* in the nucleus of lung and liver cells. (2) The reactive metabolites of the GST pathway are too short-lived to cross the nuclear membrane. (3) In order to produce a carcinogenic effect, reactive metabolites must be produced inside the nucleus in proximity to the DNA. (4) Because the mouse has high concentrations of these enzymes in the nucleus (and rats and humans do not), the mouse is uniquely susceptible to lung and liver cancer after exposure to MC. (5) Therefore, there is no risk of humans developing cancer after exposure to MC.

Some commenters [Exs. 126-12, 126-30, 126-33] maintained that HSIA's submitted studies do not support this argument. As discussed subsequently, the probe used in these experiments measured GST T1-1* mRNA, not the isozyme itself. There is not necessarily a correlation between the intracellular concentration of mRNA and the concentration of enzyme at a specific locus. In addition, one would expect there to be higher mRNA outside the nucleus (since that is where the enzyme is transcribed from the mRNA), even if the enzyme were subsequently concentrated inside the nucleus. Additionally, as discussed previously, some of the evidence presented by HSIA suggests that the metabolites can be generated outside the cell (not simply outside the nuclear membrane) and interact with the DNA. Specifically, Dr. Dale Hattis [Ex. 126-12] has remarked that,

* * * as long as these reaction and detoxification processes are not infinitely fast (and in principle they cannot be infinitely fast), a finite fraction of the activated metabolite molecules must reach the DNA and react. Even though this chain of events is required by our basic understanding of the relevant kinetic processes, in this case we also have direct empirical evidence that active metabolites need not be generated in a cell's nucleus in order to reach DNA and do damage. The DNA sequence mutations of Graves and Green [Ex. 122] and Graves et al. [Ex. 123], and the DNA single strand breaks reported by Graves et al. [Ex. 120] for CHO cells were all produced by exposing mammalian cells to a tissue culture medium that had been supplemented with mouse metabolizing enzymes and methylene chloride. The active metabolites in those cases were necessarily generated from outside of the cells, not just in the cytoplasm of the cells that manifested the DNA damage. Therefore, the claim that the active glutathione transferase metabolite(s) must be generated in the nucleus and would be ineffective if generated in the cytoplasm is flatly contradicted by HSIA's own evidence.

HSIA [Ex. 126-29] strongly disagreed that their results should be interpreted in this way and countered as follows:

The investigators had to use a suspension assay to maximize the concentration ratio of methylene chloride to cells to about 10(14), and to optimize the GST activity from mouse liver preparation. Only under these extreme nonphysiological conditions with a transformed cell line could any increase in mutation frequency be observed. There is absolutely no justification for assuming similar conditions in humans, where GST activity is absent or at very low levels in the cytoplasm and absent in the nucleus.

OSHA disagrees with HSIA, however, and finds Dr. Hattis' and the other commenters' reasoning more sound. The results of these experiments indicate that the metabolites of MC are stable enough to cross the cellular and the nuclear membrane to interact with DNA. The Agency recognizes that these are not physiological conditions, but the conditions of the experiment do support the common-sense assumption that enzymatic metabolism takes place in the cytoplasm of mouse cells and show that some fraction of the GST metabolite(s) is stable enough to cross membranes in the cell. Thus, the Agency believes that the observed tumorigenesis in the mouse is not the exclusive result of nuclear MC metabolism.

Other issues pertaining to genotoxicity. The remaining comments on these studies focused on more general issues such as the genotoxicity of MC and other factors related to the GST metabolic pathway and MC-induced carcinogenesis. Dr. Melnick [Ex. 126-33] remarked:

Some fundamental questions related to this mechanism and its uniqueness to mouse liver and mouse lung carcinogenesis are also not addressed by the present research. For example, why do tumors not develop in other organs in mice that also have high levels of GST theta (e.g., kidney)?

OSHA believes this is an important question that reduces the strength of HSIA's contention that the mouse responds in a unique way to MC. The investigators have attempted to explain differences in potency of MC with respect to liver and lung carcinogenesis by invoking differences in DNA repair rates and GST metabolism within the nuclei of critical cells. However, there are other tissues which, based on the HSIA hypothesis, ought to be prime candidates for carcinogenesis. The kidney, besides having high levels of GST theta, also has a slower rate of DNA repair than the liver. It would appear to be a logical site of carcinogenesis if HSIA's hypothesis is correct. OSHA believes that the lack of tumor response in this organ (and perhaps other logical sites) indicates that the hypothesis proposed by HSIA fails to account for all relevant observations.

(2) Non-genotoxic mechanisms of carcinogenesis. Non-genotoxic mechanisms of action have also been hypothesized for MC. Increased cell turnover, due to cell death caused by MC toxicity, could theoretically increase the available number of sites for mutation and subsequent tumor formation. However, there is only limited evidence of increased cell turnover after MC exposure. Casanova and Heck [Ex. 21-16] observed increased DNA synthesis in lung tissue of mice exposed to MC. Green et al. [Ex. 105] observed Clara cell vacuolation, and both studies measured increased DNA synthesis on the first day of exposure to MC, but not on subsequent days of exposure. Clara cells may be targets of MC-induced toxicity because they contain higher levels of MC-metabolizing enzymes and are therefore more likely to generate toxic MC metabolites (for example, carbon monoxide is known to poison MFO enzymes). Green et al. suggested that the Clara cell was the cell of origin of the lung tumors observed in the NTP mouse study, because of the metabolic properties of these cells and the increased cell turnover observed within a day of MC exposure (in addition to the DNA damage described above under the section entitled, "Genotoxic mechanisms of carcinogenesis").

Green et al. further suggested that if the cell of origin of the mouse lung tumors was the Clara cell, humans would be at substantially less risk of lung cancer, because humans have proportionally fewer Clara cells than mice do. However, OSHA believes that there is no clear evidence confirming that Clara cells were the cell of origin of the mouse lung tumors (see discussion above). Other cell types in the lung, such as the Type II lung cell, also have relatively high metabolic activity and could be the site of origin of lung tumors. These cells have not been studied separately. Further studies are needed to clarify the role of the Clara cell and other lung cell types and cells in other tissues in MC carcinogenesis.

(i) Increased cell division. In 1994, Foster et al. [Ex. 119] investigated increased cell division as the mechanism of action of MC in mouse lung cells. Specifically, they examined the mechanism of MC action on the transient vacuolation of bronchiolar cells observed following single exposures to MC. In mice exposed to 2000 and 4000 ppm MC, they observed increased numbers of vacuolated cells in the bronchiolar epithelium. Pretreatment of mice with a cytochrome P450 inhibitor decreased the number of vacuolated cells, while pretreatment with a glutathione depletor did not. In a replication of the observation made by Green et al. and described above, the authors found increased cell division (measured as incorporation of [3H]-thymidine) in Clara cells isolated from mice exposed to 4000 ppm MC. They concluded:

We believe that these results strongly support the supposition that the vacuolation of the Clara cells is due to a toxic metabolite produced by the CYP [cytochrome P-450] pathway of metabolism. Furthermore the most likely candidate for inducing the change is thought to be formyl chloride.

OSHA agrees that these observations indicate that increased cell turnover occurs in Clara cells of mice. This may possibly be a partial explanation of the mechanism, but only a partial one. In cases where cytotoxicity has been considered to be an explanation for risk occurring only at "high" doses, this argument is confined to chemicals believed to act non-genotoxically. MC is likely to be a genotoxic carcinogen, so even if cell proliferation is a factor, the genotoxic mechanism would be the primary mechanism of concern. Genotoxic carcinogens are not generally believed to have a threshold and the dose-response function is believed to be approximately linear at low doses. In addition, the study focused on one type of cell, which may not be the cell of origin for lung tumors. Carcinogenicity in humans (as well as in mice and rats) seems to originate from various cell types in various tissues.

(3) Metabolism of MC. As described above, the mechanism of carcinogenesis for MC is not known. Numerous studies over many years have explored numerous possible mechanisms and have provided substantial information regarding the metabolism and the probable metabolite responsible for the carcinogenic effect. As discussed in the Health Effects section, MC is metabolized by two pathways: the mixed function oxidase pathway (MFO) and the glutathione S-transferase (GST) pathway. Both pathways produce reactive intermediates which potentially could contribute to a genotoxic mechanism of carcinogenicity. During development of the PBPK model for MC, Reitz et al. found that tumor incidence correlated with the estimated amount of GST metabolite, as well as with the amount of parent compound administered, but not with the amount of MFO metabolite [Ex. 7-225]. The parent MC is not likely to act as a genotoxic carcinogen because it is a fairly non-reactive compound. In addition, MC blood levels in mice were lower than in rats, so if MC was the carcinogenic moiety, one would expect the risk of cancer in rats to be higher than mice, whereas the opposite was observed. Consideration of these factors has led many investigators to conclude that the GST pathway is responsible for carcinogenesis and that it is likely to produce a genotoxic carcinogenic moiety. OSHA has reviewed the data available on mechanism of action and has concluded that the most plausible assumption is that the GST pathway is responsible for the carcinogenic action of MC and that this should be taken into account in the quantitative risk assessment. This represents a case-specific departure from the default assumption that the administered dose of the parent compound is the relevant metric for exposure.

(i) Specific GST isozyme(s) responsible for MC metabolism to the carcinogenic metabolite. Recent work sponsored by the HSIA was directed at further characterization of the metabolism of MC by the GST pathway [Exs. 121, 124, 124A]. Specifically, the HSIA work on MC metabolism has focused on the isolation and description of isozymes in the GST theta class of enzymes, which HSIA believes are responsible for the metabolism of MC to the carcinogenic metabolite in mice. Mainwaring et al. have shown that the GST isomer with the greatest specific activity for MC is a member of the theta class of GST. [Ex. 121] In rats, three members of the theta class have been identified, GST 5-5, GST 12-12 and GST 13-13. In humans, two theta class enzymes have been identified, GST T1-1 and GST T2-2 and in mice, two theta enzymes have been described, GST T1-1* and GST T2-2* (also known as GST MT-1 and GST MT-2). According to Mainwaring et al. [Ex. 121], rat GST 5-5 and mouse GST T1-1* have similar specific activity toward MC and sequencing studies have shown "* * *that rat 5-5, mouse T1-1* and human T1-1 are orthologous proteins as are rat 12-12 and mouse T2-2* and human T2-2" [Ex. 124A].

The hypothesis under investigation in this work was that the enzyme similar to rat GST 5-5 (mouse T1-1* and human T1-1) was the critical enzyme responsible for metabolism of MC to the carcinogenic metabolite, and that differences in the interspecies intra- and inter-cellular distributions of this isozyme and differences in genotoxicity would be important for characterizing the risk of carcinogenesis after exposure to MC.

In order to examine the distribution of the GST isozymes of interest, the investigators used DNA oligonucleotide anti-sense probes complementary to three regions of the protein nucleotide sequences of rat GST 5-5, mouse GST T1-1* and human GST T1-1 to localize specific mRNA sequences in mouse, rat and human liver and lung tissue. They also used an antibody raised against rat GST 12-12 to localize the protein itself [Exs. 124, 124A]. In the full paper describing these experiments [Ex. 124A], Mainwaring characterized the results of this study, as follows:

The mouse enzymes [T1-1* and T2-2*] were present in significantly higher concentrations in both liver and lung than the equivalent enzymes in rat and human tissues. In mouse liver, both enzymes were localized in limiting plate hepatocytes surrounding the central vein, in bile duct epithelial cells and in the nuclei of hepatocytes. In rat liver the distribution of GST 12-12 was comparable to that seen for T2-2* in the mouse. GST 5-5 was not localized in limiting plate hepatocytes or in nuclei of rat liver. The levels of human transferase T1-1 in the liver were very low, with an even distribution throughout the lobule. The GST 12-12 antibody did reveal high concentrations of this enzyme in human bile ducts. The relative amounts of the theta enzymes in the lungs of the three species followed the pattern seen in the liver, with very high concentrations in Clara cells and ciliated cells of the mouse lung and much lower levels in the Clara cells only of rat lung. Low levels of human transferase T1-1 were detected in Clara cells and ciliated cells found at the alveolar/bronchiolar junction of one human lung sample. The enzyme was entirely absent from the large bronchioles.

Mainwaring et al. concluded that:

This study has demonstrated a highly specific distribution of the theta class GSTs 5-5 and 12-12 in liver and lung tissue from mice, rats and humans. * * *it was apparent from these studies that both the distribution and concentration of theses enzymes differed markedly between the three species. Whilst neither mRNA levels nor protein concentrations necessarily correspond to active enzyme, the distribution shown by the mRNA for GST 12-12 was quantitatively reflected by the antibody to the protein of this enzyme, suggesting that these techniques do, in this case, reflect the distribution of active enzyme. Although an antibody to GST 5-5 is not available, it is reasonable to assume that mRNA levels for this enzyme are similarly representative of the distribution of active enzyme.

An understanding of the cellular and sub-cellular distribution of GST 5-5 has provided an explanation for the species specificity of the mouse lung and liver carcinogen methylene chloride, and has provided reassurance that humans are not at risk from exposure to this chemical.

(ii) Issues raised pertaining to metabolic studies. Many commenters commended the HSIA for providing new information on the mechanism of action of MC and for confirming previous quantitative studies of the interspecies differences in MC metabolism. However, commenters also raised several specific issues regarding the conduct and interpretation of these experiments.

Correlation of mRNA concentrations with enzyme concentrations.

Mainwaring et al. [Ex. 124A] correlated the inter- and intra-cellular distribution of the mRNA for GST 12-12 in the rat with the distribution of the antibody for GST 12-12. They stated that it is reasonable to assume that since the protein and mRNA for the 12-12 isomer have similar distributions, the protein for the 5-5 isomer would distribute in the same manner as the mRNA for the 5-5 isomer. In support of their assumption, they noted that there is 80% homology between the 5-5 and 12-12 isomer. Some commenters believed that this was not a reasonable assumption and that there was no reason to believe that the distribution of the GST 5-5 isomer protein would correlate with the distribution of the GST 5-5 mRNA simply because there seemed to be a correlation in the 12-12 isomer protein and mRNA distributions [Exs. 126-7, 126-16]. OSHA concurs with these commenters, and until there is actual measurement of the GST 5-5 protein, OSHA does not believe that the question of the actual distribution of GST 5-5 isozyme will have been settled.

More importantly, several commenters stressed that it was mRNA that was actually observed in these studies, and mRNA levels do not necessarily correspond to either protein levels or protein activity within a cell [Exs. 126-7, 126-16, 126-28, 126-30, 126-32]. Although Mainwaring et al. acknowledge this fact [Ex. 124A], the conclusions reached by the authors still suggest that measurement of mRNA is equivalent to measurement of enzyme activity. Referring to the conclusions drawn by Mainwaring et al., Dr. Lorenz Rhomberg [Ex. 126-16] commented:

This interpretation of mRNA distribution is profoundly in error and contradicts some of the most well established and fundamental principles of molecular biology.* * * Finding mRNA in the nucleus is unsurprising and uninformative about the eventual location of the protein products. Detecting mRNA only reveals that the cell may be presumed to be manufacturing the corresponding protein.

Dr. Rhomberg was also concerned that the concentration of GST T1-1* in the nucleus of mice may be an artifact of the experimental conditions, resulting, perhaps, from a burst of mRNA synthesis. The concern that the apparent nuclear concentration of GST may be an artifact was echoed by Dr. Douglas A. Bell of the National Institute for Environmental Health Sciences [Ex. 126-26]. He stated:

Why the [intracellular] distribution should be different among species is unclear and unusual. Differences in processing of the nuclear RNA transcript from full length pre-mRNA may be the underlying cause of this phenomenon (or perhaps there is a transcribed pseudogene that is complicating the process).

Because of the specific cellular mechanisms that would be required to concentrate a protein in the nucleus, Dr. Rhomberg [Ex. 126-16] indicated that translocation of the GST 5-5 protein to the nucleus only in mice seemed unlikely. He stated:

It seems implausible * * * that for a series of orthologous proteins, such localization would be found in a particular species and not in other species.

OSHA agrees with the comments made by Dr. Rhomberg and Dr. Bell on this issue, and concludes that the concentration of mRNA at a particular cellular site does not necessarily correlate with concentration of the enzyme itself. OSHA believes that caution should be used when interpreting the results of these experiments.

Attribution of GST metabolizing activity to a single GST isozyme. Concern was also raised about the validity of attributing all of the glutathione S-transferase metabolism of MC to one isomer of the theta class [Exs. 126-7, 126-12]. In particular, Dr. Dale Hattis noted that there was less enzyme activity eluting coincident with the peak identified as the 5-5 form than that eluting at pH 8, which was not believed to correspond to the 5-5 form. Dr. Ronald Brown described results from a paper by Blocki (1994) [Ex. 127-22] which showed that "expression of the [5-5] isozyme contributes 50% of the total GST activity toward this substrate." This leaves the question open as to whether isozymes which may have lower specific activity for MC but which may be expressed in much greater abundance (particularly u 4-4), could contribute as much as the remaining 50% of the total GST metabolism (see Table VI-1, reproduced below from Dr. Brown's comment [Ex. 126-7], original source Blocki et al. (1994) [Ex. 127-22]).

   Table VI-1. -- Relative Contribution of Different Rat Liver
        Glutathione S-Transferases in Dichloromethane Metabolism to
                      |                |          |
                      |                |          |
Comparative parameter |                |          |
 (units)............. | 1-1+1-2+2-2    |    3-3   |    3-4
Specific activity     |                |          |
 (nmol/min/mg of      |                |          |
 protein)............ | < 0.1           |    7     |   11
% Cytosolic protein   |                |          |
 (% of total in liver)| 6.4            |  0.7     | 0.3
Total activity        |                |          |
(nmol/min/g of liver  |                |          |
protein)..............| < 10            |   49     |  33
% Total activity(c)...| < 1.5           |   11     |   7

   Table VI-1. -- Relative Contribution of Different Rat Liver
        Glutathione S-Transferases in Dichloromethane Metabolism to
                          Formaldehyde -- Continued
                      |           Glutathione S-transferases
                      |    alpha     |       u       |      theta
                      |    Class     |     Class     |      Class
Comparative parameter |              |               |
 (units)............. |         4-4  |   (b)5-5      | (b)13k
Specific activity     |              |               |
 (nmol/min/mg of      |              |               |
 protein)............ |        23    |   11,000      |      9
% Cytosolic protein   |              |               |
 (% of total in liver)|       0.6    |        0.002  |      0.005
Total activity        |              |               |
(nmol/min/g of liver  |              |               |
protein)..............|       138    |       22      |      0.45
% Total activity(c)...|        32    |       50      |      0.1
  Footnote(a) Data from Meyers et al., 1991.
  Footnote(b) Data for 13,000 molecular weight glutathione
transferase from Blocki et al., 1992.
  Footnote(c) Assuming Vmax conditions for each.

In addition, Mainwaring et al. [Ex. 124A] noted that the "substrate specificity of GST 12-12 is currently poorly characterized," although the purified enzyme has no activity toward MC. As described above, these enzymes appear to be very labile upon purification. Therefore, it is unclear how much the 12-12 isomer itself may contribute to MC metabolism. As Dr. Kenneth T. Bogen stated, "* * * while the substrate specificity of GST 12-12 may currently be poorly characterized, current data do not appear to rule out GST 12-12 specificity toward MC."

Limited human samples and human polymorphism in the GST theta genes. Several commenters expressed concern for the limited number of human samples (one pooled lung sample and less than 40 human liver samples have been assayed) and the potential effect of a known human polymorphism for the glutathione S-transferase theta class genes on risk estimations [Exs. 126-7, 126-16, 126-26, 126-35]. Specifically, commenters raised concerns that there may be a large subpopulation of GST conjugators who may be at increased risk from MC exposure that has not been adequately characterized in the limited number of human samples (especially lung samples) that have been tested. HSIA objected to these comments, stating,

The human tissue data base for the metabolism of methylene chloride by the GST pathway is one of the largest, if not the largest, available for this type of risk assessment. To discount it based on arguments concerning hypothetical polymorphisms, as these commenters urge OSHA to do, would be contrary to the message consistently put forward by the National Academy of Sciences and regulatory authorities for the past decade. * * *"

In fact, the National Academy of Sciences report cited by HSIA, "Science and Judgement in Risk Assessment" does encourage agencies to make use of biologically-based models, but cautions that using them without adequately considering human variability would be a step backwards:

EPA has not sufficiently accounted for interindividual variability in biologic characteristics when it has used various physiologic or biologically based risk-assessment models. The validity of many of these models and assumptions depends crucially on the accuracy and precision of the human biological characteristics that drive them. In a wide variety of cases, interindividual variation can swamp the simple measurement uncertainty or the uncertainty in modeling that is inherent in deriving estimates for the "average" person.

The Academy goes on to recommend specifically that making "reasonable inferences" about interindividual variation is required, rather than assuming that no such variation exists:

Even when the alternative to the default model hinges on a qualitative, rather than a quantitative, distinction, such as the possible irrelevance to humans of the alpha-2u-globulin mechanism involved in the initiation of some male rat kidney tumors, the new model must be checked against the possibility that some humans are qualitatively different from the norm. Any alternative assumption might be flawed, if it turns out to be biologically inappropriate for some fraction of the human population.

When EPA proposes to adopt an alternative risk-assessment assumption * * * it should consider human interindividual variability in estimating the model parameters or verifying the assumption of "irrelevance." If the data are not available that would enable EPA to take account of human variability, EPA should be free to make any reasonable inferences about its extent and impact (rather than having to collect or await such data), but should encourage interested parties to collect and provide the necessary data.

OSHA believes HSIA has misinterpreted the NAS recommendations, and further disagrees with HSIA that the polymorphism is "hypothetical." Investigators have demonstrated this polymorphism in human GST and have shown how the polymorphism varies across races [Exs. 127-7, 127-9, 127-17, 127-21, 127-23, 127-24, 127-25]. OSHA agrees with the commenters that a human polymorphism in the GST theta genes may increase concern for individuals that may be at higher risk from exposure to MC due to their genetic make-up. The Agency has considered sensitive subpopulations in the development of health standards, including this rulemaking. For example, the subpopulation of workers with silent or symptomatic heart disease was considered in assessing the cardiac risks of MC (due to its metabolism to carbon monoxide). The variation in enzyme activity raises additional uncertainty in the use of human data to support the hypothesis that mice are uniquely sensitive to MC carcinogenicity. However, for purposes of quantitative analysis, the Agency has not attempted to systematically adjust the risk estimates based on a "high GST metabolizing" individual because the frequency and impact of such polymorphisms have not been clearly worked out.

Target site of MC carcinogenesis in mice versus humans.

Drs. Brown and Melnick [Exs. 126-7, 126-33] also raised the possibility that the target site for MC carcinogenesis may be different in humans than in mice or rats. Specifically, research on the occurrence of theta isomers of GST in human blood was described. The characterization of GST metabolism in human erythrocytes [Exs. 127-11, 127-12] suggests the possibility of the bone marrow as a potential target of MC carcinogenesis and also the potential for metabolism in the blood and translocation of the metabolites to a variety of potential targets. The HSIA discounted human blood metabolism of MC, stating,

The `very high capacity to conjugate methylene chloride' mentioned by Brown is in fact very low, approximately 40-fold lower than the highest activity detected in human liver.

OSHA believes that although the specific activity in the blood may be lower than the human liver activity, the total activity of the GST enzymes in blood and marrow may be significant when one also considers the volume of these compartments. OSHA also notes that interspecies tumor site concordance is not necessarily expected, and it is prudent to consider any human tissues which have the potential to metabolize MC to the putative carcinogen.

Concentration of protein complementary to rat GST 12-12 in human bile ducts.

Dr. Bogen [Ex. 126-15] commented specifically on the human liver protein complementary to the antibody to rat GST 12-12 protein. In particular he was concerned that high concentrations of this enzyme were reported in bile ducts of the human liver. He noted,

With regard to potential human carcinogenicity of MC relative to its known carcinogenic potential in mice, it seems to me that these particular data ought not to reduce regulatory concern, but rather ought to increase regulatory concern, in view of the fact that bile duct epithelium cells are the most likely stem cells for hepatocytes.

* * * Thus hepatocellular bile-duct epithelial cells are likely to play an important role in liver carcinogenesis in both mice and humans.

OSHA agrees with Dr. Bogen's concerns and also notes that in the cohort study of textile workers conducted by Hoescht-Celanese [Ex. 7-260], an excess of biliary cancers was observed in those workers exposed to the highest concentrations of MC and those with the longest latency period between exposure and disease. If the HSIA theory is correct (i.e., a single isozyme is the culprit), then finding high levels of this isozyme in human bile duct is strong evidence implicating MC in human carcinogenesis.

Interpretation of data as qualitative versus quantitative differences.

Perhaps most importantly for the purposes of MC risk assessment, several commenters remarked that OSHA should use caution when interpreting the data from the HSIA submissions, because any interspecies differences are rightly considered first as quantitative rather than qualitative ones. In part, the commenters cautioned that one should pay special attention to the threshold of detection in all assays. As Dr. Andrew Salmon stated,

Green and co-workers have consistently confused their inability to measure a result or parameter value due to its magnitude or frequency of occurrence being below their threshold for practical detection, with a true zero value for the parameter or zero risk of an occurrence [Ex. 126-36].

OSHA agrees that caution should be used when attempting to characterize a difference between species as an absolute qualitative difference. A much higher burden of proof is required to support a claim of zero risk than of diminished risk. (This higher burden is due to the need to consider assay sensitivity and other factors; the fact that the consequences of incorrectly concluding that humans are at zero risk are particularly dire only adds to the already high threshold of scientific evidence needed to successfully make such a claim). In the case of MC, humans clearly have the ability to metabolize MC via the GST pathway [Exs. 21-53, 127-16]. Even if the enzyme concentration of GST T1-1* itself actually occurs only in the nuclei of mouse lung or liver (as opposed to the concentration of mRNA, which may or may not be localized differently within mouse cells), it is still unclear what impact (if any) this fact would have on the characterization of human cancer risks for MC. OSHA believes that the statement that there are absolute species differences in the activity and intracellular distribution of GST 5-5 is highly speculative and is not supported by the data presented to date, because the data presented refers to the distribution of mRNA for GST 5-5, not the enzyme concentrations or activity levels of the enzyme; there is no quantification of the intracellular levels of the mRNA or enzyme levels, only photographic representations; and there is no evidence that any potential difference in enzyme activity (when those experiments are completed) would be greater than the difference already predicted from allometric scaling considerations.

Conclusions reached by the HSIA.

HSIA concluded from these studies that because of a qualitative inter-species difference in the distribution of the GST theta enzyme responsible for MC carcinogenesis, humans would not be at risk of developing cancer under "foreseeable conditions of exposure." Although some commenters agreed with the conclusions reached by the HSIA [e.g., Exs. 126-10, 126-13, 126-20], many commenters strongly disagreed with this interpretation of these data pertaining to the risk assessment for MC. These commenters [e.g., Exs. 126-7, 126-11, 126-12, 126-15, 126-16, 126-22, 126-26, 126-30, 126-36] were concerned that the question was in reality an issue of quantitation of enzyme, not a qualitative difference in metabolism. Dr. Lorenz Rhomberg commented:

The question is, is there any basis for believing that the species difference in activity suggested by the mRNA data is greater than has been previously supposed? It should be emphasized that some degree of species difference in metabolic activity is expected even under the default cross-species extrapolation methods. That is, in keeping with the general pattern of scaling of physiological processes across species, general metabolic rates are presumed to be lower on a per unit of tissue basis in larger animals. As a default, this pattern can be presumed to apply to individual metabolic pathways as well, although data on species-species activities can be used in place of such defaults if available.

If species-species activities are discovered by experiment to be less in humans than in mice to the degree already anticipated by allometry, then the experiments are simply confirming the default and no change in the human risk estimates is warranted. If humans have a metabolic activity different than the allometric prediction, the incorporation of such estimates into PBPK models can show different human risks from those predicted under the default. The allometric prediction is that, on a per unit of tissue basis, humans should have about 7-fold lower activity than mice and about 4-fold lower activity than rats.

Given the limit of detection of the assay methods, human metabolic activity (or mRNA levels) only a bit less than the allometric expectation of 7-fold less than mice are often difficult to distinguish from zero. That is, claims that humans have no activity (or no mRNA production) in certain tissues must be judged in the light of the fact that only a small change from the already acknowledged allometric difference can often make the human activity undetectable. A 20-fold mouse-human difference, for example, really only represents a 3-fold exaggeration of the 7-fold allometric pattern, yet many assays may fail to reliably characterize a 20-fold difference as a quantitative difference rather than a qualitative difference.

For the above reasons, claims that human metabolic activity in activating methylene chloride are so low as to be essentially qualitatively different than mice should be interpreted with great caution. In fact, existing assays have great difficulty in detecting species differences in metabolic activity great enough to markedly challenge existing risk assessments.

Another commenter discussed the fact that cellular levels of the GST 5-5 isoenzyme would be expected to be distributed unevenly across cells, putting some cells at greater or lesser risk. This would tend to average out over a tissue and would be best described by tissue metabolism data. Other commenters remarked that there was no need to adjust the risk estimates based on these studies because current pharmacokinetic models already account for interspecies differences in metabolism. Although OSHA has incorporated data from these studies, especially in its "alternative analysis," OSHA agrees with Dr. Rhomberg and the other commenters who have taken exception to the HSIA conclusions.

The Agency does not accept the HSIA characterization of the results of the summarized studies. OSHA has determined that no evidence has yet been presented that demonstrates that humans are not at risk of developing cancer after exposure to MC. At most, the presented studies suggest a quantitative inter-species difference in MC metabolism, which was established in previous scientific reports and is already accounted for by PBPK modeling. As discussed extensively in this document, OSHA has concluded that HSIA has undervalued certain strong evidence and has overemphasized some more speculative hypotheses. However, as is clear from this discussion OSHA has carefully considered all of the evidence. Substantial evidence in the record clearly supports OSHA's conclusions. Consequently, OSHA's approach of relying on the NTP mouse tumor data as the basis of its quantitative risk assessment continues to be the best approach to risk estimation.

c. Conclusions regarding the carcinogenesis of MC. The HSIA submitted these documents to OSHA with a request that the Agency consider the mouse tumor data in light of these additional studies and reject use of the mouse tumor response data as the basis of the Agency's quantitative risk assessment. OSHA believes it has given proper weight to all the evidence, giving greater weight to that which is of the highest scientific quality. However, in light of HSIA's request, the Agency reopened the rulemaking record and reviewed all the new data. After submitting these documents for review, the HSIA [Ex. 126-29] remarked on comments submitted to the docket by other scientists,

In general, the comments submitted by R. Maronpot, R. Brown, L. Rhomberg, K. Bogen and D. Hattis exhibit a reluctance to use the large body of mechanistic data now available in assessing the potential carcinogenic risk posed by methylene chloride, even though most other commenters agree that the pathway responsible for its observed carcinogenicity in mouse liver and lung, as well as species variations in activity of this critical pathway, have now been identified. Much of the comment addressed here appears to be motivated by a desire to maintain the "status quo" for assessing carcinogenic risk based on default principles that were developed twenty years ago.

The HSIA goes on to say,

Many of the conclusions reached by the commenters * * * are based, often erroneously, on single aspects of one or the other of these publications, rather than on the entire data base, as a "weight of evidence" approach would demand and as is necessary to understand the results.

OSHA finds it difficult to understand why HSIA believes that the scientists they listed are primarily interested in preserving the "status quo." Dr. Maronpot conducted the mechanistic studies on MC at NIEHS, which have generated mechanistic information useful to the risk assessment process. Dr. Rhomberg was instrumental in developing the pharmacokinetic approach used by the Environmental Protection Agency in its risk assessment of MC (an approach never used by the Agency previously). Dr. Hattis, Dr. Bogen and Dr. Brown are all experts in the application of pharmacokinetic modeling to risk assessment and have repeatedly called for incorporating more mechanistic and physiological data into pharmacokinetic models. These highly respected scientists, among others, reviewed the HSIA submissions critically and independently and reached conclusions different from those of the HSIA, conclusions which themselves depart significantly from the "status quo." This does not suggest to OSHA that they are trying to preserve some status quo in risk assessment, and OSHA finds nothing in the comments of these experts to suggest that this is the case.

In order to respond to HSIA's desire to have OSHA further review all of the data, the Agency has reviewed each submitted study carefully and critically on its own merits to determine how each piece of data fits into the overall picture of the mechanism of action for MC. OSHA believes that in this process the critical issues raised by the HSIA have received a full airing and the hazard identification and the risk assessment for MC have been improved because of it. OSHA believes, however, that looking only at the new studies submitted by HSIA, and examining them uncritically, would contradict every principle of scientific analysis.

In summary, in order to accept the HSIA's supposition that MC is not carcinogenic in humans, one must believe the following:

1. GST 5-5 is the only isozyme which can metabolize MC to the carcinogenic metabolite.

2. DNA single strand breaks are relevant and a sufficient measure of the tumorigenicity of a compound.

3. The absence of detectable increases in DNA ss breaks in a single experiment means that there are in fact no additional ss breaks.

4. The limited number of human samples (one sample of pooled lung tissues being the absolute extreme of "limited" data) used to determine metabolic parameters are truly representative of the range of human variability.

5. An apparent correlation in the distribution of the GST 12-12 protein and GST 12-12 mRNA means that the distribution of GST 5-5 protein will correlate similarly with the distribution of GST 5-5 mRNA.

6. Visual interpretation of photomicrographs staining for GST mRNA gives a true and accurate measure of GST activity in the cell.

And one must also ignore the following contradictory observations and conclusions about the mechanism of action (in addition to ignoring the suggestive epidemiologic evidence):

1. Metabolites of GST can cross cell and nuclear membranes and interact with DNA to induce DNA ss breaks and mutations.

2. GST mRNA and protein stain heavily in human bile duct cells (believed to be precursors of hepatocytes).

3. Human lung tissue has been shown to stain for GST mRNA. 4. Only 50% of the GST metabolism of MC can be accounted for by the GST 5-5 isozyme.

5. The metabolic capacity of GST 12-12 for MC has not been characterized.

OSHA concludes that these studies, even putting aside all technical objections to the methodology and interpretation of individual studies, do not change the conclusion that substantial evidence supports the carcinogenicity of MC. The bioassay results in mice are still qualitatively and quantitatively relevant to humans. Once the HSIA studies have been replicated and key components quantified (like the intracellular enzyme activity (instead of mRNA levels) of GST towards MC), the HSIA data may be useful in characterizing quantitative interspecies differences in MC GST metabolism. In particular, it would be useful to determine whether all of the evidence that HSIA submitted is consistent with an allometric difference (a difference expected based on the size of the animal) in sensitivity to MC or with a greater interspecies difference in sensitivity. (The specific activity of GST toward MC in mice is estimated to be about 7-fold that of humans, based on allometric considerations.) OSHA believes that its final risk assessment, which relies on an analysis of all available PBPK data, addresses both possible interpretations.

B. Selection of Database for Quantitative Risk Assessment

1. Animal Bioassays

The first step in performing a quantitative assessment of carcinogenic risk based on animal data is to choose a data set or sets from which to define the dose-response relationship. In its NPRM, OSHA had chosen the NTP female mouse lung and liver tumors to determine its estimates of risk. OSHA chose these responses because they provided clear dose-response relationships, had low background tumor rates and were more sensitive measures of dose-response than corresponding male mouse tumor sites.

The EPA, the CPSC and the FDA chose to use the combined incidence of adenomas and carcinomas of the lung and liver as the basis for their risk assessments. Specifically, the EPA [Exs. 25-D, 28] placed emphasis on the experimental species and sex group showing the highest risk: the number of female mice showing either adenoma or carcinoma in either lung or liver (or both). The CPSC [Ex. 25-I] pooled benign and malignant tumors of either the mammary gland, lung or liver and averaged male and female estimates to derive an overall risk estimate. The FDA [Ex. 6-1] used benign and malignant responses of female mice. The Crump report [Ex. 12] noted that it may be reasonable to combine lung and liver responses to give an indication of the potency of MC, due to the fact that metabolism of MC occurs by the same pathway in both lung and liver and thus results in the same ultimate metabolites. However, the report added that since both tissues have different background responses, combining responses may tend to complicate the interpretation of risk estimates.

In OSHA's final rule, the NTP study (rat and mouse, inhalation) was chosen for quantitative risk assessment because it provided the best toxicological and statistical information on the carcinogenicity of MC [Exs. 12, 7-127] and because the study was of the highest data quality. In the NTP study, MC induced significant increases both in the incidence and multiplicity of alveolar/bronchiolar and hepatocellular neoplasms in male and female mice. In rats, dose-related, statistically significant increases in mammary tumors were also observed. OSHA chose the female mouse tumor response as the basis of its quantitative risk assessment, because of the high quality of data, the clear dose response of liver and lung tumors and the low background tumor incidence. Although the female rat mammary tumor response was also dose-related, the data of high quality and amenable to quantitative risk assessment, the mouse data set had a clearer dose-response in both liver and lung tumors than the rat mammary tumor response and the mouse background tumor incidence was lower than in the rat. Therefore the mouse data set was chosen for quantitative analysis.

OSHA included the lung adenomas in the quantitative analysis. The evidence suggests that the presence of benign tumors with the potential to progress to malignancies should be interpreted as representing a potentially carcinogenic response. This belief is supported by the OSTP's views on chemical carcinogenesis (50 FR 10371). OSTP stated that at certain tissue sites, such as the lung, most tumors diagnosed as benign really represent a stage in the progression to malignancy. Additionally, NIOSH, the EPA, the CPSC and the FDA have also included benign responses in their assessments. Therefore, it is appropriate and sometimes necessary to combine certain benign tumors with malignant ones occurring in the same tissue and the same organ site. In particular, OSTP also stated that "the judgement of the pathologist as to whether the lesion is an adenoma or an adenocarcinoma is so subjective that it is essential they be combined for statistical purposes." (50 FR 10371).

OSHA chose female mouse lung tumors as the specific tumor site for its final quantitative risk assessment. There is no a priori reason to prefer the mouse lung tumor response over the liver tumor response, because both data sets were of high quality, showed a clear dose-response relationship and had low background tumor incidence. In fact, in the NPRM, the Agency reported estimates of risk generated using both sites. However, to reduce the complexity of the final PBPK analysis, which required highly intensive computations, OSHA chose one site (the female mouse lung tumor response) for its final risk estimates. The risks calculated using the female mouse liver response would likely be slightly lower than those calculated using the lung tumor response. On the other hand, pooling the total number of tumor-bearing animals having either a lung or liver tumor (or both) (which is the procedure EPA advocates [see its 1986 Guidelines for Cancer Risk Assessment]) would have yielded risk estimates higher than OSHA's final values.

The NTP study has been described in the Health Effects section and, above, in the discussion regarding hazard identification.

2. Epidemiologic Data

The epidemiology data are not as useful for quantitative risk assessment as the animal data because the animal data provide a clear dose-response, with fairly precise indices of exposure, which cannot be derived from the epidemiology data. All other things being equal, risk assessors would prefer to use epidemiologic data to assess cancer risk in humans over data from animal studies whenever good data on human risk exist. However, the uncertainty inherent in epidemiologic studies must be accounted for; in particular, "positive" studies often have lower confidence limits that do not rule out the no-effect hypothesis, while ostensibly "negative" studies often have UCLs that would support a substantial positive effect. OSHA believes (see discussion below) that the latter circumstance applies to some of the MC studies. Other factors, such as duration and intensity of a chemical exposure (which can rarely be controlled and accurately measured in an epidemiological study), difficulty in accurately defining the exposed population, and other confounding factors diffuse a study's predictive power of true risks.

Frequently, animal studies indicate a positive response to a particular chemical when epidemiologic studies of exposures to the same chemical fail to exhibit a statistically significant increase in risk. When animal studies show a substance to be a carcinogen but epidemiologic studies are non-positive, the minimum risk which could be detected by the human study should be estimated to assess the strength of the epidemiologic study and justify its importance in the risk assessment process. Similarly, the animal-based potency estimate can be used to predict the number of human deaths investigators would likely have seen in an epidemiologic study if the animal-based estimate was correct; if the observed number of human deaths is markedly inconsistent with this predicted number, the relevance of the animal-based estimate might well be called into question. If the human data are equivocal, or the epidemiologic study is not sufficiently sensitive to identify an increased risk predicted by a well-conducted animal bioassay, it is necessary to consider the animal data to protect workers from significant risk. OSHA concludes that the MC epidemiology studies do not have adequate information upon which to base a quantitative risk assessment. OSHA has, however, used the analyzed epidemiological data to determine whether the results are consistent with those estimated using the rodent models. This is discussed later in the document.

3. Conclusions

After reviewing the animal data and the quantifiable epidemiology data, OSHA has determined that the NTP female mouse lung tumor response is the appropriate data set on which to base its quantitative risk assessment, and has determined that the most scientifically-appropriate way to use these data involves constructing a PBPK model to extrapolate from animals to humans. OSHA believes that the non-positive epidemiology data, in particular those from Kodak, are of in sufficient power to rule out the risk estimates derived from the animal data.

C. Choice of Dose-Response Model

Several approaches have been used to estimate cancer risk from exposure to toxic agents. A standard approach uses mathematical models to describe the relationship between dose (airborne concentration or target tissue dose surrogate) and response (cancer). Generally, mathematical functions are fit to the data points observed at different exposure levels and these functions are used to estimate the risk that would occur at exposure levels below those observed. The shapes of these curves vary, ranging from linear extrapolations from the observed points through the origin (zero exposure and zero risk) to curves which may deviate far from linearity at the very highest or lowest doses. The use of a particular model or curve can be justified in part by statistical measures of "goodness-of-fit" to observed data points. That is, there are various statistical tests which measure how closely a predicted dose-response curve fits the observed data.

The most commonly used model for low-dose extrapolation is the multistage model of carcinogenesis. This model, derived from a theory proposed by Armitage and Doll in 1961, is based on the biological assumption that cancer is induced by carcinogens through a series of independent stages. The Agency believes that this model conforms most closely to what we know about the etiology of cancer. There is no evidence that the multistage model is biologically inappropriate, especially for genotoxic carcinogens, which MC most likely is. The most recent data submitted by the HSIA [Exs. 117-124A] clearly add substantial support to the previous body of evidence indicating that one or more metabolites of MC is a genotoxic carcinogen. The low-dose linearity feature of this model is scientifically required for any exposure that confers additional risk upon a pre-existing background level of risk produced by a similar or equivalent mechanism. Given the underlying connection between DNA mutations and cancer and the obvious background incidence of cancer in the human population, the overwhelming scientific consensus is that genotoxins follow low-dose linear functions.

The multistage model is generally considered to be a conservative model because it is approximately linear at low doses and because it assumes no threshold for carcinogenesis, although there are other plausible models of carcinogenesis which are more conservative at low doses. "No threshold" means that any incremental amount of exposure to a carcinogen is associated with some amount of increased risk. "Approximately linear at low doses" means that one unit of change in dose will result in one unit of change in risk at low doses.

The most common approach for setting the parameters in the multistage model is to assume that the dose-response curve is described by a polynomial of k-1 degrees, where k is the number of dose groups tested. The multistage model thus takes the form

P(Cancer) = 1 -- exp(-f(dose)),

with f(dose) given by:

f(dose) = a + b(1)(dose) + b(2)(dose)(2) + ...+ b(k-1)(dose)(k-1).

The number of stages is specified by k-1, and the parameters a (the background risk) and b(i) are estimated from the observed data.

Alternatives to the multistage model include the tolerance distribution models such as the probit model, the logit model and the Weibull model. The tolerance distribution models generally predict dose-response relationships which are sigmoid in shape. Thus, these models will approach zero more rapidly than a linear multistage model. This means that at low doses, these models will predict lower risks than will a linear multistage model.

In the MC rulemaking, most of the risk assessments submitted to the Agency used the linearized multistage model to predict risk. The differences in risk estimates were not generally due to the dose-response model used, but to whether the risk assessor used pharmacokinetic modeling to estimate target tissue doses, and what assumptions were used in the pharmacokinetic modeling.

D. Selection of Dose Measure

1. Estimation of Occupational Dose

The purpose of low dose extrapolation is to estimate risk of cancer at a variety of occupational exposures. This requires that the doses be converted into units comparable to those in which the experimental dose is measured.

In its NPRM, OSHA first converted the experimental dose, measured in ppm, to an inhaled dose, measured in mg/kg/day. The female mouse body weight used in these calculations was 0.0308 kg. The breathing rate for mice was 0.05 m(3)/day. The Agency then assumed that equivalent doses in mg/kg/day would lead to equivalent risk. Once the experimental dose (in mice) had been converted to mg/kg/day, it was then converted to ppm using the human breathing rate of 9.6 m(3)/workday and human body weight of 70 kg in order to estimate risks at various potential exposure levels. To determine the dose to humans corresponding to the risk estimated from the mouse data, OSHA used the following equations:

                      Dose(M)(84.9g/mol)(1000 mg)(1000 L)
  Dose(M)(mg/m(3)) =  ___________________________________
                         24.45 L/mol        (g)(m(3))

   Dose(M)(mg/kg/d) = _____________________________________________
                                       (0.0308 kg BW)

OSHA assumed that risk estimates derived for mice at a given mg/kg/d would be equivalent to risks experienced by humans at that mg/kg/d. Doses in mg/kg/d in humans were converted to ppm to determine risk at various potential workplace exposures using the following equations:

      Dose(H)(mg/m(3)) = ___________________________________

       Dose(H)(ppm) = Dose(H)(mg/m(3))(24.45L/mol)/(84.9 g/mol)

This process was used by K.S. Crump et al. in their risk assessment submitted to OSHA [Ex. 12]. Use of mg/kg/d as a measure of dose has been criticized by Mr. Harvey Clewell, representing the U.S. Navy [Ex. 19-59]. He stated,

Strictly speaking, the concept of a mg/kg/day dose applies only to exposures for which the term "administered dose" is well defined, which does not include inhalation exposure to a volatile, lipophilic chemical such as MC....If a non-pharmacokinetic dose surrogate is desired, the choice should be time-weighted average concentration (ppm) as used by the FDA.

Mr. Clewell preferred use of dose surrogates calculated in the PBPK models to estimate human risk. OSHA has given careful consideration to the issues raised by Mr. Clewell and, in the risk assessment presented here, considered dose surrogates estimated in PBPK models and time-weighted average concentration in addition to the mg/kg/d dose presented in the NPRM.

For all dose measures used to estimate human risk, the assumptions used by OSHA for body weights and exposure times and rates were those described above. In OSHA's final risk assessment, a Bayesian analysis was used and the prior distribution for breathing rate was centered on OSHA's preferred value of 9.6 m(3)/d.

2. mg/kg/d Versus Other Measures of Exposure

Quantitative risk assessments based on animal data are conducted under the assumption that animals and humans have equal risks from lifetime exposures to a chemical when exposure is measured in the same unit for both species. Opinions vary, however, on what is the correct measure of exposure. For site-of-contact tumors, a ppm-to-ppm conversion is a generally accepted measure of dose. For systemic tumors, commonly used dose conversions include mg/kg/day (as used by OSHA in its MC NPRM), mg/surface area/day (with surface area approximated by BW(2/3)), mg/BW(3/4)/day, and mg/kg/lifetime. When adequate and appropriate pharmacokinetic or metabolic data are available, these data are sometimes used to estimate internal dose. In the case of MC, metabolic data have been gathered and pharmacokinetic models have been used by various investigators to estimate target tissue doses for MC.

Some commenters [Exs. 19-28, 19-57] had expressed concern that OSHA used a surface area correction factor in its risk assessment in the NPRM. In fact, in the NPRM, OSHA extrapolated from mice to humans based on body weight rather than surface area. However, the Agency requested comment on which species conversion factor would be appropriate to use in OSHA's final risk assessment and whether incorporation of pharmacokinetic information should influence the choice of the conversion factor. Two commenters [Exs. 19-83, 23-21] referred to the interagency document on interspecies scaling which ultimately recommends BW(3/4) as the appropriate extrapolation factor in the absence of appropriate pharmacokinetic information, although the document also indicates that extrapolation factors based on BW or BW(2/3) would also be consistent with the available data (EPA Draft Report: "A cross-species scaling factor for carcinogen risk assessment based on equivalence of mg/kg(3/4)/day." 57 FR 24152, June 5, 1992).

There was also considerable discussion as to whether it was appropriate to apply an extrapolation factor such as BW(3/4) or BW(2/3) in addition to PBPK modeling of dose, to account for pharmacodynamic differences between species (such as differences in DNA repair rates and other non-metabolic differences in interspecies susceptibility to an agent). The EPA applied the BW(2/3) extrapolation factor after incorporation of the PBPK data for MC in their 1987 draft update of the MC risk assessment. In their previous risk assessment, which did not incorporate PBPK data, EPA also used BW(2/3) as the extrapolation factor. Since OSHA has preferred the BW extrapolation in other chemical-specific risk assessments and has used BW as the extrapolation factor in its best estimate of risk in the NPRM for MC, OSHA agrees with Dr. Lorenz Rhomberg's assessment [Ex. 28] that OSHA should continue to use body weight as its extrapolation factor in its final MC risk assessment. Thus, OSHA's risk estimate does not make any allowance for possible pharmacodynamic differences between rodents and humans, or within the diverse human population.

3. Pharmacokinetic Modeling of Dose

OSHA discussed issues relating to the use of pharmacokinetic data in its NPRM. These issues were further explored during the hearings and in pre-hearing and post-hearing comments. In response to the ANPR [51 FR 42257], Dow Chemical submitted documentation of a physiologically-based pharmacokinetic model (PBPK) [Exs. 8-14d and 10-6a], developed for MC by Reitz and Anderson, which described the rates of metabolism of the MFO and GST pathways and the levels of MC and its metabolites in various tissues of rats, mice, hamsters and humans. This model was presented as a basis for converting an applied (external) dose of MC to an internal dose of active metabolite in the lung and liver in various species under various MC exposure scenarios. Since publication of the NPRM, several parties have submitted pharmacokinetic models or comments on modeling to the rulemaking record. These are discussed in detail below.

a. General issues in PBPK modeling. Physiologically-based pharmacokinetic modeling can be a useful tool for describing the distribution, metabolism and elimination of a compound of interest under conditions of actual exposure and, if data are adequate, can allow extrapolation across dose levels, across routes of exposure and across species. One limitation of using PBPK modeling is a widespread lack of adequate and appropriate physiological and metabolic data to define the model. In particular, difficulties arise in attempting to define a model for which the mechanism of carcinogenesis has not been established, when it is unclear whether there would be tumor site concordance across species, and when the metabolic pathway(s) responsible for carcinogenesis has not been determined.

The concentration of a chemical in air or the total inhaled dose (mg/kg/d) may not be the most biologically relevant dose to use in comparing toxicity across doses or across species. The dose measure that would be most useful in risk assessment is the dose to the target tissue of the chemical or metabolite that is known to directly cause the toxic effect. Generally, this quantity is unknown in almost every case because the proximate carcinogenic moiety is usually highly reactive, and therefore very difficult to measure in biological systems. Since the proximate toxic agent is unlikely to be a quantity readily measured in the laboratory, it is sometimes desirable to use dose surrogate concentrations, calculated by methods such as PBPK modeling, to obtain a more direct estimate of a dose-response relationship. Examples of dose surrogates that may be relevant to the toxic mechanism of action of a chemical are peak concentrations of a particular metabolite at a target tissue site, area under the concentration-time curve of a metabolite at a target site, and blood concentration of the parent chemical or a relevant metabolite.

If the dose surrogate chosen is directly relevant to the mechanism of action of a chemical, there is greater confidence in the risk estimates generated using the dose surrogate than those generated using total inhaled concentration. If the mechanism of action of a chemical is uncertain, and therefore the relevance of the dose surrogate to carcinogenicity is in question, there is proportionally less confidence in the predicted risks estimated using that dose surrogate. Risk estimates from PBPK modeling can also be limited by the quality and quantity of available metabolic data. Since risk estimates are directly dependent upon the dose or dose surrogate chosen, reliable measures of all relevant physiological parameters and all relevant metabolic pathways in all target tissues from all species under investigation are critical. In addition, measures of the uncertainty and inter-individual variability of these parameters must be generated.

In its NPRM, OSHA solicited information on the appropriateness of physiologically-based pharmacokinetic modeling for the MC risk assessment. Specifically, OSHA asked the following questions:

(a) How can pharmacokinetics be best applied to the risk assessment of MC and what are the current limitations of this approach in the quantitation of health risks? What weight should OSHA give to pharmacokinetic data in its risk assessments and why? (b) Given that five separate risk assessments have utilized the pharmacokinetic models for MC in five different ways (resulting in from 0 to 170 fold reduction in the final risk when compared with assessments not utilizing pharmacokinetic data), how can OSHA best utilize the existing pharmacokinetic data and still be certain of protecting worker health? (c) Which parameters in the pharmacokinetic models are most sensitive to errors in measurement or estimation? Can an increased database reduce the uncertainties in these parameters? (d) How much confidence can be placed in the human in vitro MC metabolism data, especially that for lung tissue? How will human variability in these parameters affect the extrapolation of risk from rodent species? (e) Are there any studies in progress which attempt to verify the predictive ability of the model in vivo, (e.g., by giving doses in a lifetime bioassay which will produce cancer in a species other than the B6C3F1 mouse and the F344 and Sprague-Dawley rats)? (f) OSHA recognizes the large areas of uncertainty which exist in applied dose risk assessment procedures. If pharmacokinetic modeling reduces these uncertainties, can the reduction in uncertainty be quantified? Are additional uncertainties introduced into the risk assessment process by the use of pharmacokinetic models? (g) By using the pharmacokinetic models in the risk assessment process, one is making an assumption about the carcinogenic mechanism of action of methylene chloride. Are there any new studies on the carcinogenic mechanism of action of MC which would support or refute this assumption? (h) If the carcinogenic process is, in fact, not the result of the metabolite(s) from the GST pathway alone, but is due to a combination of metabolites or a combination of the parent compound plus the metabolites, how would the pharmacokinetic model and the subsequent risk assessments be affected? Can these effects be quantified? (i) One of the assumptions made in the pharmacokinetic model is that the target tissues for MC are liver and lung. Can this model predict cancer incidences at other sites? If not, is there a way to factor in consideration of possible MC-induced human cancers at other sites than liver and lung? (j) OSHA solicits information supporting or refuting interspecies allometric scaling based on body weight or body surface area.

OSHA reviewed comments and testimony on these issues from an expert witness [Ex. 25-E]; representatives of other U.S. government agencies, including NIOSH [Exs. 19-46, 41], EPA [Exs. 25-D, 28], CPSC [Ex. 25-I] and U.S. Navy [Exs. 19-59, 96]; the State of California [Ex. 19-17]; the Halogenated Solvents Industry Alliance (HSIA) [Exs. 19-45, 19-83, 105]; and the UAW [Exs. 19-22, 23-13, 61]. Comments and testimony from the expert witness, the other government agencies and the Halogenated Solvents Industry Alliance generally reflected the opinion that the pharmacokinetic information was sufficiently developed in the case of MC to justify its use in estimating human cancer risks. The predominant view among these commenters and hearing participants was that the data collected for MC and the pharmacokinetic model developed by Reitz and Andersen adequately represented the metabolism of MC in mice. Many commenters also believed that it was reasonable to conclude that the lung and liver tumor incidence in the B6C3F1 mice was the result of the GST metabolite. As described in further detail below, OSHA generally agrees that the PBPK approach is reasonable to assess cancer risks of MC. In fact, the Agency has evaluated the submitted PBPK models, determined that there were several deficiences in each of those models, and improved upon those in its final quantification of risks.

One rulemaking participant was strongly opposed to using pharmacokinetic data in the MC risk assessment. Dr. Franklin Mirer [Ex. 61], representing the UAW, stated:

The pharmacokinetic model advanced for methylene chloride carcinogenesis is incorrect and should not be used for quantitative risk assessment.

Dr. Mirer was particularly concerned that the PBPK model ignored the rat cancer bioassay data and that the model was based on a "mechanistic hypothesis."

Dr. Mirer reiterated his concerns in response to the October 24, 1995 reopening of the rulemaking record [Ex. 126-31], stating,

The simple message is that OSHA should give no additional weight to the pharmacokinetic argument. For OSHA to give the argument any additional weight would mean that OSHA was ignoring a substantial body of evidence regarding carcinogenicity of methylene chloride in additional animal species.

Dr. Mirer continued,

The pharmacokinetic hypothesis is unconvincing even as an explanation of the differences in lung and liver tumors in mice and rats.

OSHA shares Dr. Mirer's concerns that the mechanism of carcinogenicity for MC has not been clearly established and that using pharmacokinetic modeling may lead to risk estimates which ignore the rat tumor data. The Agency notes that it has used the NTP rat data in its hazard identification for MC. OSHA has also determined, however, that the mouse data represent the strongest data set on which to base a quantitative risk assessment, and notes that risk estimates based on the rat data (without PBPK-based adjustment of dose) are similar to OSHA's final risk estimates using mouse data and a PBPK analysis.

The determination that the mouse data set was the strongest on which to base a quantitative risk assessment was made without regard to the availability of information on pharmacokinetics. Incorporating pharmacokinetic modeling into the risk assessment for MC is a logical extension of OSHA's risk assessment decisionmaking process and reflects the Agency's review of the totality of data on tumor incidence, metabolism and mechanism of action. The extensive data base on MC metabolism and mechanism of action, although by no means complete, was the determining factor in the decision to incorporate pharmacokinetics into its final risk assessment. The Agency is aware of very few chemicals of regulatory interest for which the available data could match this body of information. The specific criteria utilized by the Agency in making this determination are enumerated below.

Comments on the specific issues enumerated above are discussed under the appropriate topics in the sections that follow.

b. Criteria for using PBPK in quantitative risk assessment. OSHA evaluated several criteria before deciding to use PBPK analysis in its final quantitative risk assessment for MC. In future rulemakings in which the use of pharmacokinetic information in risk assessment is at issue, it will be necessary to evaluate at least the criteria described below before reaching conclusions, in order to avoid adopting an alternative hypothesis that is less (rather than more) reflective of the true situation than the more generic applied-dose assumption. Further, it may be appropriate to evaluate additional criteria in some cases, depending on the metabolism and mechanism of action of the chemical. The criteria which OSHA considered before incorporation of PBPK in the final risk estimate for MC were:

(1) The predominant and all relevant minor metabolic pathways must be well described in several species, including humans. (Two metabolic pathways are responsible for the metabolism of MC in humans, mice, rats and hamsters).

(2) The metabolism must be adequately modeled (Only two pathways are responsible for the metabolism of MC as compared to several potential routes of metabolism for other compounds, such as benzene and the dioxins. This simplified the resulting PBPK models).

(3) There must be strong empirical support for the putative mechanism of carcinogenesis (e.g., genotoxicity) and the proposed mechanism must be plausible.

(4) The kinetics for the putative carcinogenic metabolic pathway must have been measured in test animals in vivo and in vitro and in corresponding human tissues (lung and liver) at least in vitro, although in vivo human data would be the most definitive.

(5) The putative carcinogenic metabolic pathway must contain metabolites which are plausible proximate carcinogens (for example, reactive compounds such as formaldehyde or S-chloromethylglutathione).

(6) The contribution to carcinogenesis via other pathways must be adequately modeled or ruled out as a factor. For example, there must be a reasonable analysis of why reactive metabolites formed in a second pathway would not contribute to carcinogenesis (e.g., formyl chloride produced via the MFO pathway is likely to be too short-lived to be important in MC carcinogenesis).

(7) The dose surrogate in target tissues (lung and liver in the case of MC) used in PBPK modeling must correlate with tumor responses experienced by test animals (mice, rats and hamsters).

(8) All biochemical parameters specific to the compound, such as blood:air partition coefficients, must have been experimentally and reproducibly measured. This must be true especially for those parameters to which the PBPK model is most sensitive.

(9) The model must adequately describe experimentally measured physiological and biochemical phenomena.

(10) The PBPK models must have been validated with data (including human data) which were not used to construct the models.

(11) There must be sufficient data, especially data from a broadly representative sample of humans, to assess uncertainty and variability in the PBPK modeling.

In the case of MC, to a large extent these criteria were met. This made evaluation of existing PBPK models and further development of the modeling strategy a viable option. Therefore, the Agency evaluated existing PBPK models and then contracted with Drs. Andrew Smith, Frederic Bois, and Dale Hattis to help OSHA improve on the MC PBPK model in the record, which would extend the application of modeling techniques beyond those models which had been submitted to the Agency and incorporate all of the data available and appropriate for quantitative analysis in the record. OSHA's evaluation of existing PBPK models, the development of a modified MC PBPK analysis, and OSHA's final risk assessment are described later in this document.

c. Choice of GST metabolic pathway as dose surrogate. The choice of "dose surrogate" for the MC PBPK model is a critical factor in estimating PBPK-based risks. The dose or "dose surrogate" used in a risk assessment should be a biologically-important quantity, should have a plausible mechanism of action at the target tissue and should correlate with the response of interest. The simplest choice of dose is the applied dose or ambient concentration of the contaminant measured as ppm or as the inhaled quantity in mg/kg/day (as used in the Preliminary Quantitative Risk Assessment in the NPRM). Such quantities have the advantage of being easily and directly measurable during the bioassay. Other meaningful dose surrogates could include the concentration of parent compound in the target organ, the concentration of specific metabolites in the target organ, the area under the time-concentration curve (integrated dose) of each metabolite and the parent compound, or peak blood or target organ levels of each metabolite and parent compound. These quantities are not as easily measured. Often only indirect measurements or computer modeling of these dose surrogates are available.

In the PBPK model developed by Reitz et al. [Ex. 7-225], the dose surrogates that correlated with the tumor response were the parent compound (MC) concentration and the amount of GST metabolites formed in the lung and liver. Reitz et al. discounted the parent compound as the dose surrogate because MC is not a chemically reactive compound and direct-acting carcinogens (and metabolites of carcinogenic compounds) are generally hypothesized to be reactive (usually, electrophilic). They also discounted the parent compound as a relevant dose surrogate because parent MC concentration was higher in the rat blood than in the mouse for any dose of MC, while the cancer response of the mouse was greater than the rat. If parent MC were the critical compound for MC carcinogenesis, one would expect the cancer response across species to correlate with blood levels of the compound.

(1) Metabolism via GST versus MFO pathway. Human metabolism of MC has been well studied. One clear finding from the human metabolic studies is that humans metabolize MC by both the MFO and GST pathways, as do mice, rats, and hamsters. Although human metabolism via the MFO pathway has been measured in vivo as well as in vitro, human MC metabolism via the GST pathway has been measured only in vitro. Metabolic data on the human GST pathway have been collected from several liver samples and one pooled lung sample (combined samples from four human subjects). However, it has not been possible to measure human GST metabolism of MC in vivo.

Reitz et al. measured the metabolic constants (K(m) and V(max)) in vitro for the GST and the MFO metabolic pathways. Enzyme activities were determined by measuring the conversion of (36)Cl-labeled MC to water-soluble products. Metabolic constants were then compared across species (mouse, rat, hamster and human). In the liver, the MFO activity was highest in the hamster, followed by the mouse, human and rat. Human values were much more variable than those of the rodent species. Human V(max) for the liver MFO pathway ranged approximately an order of magnitude and human K(m) varied approximately three-fold. GST activity in the liver was determined for mouse and human tissues only. Mouse liver had approximately 18-fold greater activity (V(max)) than human liver, but the human tissue had about a three-fold greater affinity constant (K(m)) for MC than the mouse.

In the lung, the activity of the MFO and GST enzymes was determined for a single substrate concentration. For the MFO pathway, mouse tissue had the highest activity, followed by hamster and rat. No MFO activity specific for MC was detected in the human lung tissue, although other MFO isozymes were demonstrated to be active in the tissue. For the GST pathway in lung, mouse tissue was the most active, followed by rat and human. No GST activity was detected in the hamster lung.

In humans, the MFO pathway has been measured in vivo as well as in vitro [Ex. 7-225]. Human in vivo experimentation was conducted by several investigators. Metabolism via the MFO pathway is relatively easy to measure because the end product is carbon monoxide [Ex. 7-24]. The metabolic rates measured in vitro were not similar to those measured in vivo after exposure to known concentrations of MC, which means that in vitro measurements in human tissue (in particular for the GST pathway for which there are no human in vivo data) could not be used directly as a measure of metabolism. Human in vivo and in vitro MFO metabolism data were important in developing the pharmacokinetic models because they provided human data for MC-specific metabolism which could be used to help validate the models. Unfortunately, the modeling of the putative critical pathway for carcinogenesis (the GST pathway) could not be validated for humans. This is a weakness in the PBPK modeling for MC shared by all of the models, including OSHA's final PBPK analysis.

In the PBPK models submitted to OSHA, the human rate of metabolism of MC, particularly via the GST pathway, was based on data gathered from four liver samples and one pooled lung sample. Although the liver metabolic data were of the same magnitude as those collected by Green et al., Green's data were not considered in Reitz's model and the variability of those data was not assessed. Therefore, the estimates of the dose surrogates in Reitz's model were based on the average of four liver samples. Four liver samples are not nearly enough data to confidently estimate and account for human variability. Considerations of the variability and uncertainty of these data are discussed in more detail later in this document.

The human lung data were even more limited. Four human lung samples were pooled to provide a single data point. This lack of lung tissue data is particularly critical in PBPK modeling when calculating the ratios of A1 and A2 (the distribution of metabolism between liver and lung tissue in humans). Errors in calculating these ratios will significantly affect the final risk estimates, as discussed by Mr. Harvey Clewell for the U.S. Navy [Ex. 96].

HSIA submitted additional data on the human metabolism of MC in the form of a study of GST metabolism in human liver samples conducted by Bogaards et al. [Ex. 127-16]. The human GST liver metabolism data collected in this study were not directly comparable to the data collected by Reitz or Green, becausethe Bogaards data were measured using a colorimetric method which was not as sensitive as the (36)Cl method. Under contract to OSHA, Dr. Andrew Smith and Dr. Frederic Bois compared the data from different laboratories and collected under different methodologies and developed a correction factor across methodologies so that they could use all of the human metabolic data available in OSHA's final PBPK model [Ex. 128]. There are now over 30 data points for human liver in vitro metabolism by the GST pathway and 5 human lung data points (the additional lung data points were reported in Green et al., Ex. 124A). OSHA determined that it was important to use as much of the available human data in its PBPK model for MC as scientifically justifiable. These data were used to estimate the variability and uncertainty surrounding the measures of human GST metabolism. Although the methodologies differed across studies, OSHA has adjusted and incorporated all of the available human data in its PBPK model.

(2) Parallelogram approach. When the metabolic rates for the MFO pathway measured in vivo and in vitro within each species were compared, it was determined that those rates were not equivalent. This meant that, unlike the case for some other chemical compounds, the in vitro GST data could not substitute directly for an in vivo measurement of metabolism. Reitz and Andersen [Ex. 7-225] suggested a "parallelogram" approach to the problem of non-comparability of in vitro and in vivo rates. This approach makes the assumption that the ratio of in vivo to in vitro measurements is roughly comparable across species (including humans). They measured metabolic rates of both pathways in vitro and in vivo in rodents and then used the average ratio of the in vitro to in vivo metabolic rate in three rodent species to extrapolate from in vitro rates in humans [Ex. 7-225] to an estimated in vivo value.

(For Figure VI-1, Click Here)

Figure VI-1: Schematic diagram of the parallelogram approach.

                      rodent (in vitro)   human (in vitro)
         Assumption: _________________  = ________________
                      rodent (in vivo)     human (in vivo)

                          rodent (in vivo) x human (in vitro)
    or, human (in vivo) = ____________________________________
                                      rodent (in vitro)

Ron Brown [Ex. 25-E], an expert witness for OSHA, was concerned that "...the methodology used to extrapolate the in vitro data to the in vivo state is problematic and the accuracy of the human in vitro measurement of GST activity toward MC is uncertain." This may be due to the small sample size, variability in the laboratory analysis or inadequacy of the in vitro model. OSHA believes that this is a critical point of uncertainty in using the PBPK model for risk assessment. The Agency also notes that in the risk assessments using PBPK models submitted during the MC rulemaking, none used the parallelogram approach as the basis of determining human in vivo metabolic rates. Instead, allometric scaling was used to estimate human values. OSHA has conducted risk assessments using both the allometric approach (OSHA's final risk estimates) and the parallelogram approach (OSHA's alternative analysis). The Agency did this in order to determine what the risk estimates would be if all possible quantitative data were used to the fullest extent, regardless of the uncertainties in the data.

OSHA agrees that evidence presented in the record generally supports the GST pathway as a plausible carcinogenic mechanism of action of MC. The Agency remains concerned, however, that sole reliance on the GST pathway may show insufficient consideration for potential contributions of the parent compound and/or metabolites of the MFO pathway to the carcinogenesis of MC. It is clear that ambient MC concentration is dose-related to tumor response. It has not been shown with any certainty that MC GST metabolites are related to tumor response across species. Thus, there is greater confidence that the lifetime bioassays predict MC carcinogenicity in humans than there is that cancer occurred through a specific mechanism, and even less confidence that the metabolic rates measured in vitro accurately measure differences in species that correlate to tumor development. This is particularly true for lung metabolism where only one pooled and five individual human samples were analyzed. Notwithstanding the uncertainties described above, the Agency believes that the hypothesis that GST is the carcinogenic pathway presents a plausible mechanism of action for MC and is sufficiently well-developed to warrant the use of PBPK modeling of the GST pathway as the dose surrogate of choice in the quantitative risk assessment for MC.

d. Structure of the MC PBPK model. The PBPK models described below are based on the model originally submitted by Dr. Reitz on behalf of HSIA in 1992 [Ex. 7-225]. Over the years since the first submission of a MC PBPK model to OSHA, significant improvements have been made in model structure and in the data collected for PBPK modeling, especially in how the uncertainty and variability in the data are treated. The general structure of the models submitted to OSHA are described below, followed by a description of the parameters used in the various models. Next follows a description of how the variability, uncertainty, and sensitivity of the models to uncertainty have been assessed, noting the improvements that have been made in developing methods to handle these issues. This is followed by a comparison of the risk estimates generated by these models. Finally, OSHA's final risk assessment is described. This risk assessment incorporates lessons learned from previous models and uses all of the available, appropriate, quantifiable data in a Bayesian approach to modeling the dose metric for MC.

In the PBPK model submitted by Dr. Reitz of HSIA [Ex. 7-225], a series of differential equations was used to model the mass balance of MC and its metabolites in five physiologically-defined compartments, including the lung, liver, richly perfused tissue, slowly perfused tissue, and fat. Metabolism via the MFO pathway was described by saturable Michaelis-Menten kinetic equations and GST metabolism was modeled using first-order nonsaturable kinetics. With the exception of the PBPK model sumitted by ICI [Ex. 14A], all of the PBPK models submitted to the Agency followed these assumptions regarding the metabolism of MC. The rate constants for the metabolic equations were estimated based on measurements of the partition coefficients, allometric approximations of the physiological constants (e.g., lung weight), and estimations (i.e., allometric scaling of rodent data, estimations made using the parallelogram approach, etc.) of the biochemical constants (e.g., Michaelis-Menten constants).

NIOSH presented a PBPK model in 1993 [Ex. 94], also structurally based on the Reitz-Andersen model, but with modifications to the human breathing rate and cardiac output to account for uptake of MC in physically active workers, rather than at-rest humans or humans involved in light activity, and including an analysis of the variability of the human metabolic parameters. Specifically, NIOSH compared estimates derived from the arithmetic average of the human GST metabolism data with the individual human liver data points to estimate the uncertainty in an individual's risk of cancer from occupational MC exposure. This approach began to incorporate some necessary features, such as a special focus on physically active workers and the variability of human metabolic parameters, but did not attempt to quantify the uncertainty and variability of the individual parameters and their contribution to the uncertainty associated with the PBPK model.

Mr. Harvey Clewell, representing the U.S. Navy, also submitted several PBPK models to OSHA. In his initial submission (1992), Mr. Clewell modified an existing PBPK model [Ex. 7-125] to include more recent data on the mouse blood/air partition coefficient [Ex. 19-59]. In a second PBPK model, he "started from scratch" to construct a model based on data derived from sources independent of the previous work of Reitz and Andersen [Ex. 23-14], which was described in Mr. Clewell's testimony [Tr. 2361,10/15/92]. This model was structurally similar to the model presented by HSIA with the following exceptions: it featured three lumped compartments (slowly perfused, moderately perfused and rapidly perfused) based on tissue kinetic constants rather than the earlier two lumped compartment models based on tissue blood volumes; and the mouse blood/air partition coefficient was corrected to 19.4 instead of the earlier 8.29 on the basis of more recent data. A third model submitted by Mr. Clewell was identical in structure to the Reitz/Andersen model, but incorporated the more recent experimental data on the partition coefficients and the more recent mouse metabolism data [Ex. 96]. OSHA used Mr. Clewell's third model in its comparison of PBPK-derived risk estimates because of its similarity in structure to the original Reitz model and its incorporation of the most recent experimental data.

In his third model, Clewell either derived probability distributions for each parameter from the literature or estimated distributions for those parameters for which data were not available, and conducted Monte Carlo simulations to derive output distributions for the dose surrogates. These distributions of dose surrogates were then used to derive four risk estimates: the doses input into the multistage dose-response analysis of the tumor bioassay were derived either from the mean or from the 95th percentile of the output distribution of PBPK parameters, and these in turn were coupled with the either the MLE or the UCL of the distribution of possible values of the multistage model parameters. This analysis was an advance over that of previous models because it took into account some of the uncertainty and variability known to be associated with the data used in the PBPK model.

After evaluating these submitted models, OSHA determined that Clewell's model provided the best prototype on which to base its final PBPK modeling approach for MC. Therefore, the Agency worked with Drs. Smith and Bois to review Clewell's model and with the assistance of Dr. Hattis, to develop a refined PBPK modeling approach with a more sophisticated analysis of variability and uncertainty (and other refinements as described below). In this way the Agency developed an approach which would incorporate what was learned in the development of earlier PBPK models and make use of as much of the available physiological and metabolic data in the record as possible. Clewell's model was chosen for comparison, because this was the only model to provide a systematic analysis of the uncertainty, variability and sensitivity of the model using Monte Carlo techniques. OSHA's final risk assessment approach is described in greater detail below.

e. Choice of parameters for PBPK modeling. The definitions of the parameters used in the models described above are contained in Table VI-2. Note that not all parameters were used in each model and slightly different variable names were used by different investigators. For example, OSHA's final analysis contains a bone marrow compartment, while Clewell's model did not. OSHA refers to the blood flow for poorly (or slowly) perfused tissues as "QppC," while Clewell used "QSC."

      Table VI-2. -- Definitions of Pharmacokinetic Parameters
      Parameter (units)  |              Definition
BW (kg)..................|Body weight in kg. Human body weights
                         | were assumed to be 70-kg (Reference
                         | Man). Mouse body weights were the
                         | average weight of mice in the NTP
                         | bioassay.
QPC unscaled             |Breathing rate. QPC = QP(1/hr)/BW(.75)
  (1/hr, 1 kg BW)........| where QP = alveolar ventilation rate.
                         | Human QP was based on rate of 9.6
                         | m(3/8-) hr (converted 1/hr and adjusted to
                         | alveolar ventilation (= 0.70 total
                         | ventilation) except in NIOSH and
                         | OSHA-modified models. Mouse QP
                         | = (24.3 1/hr)(0.70 alveolar/total).
QCC unscaled             |Cardiac output. QCC = QC(1/hr)/BW(.75)
  (1/hr, 1 kg BW)........| where QC = cardiac output in 1/hr. Reitz
                         | set QC = QP. Clewell and NIOSH based
                         | human QC on Astrand et al. [Ex. 7-120]
                         | data on cardiac output and breathing
                         | rate vs. workload.
VPR (ratio, unitless)....|Alveolar ventilation/perfusion ratio.

                                Blood flows to tissues

QGC or QgiC (fraction of |Blood flow to gastrointestinal tract as a
 cardiac output)........ | fraction of cardiac output. QGC = QG/QC.
QLC or QliC (fraction of |Blood flow to liver as a fraction of
 cardiac output)........ | cardiac output. QLC = QL/QC.
QFC or QfatC (fraction of|Blood flow to fat as a fraction of
 cardiac output)........ | cardiac output. QFC = QF/QC.
QSC or QppC (fraction of |Blood flow to slowly (or poorly) perfused
 cardiac output)........ | tissues as a fraction of cardiac output.
                         | QSC = QS/QC.
QRC or QwpC (fraction of |Blood flow to rapidly (or well) perfused
 cardiac output)........ | tissues as a fraction of cardiac output.
                         | QRC = QR/QC.
QmarC (fraction of       |Blood flow to bone marrow as a fraction
 cardiac output)........ | of cardiac output.

                                 Tissue volumes

VGC or VgiC (fraction of |Volume of GI tract as a fraction of body
 body weight).           | weight. VGC = VG/BW.
VLC or VliC (fraction of |Volume of liver as a fraction of body
 body weight).           | weight. VLC = VL/BW.
VFC or VfatC (fraction of|Volume of fat as a fraction of body
 body weight).           | weight. VFC = VF/BW.
VSC or VppC (fraction of |Volume of slowly (or poorly) perfused
 body weight).           | tissues as a fraction of body weight.
                         | VSC = VS/BW.
VRC or VwpP (fraction of |Volume of rapidly (or well) perfused
 body weight).           | tissues as a fraction of body weight.
                         | VRC = VR/BW.
VluC (fraction of body   |Volume of lung as a fraction of body
 weight).                | weight.
VmarC (fraction of body  |Volume of bone marrow as a fraction of
 weight).                | body weight.

                              Partition coefficients

PB or Pblo...............|Blood/air partition coefficient.
PG or Pgi................|GI tract/blood partition coefficient (GI
                         | tract/air divided by PB).
PL or Pli................|Liver/blood partition coefficient
                         | (Liver/air divided by PB).
PF or Pfat...............|Fat/blood partition coefficient (Fat/air
                         | divided by PB).
PS or Ppp................|Slowly (or poorly) perfused tissue/blood
                         | partition coefficient (Slowly perfused
                         | tissue/air divided by PB).
PR or Pwp................|Rapidly (or well) perfused tissue/blood
                         | partition coefficient (Rapidly perfused
                         | tissue/air divided by PB).
PLU or Plu...............|Lung/blood partition coefficient
                         | (Lung/air divided by PB).
Pmar.....................|Bone marrow:air partition coefficient.

                                Metabolic parameters

VMAXC unscaled (mg/hr,   |MFO pathway Michaelis-Menten maximum
 1 kg animal).           | velocity for MC metabolism. VMAXC = VMAX
                         | (mg/hr)/BW(.75).
KM (mg/l)................|MFO pathway Michaelis-Menten affinity
                         | constant for MC metabolism.
KFC, unscaled, (/hr,     |GST pathway 1st order kinetic rate
 1 kg animal).           | constant for MC metabolism. KFC = KF
                         | (/hr)(BW(.25)).
A1 (ratio)...............|Ratio of distribution of MFO pathway MC
                         | metabolism between lung and liver. A1 =
                         | VMAXC(lung)/VMAXC(liver).
A2 (ratio)...............|Ratio of distribution of GST pathway MC
                         | metabolism between lung and liver. A2 =
                         | KFC(lung)/KFC(liver).
B1 (ratio)...............|Ratio of lung and liver tissue content of
                         | microsomal protein.
B2 (ratio)...............|Ratio of lung and liver tissue content of
                         | cytosolic protein.
Sp--Kf...................|Allometric scaling power for body weight
                         | scaling of KFC from mice to humans.

The MC physiologically-based pharmacokinetic (PBPK) models discussed here contain the following types of parameters as defined above: body weight, breathing rate, cardiac output, blood flows to tissue compartments (as a fraction of the cardiac output), volumes of tissue compartments (as a fraction of body weight), partition coefficients, the metabolic parameters (the Michaelis-Menten parameters, Vmax and Km, for the MFO pathway and the 1st-order rate constant, Kf, for the GST pathway) and the ratio of the pathway-specific metabolic capacity between the major metabolic sites (lung and liver). Differences in model structure (such as choice of lumped tissue compartments) and differences in sources of data for individual parameters lead to differences in the parameter values used in different models.

The parameter values (point estimates) used in the PBPK models reviewed by OSHA are presented in Table VI-3. The parameter distributions used by OSHA in its analysis are presented later.

As far as OSHA could determine, the parameters chosen by HSIA were those presented in Reitz's 1989 paper [Ex. 21-53] except that OSHA's preferred values for breathing rates (based on 9.6 m(3)/workday) and 8-hour human exposures were used. The model submitted by NIOSH used the parameters and computer code from the Reitz model, except for the human breathing rate, human cardiac output and human metabolic parameters. The parameters used by Clewell were summarized in his post-hearing submission [Ex. 96], which included more recent experimental data for the partition coefficients and mouse metabolic parameters and a different scaling for human cardiac output.

    Table VI-3. -- Parameters Used in PBPK Models Reviewed by OSHA
       Model                         |     Clewell        |  NIOSH
                                     |     [Ex.96]        |   [Ex.
                                     |                    |  23-18]
     Parameter                       |  Mouse   | Human   |  Mouse
BW (kg)..............................|  0.0345  |70       | 0.0345
QPC, unscaled alveolar ventilation   |          |         |
 (1/hr, 1 kg animal).................| 29.0     |35       |29.0
QCC, unscaled cardiac output (1/hr,  |          |         |
 1 kg animal)........................| 16.5     |18       |29.0
QGC(a), flow to GI tract (fraction   |          |         |
 of cardiac output)..................|  0.165   | 0.195   | 0.0
QLC(a), flow to liver (fraction of   |          |         |
 cardiac output).....................|  0.035   | 0.07    | 0.24
QFC(a), flow to fat (fraction of     |          |         |
 cardiac output).....................|  0.03    | 0.05    | 0.05
QSC(a), flow to slowly perfused      |          |         |
 tissues (fraction of cardiac        |          |         |
 output).............................|  0.25    | 0.24    | 0.19
QRC(a), flow to rapidly perfused     |          |         |
 tissues (fraction of cardiac        |          |         |
 output).............................|  0.52    | 0.445   | 0.52
VGC, GI volume (fraction of BW)......|  0.031   | 0.045   | 0.0
VLC, liver volume (fraction of BW)...|  0.046   | 0.023   | 0.04
VFC, fat volume (fraction of BW).....|  0.100   | 0.16    | 0.07
VSC, slowly perfused tissue volume   |          |         |
 (fraction of BW)....................|  0.513   | 0.48    | 0.75
VRC, rapidly perfused tissue volume  |          |         |
 (fraction of BW)....................|  0.041   | 0.033   | 0.05
VLUC, lung volume (fraction of BW)...|  0.008   | 0.006   | 0.012
PB, blood/air part. coeff............| 23.0     |12.9     | 8.29
PG, GI tract/air part. coeff.........|  0.52    | 0.93    |NA
PL, liver/blood part. coeff..........|  1.6     | 2.9     | 1.71
PF, fat/blood part. coeff............|  5.1     | 9.1     |14.5
PS, slowly perf./blood part. coeff...|  0.44    | 0.78    | 0.96
PR, rapidly perf./blood part. coeff..|  0.52    | 0.93    | 1.71
PLU, lung/blood part. coeff..........|  0.46    | 0.82    | 1.71
VMAXC mg/hr, 1 kg animal (unscaled)..| 13.4     | 5.0     |13.2
                                     |          |         |
                                     |          |         |
                                     |          |         |
KM (mg/L)............................|  1.35    | 0.4     | 0.396
                                     |          |         |
                                     |          |         |
                                     |          |         |
KFC /hr, 1 kg animal (unscaled)......|  1.5     | 1.5     | 1.73
                                     |          |         |
                                     |          |         |
                                     |          |         |
A1 (Vmaxc(lung)/Vmaxc(liver))........|  0.41    | 0.015   | 0.416
A2 (KFC(lung)/KFC (liver))...........|  0.28    | 0.18    | 0.137

 Table VI-3. -- Parameters Used in PBPK Models Reviewed by OSHA
            Model                    |  NIOSH    |
                                     |[Ex. 23-18]|  HSIA [Ex. 19-45]
                                     |  (cont.)  |
          Parameter                  |   Human   |  Mouse  |  Human
BW (kg)..............................|  70       |  0.0345 | 70
QPC, unscaled alveolar ventilation   |           |         |
 (1/hr, 1 kg animal).................|  43.1     | 29.0    | 35.0
QCC, unscaled cardiac output (1/hr,  |           |         |
 1 kg animal)........................|  20.9     | 29.0    | 35.0
QGC(a), flow to GI tract (fraction   |           |         |
 of cardiac output)..................|   0.0     |  0.0    |  0.0
QLC(a), flow to liver (fraction of   |           |         |
 cardiac output).....................|   0.2093  |  0.24   |  0.24
QFC(a), flow to fat (fraction of     |           |         |
 cardiac output).....................|   0.040   |  0.05   |  0.05
QSC(a), flow to slowly perfused      |           |         |
 tissues (fraction of cardiac        |           |         |
 output).............................|   0.4319  |  0.19   |  0.19
QRC(a), flow to rapidly perfused     |           |         |
 tissues (fraction of cardiac        |           |         |
 output).............................|   0.3188  |  0.52   |  0.52
VGC, GI volume (fraction of BW)......|   0.0     |  0.0    |  0.0
VLC, liver volume (fraction of BW)...|   0.0314  |  0.04   |  0.0314
VFC, fat volume (fraction of BW).....|   0.231   |  0.07   |  0.231
VSC, slowly perfused tissue volume   |           |         |
 (fraction of BW)....................|   0.621   |  0.75   |  0.621
VRC, rapidly perfused tissue volume  |           |         |
 (fraction of BW)....................|   0.0371  |  0.05   |  0.0371
VLUC, lung volume (fraction of BW)...|   0.011   |  0.012  |  0.011
PB, blood/air part. coeff............|   9.7     |  8.29   |  9.7
PG, GI tract/air part. coeff.........|   NA      |  NA     |  NA
PL, liver/blood part. coeff..........|   1.46    |  1.71   |  1.46
PF, fat/blood part. coeff............|  12.4     | 14.5    | 12.4
PS, slowly perf./blood part. coeff...|   0.82    |  0.96   |  0.82
PR, rapidly perf./blood part. coeff..|   1.46    |  1.71   |  1.46
PLU, lung/blood part. coeff..........|   1.46    |  1.71   |  1.46
VMAXC mg/hr, 1 kg animal (unscaled)..|   3.98    | 13.2    |  4.9
                                     |   1.15    |         |
                                     |   9.81    |         |
                                     |   4.71    |         |
KM (mg/L)............................|   0.72    |  0.396  |  0.580
                                     |   0.55    |         |
                                     |   0.26    |         |
                                     |   0.79    |         |
KFC /hr, 1 kg animal (unscaled)......|   1.56    |  1.73   |  1.24
                                     |   0.00    |         |
                                     |   1.62    |         |
                                     |   1.79    |         |
A1 (Vmaxc(lung)/Vmaxc(liver))........|   0.00143 |  0.416  |  0.00143
A2 (KFC(lung)/KFC (liver))...........|   0.18    |  0.137  |  0.18
  Footnote(a) QGC + QLC + QFC + QSC + QRC MUST = 1.00.

f. Assessment of the sensitivity and uncertainty of the PBPK model. In the NPRM, OSHA expressed concern that, if PBPK models were used to adjust risk assessments, the uncertainty in PBPK modeling should be adequately addressed. Specifically, OSHA was concerned that the uncertainty in the mechanism of action and the lack of human lung metabolism data were the greatest obstacles to incorporation of pharmacokinetic data into the MC final risk assessment. Many of the uncertainties in model parameters have been quantified by various hearing participants and are summarized below. The quantification of these uncertainties, however, did not address OSHA's primary concerns regarding the mechanism of action and the distribution of metabolism between lung and liver. OSHA's analyses of the uncertainty and variability of parameters in the PBPK model are presented with its risk assessment later in this document.

The concepts of uncertainty, variability and sensitivity in PBPK modeling were defined in comments submitted by the U.S. Navy [Ex. 19-59]:

As it relates to the issue of using PBPK modeling in risk assessment, uncertainty can be defined as the possible error in estimating the "true" value of a parameter for a representative ("average") animal. Variability, on the other hand, should only be considered to represent true interindividual differences.

The normalized sensitivity coefficient gives the percentage change in a model output due to a percentage change in the parameter value and represents the relative importance of the parameter to the model output under the conditions of the simulation.

Each of these quantities is of concern for risk assessment and PBPK modeling. For example, we know that there is variability or inter-individual heterogeneity in the body weights of humans (and mice), yet we estimate risks for an average member of the population (70 kg in humans, average bioassay weight in mice). For many parameters, the interindividual variability may not be known and must be estimated.

Uncertainty in estimation of the value of a parameter representing an average member of a population is primarily due to laboratory measurement and related errors. Measurement errors, in many cases, can be quantified or estimated so that the potential impact of this uncertainty on the outcome of the PBPK modeling can be assessed.

The sensitivity of the model to particular parameters is useful for determining which experiments should be conducted to confirm parameters and to determine the amount of confidence that PBPK model outputs merit. For example, when a sensitivity analysis is conducted and it is determined that the model outcomes are not very sensitive to changes in the definitions of the lumped tissue volumes, it suggests that there is little need to conduct experiments to describe those relationships more precisely. Similarly, even though the lumped tissue volume does not represent a "true" biological quantity, there is confidence that its precise definition is not critically important in PBPK model outcomes. Therefore, if the only large (quantifiable) uncertainty resides in this measurement, one would have greater confidence that the model predictions were reasonably accurate. Therefore, it is instructive to understand which parameters influence the model outcomes to the greatest degree. Conversely, if the PBPK model outputs are sensitive to a parameter which has not been precisely described (such as the distribution of GST metabolism between lung and liver), the confidence in model outputs is correspondingly reduced.

Various investigators have attempted to determine the sensitivity of the PBPK models to parameter values and to characterize the uncertainty and variability within parameters in the models. The first attempt to describe the sensitivity of the Reitz's original PBPK model was performed by the Consumer Product Safety Commission (CPSC).

The CPSC conducted a sensitivity analysis of the metabolic parameters, Km, Vmax and Kf, in the "Updated Risk Assessment for Methylene Chloride" [Ex. 7-126]. They analyzed the sensitivity of the model by selecting alternative point estimates for the metabolic parameters and determining what the resulting ratio of GST metabolite at 4000 ppm vs. 1 ppm would be. This analysis shows how this ratio would vary if the metabolic parameters used in the model were higher or lower than the measured values as selected by CPSC. The results showed that the ratio of the GST metabolite in the liver at 4000 ppm to the GST metabolite at 1 ppm (or the ratio of the GST metabolite in the lung at 4000 ppm to the GST metabolite at 1 ppm) was relatively insensitive to the value of Kf (when CPSC varied Kf from 0.01 to 5.3, while Km and Vmax were held constant at Reitz-Andersen values).

HSIA presented a sensitivity analysis of the PBPK parameters from the Reitz (HSIA) model in the testimony of Dr. Reitz [Ex. 23-21A]. Results were presented for mice at 4000 ppm, mice at 1 ppm, humans at 1000 ppm and humans at 1 ppm. In the first analysis (mice at 4000 ppm), the most sensitive parameters were determined to be PB (blood:air partition coefficient) and Kf (metabolic parameter for the GST pathway). The authors observed that at high MC exposure levels the model output was at least an order of magnitude less sensitive to changes in the other sixteen parameters investigated.

When mice were exposed to lower concentrations of MC (1 ppm) Vmax and Km for the MFO pathway were the most sensitive parameters (sensitivity coefficient was over 120% for each of these parameters). In addition, several other parameters were found to exert a significant influence on model outputs: QP, QL, PB, VLu, and KF.

In humans, at high concentrations (> 1000 ppm) the results were similar to those observed in mice: the model was most sensitive to PB and KF, with sensitivity coefficients of 87% and 97%, respectively. In addition, the human model was also sensitive to the value chosen for the QP (sensitivity coefficient = 43%).

In humans, at 1 ppm MC, Km and Vmax for the MFO pathway were the most sensitive parameters out of the six parameters which had a significant effect upon model outputs: QP, QL, PB, Vmax, Km, and KF.

This type of sensitivity analysis improves on that conducted by the CPSC, because it looks at more of the parameters. It is still deficient, however, because it examines the effect of each parameter individually, and because it does not examine the effect of uncertainty in two key parameters, A1 and A2 (the ratios of distribution of the MFO and GST pathways between lung and liver), on the outcomes of the modeling.

Mr. Clewell [Ex. 19-59] also conducted a sensitivity analysis to determine the impact of uncertainty in PBPK parameters on the model outcomes. In contrast to the HSIA analysis, he examined the sensitivity of the outcomes to the ratios A1 and A2, and he chose a more realistic occupational exposure level (100 ppm). He found that for mice at 4000 ppm, the most sensitive parameters for estimation of lung tumors were KF, A2, and PB. In the liver, the most sensitive parameters were KF and PB, which agrees with the results of the HSIA analysis. For humans at 100 ppm, the most sensitive parameters for estimating lung tumors were KF and A2. Other parameters with significant effects on model outcomes were PB, QPC, BW, KM, QCC, and QLC. The most sensitive parameters for estimating liver tumors were VMAX, KF, QPC and BW, while PB, KM, QCC and QLC also produced significant effects on model outcomes.

In all of these analyses, the PBPK models were clearly sensitive to the values chosen for the metabolic parameters, especially the GST metabolic parameter (KF). Other parameters with consistently significant impact on the outcomes of the model included breathing rate (QP) and distribution of GST metabolism between lung and liver (A2). These analyses suggest that additional studies to quantify the metabolic parameters (KF, KM and VMAX), breathing rates (QP) and distribution of GST metabolism between lung and liver (A2) would increase confidence in the model outcomes. Characterization of the distribution of metabolism between lung and liver is particularly critical because estimates for human lung metabolism were initially based on one pooled sample of lung tissue, and the variability and uncertainty of the value of this parameter has not been quantified.

Some analysts [Ex. 21-52] have suggested that the uncertainty is increased in risk assessments based on PBPK as compared to applied-dose risk assessments, because some methods of quantifying the uncertainty result in rather broad distributions of uncertainties. OSHA, in contrast, agrees with most commenters that quantifying uncertainty in a PBPK model or risk assessment does not increase the uncertainty. The Agency stresses that the appearance of increasing uncertainty with the identification of sources of uncertainty almost certainly means that the original uncertainty was underestimated. (In fact, since many assessors have not attempted even to quantify the uncertainty in applied-dose risk assessments, the uncertainty has often been infinitely underestimated.) When conducting a risk assessment using PBPK that appears to increase the uncertainty over delivered-dose methodologies, the investigator should go back and recalibrate what the uncertainty in the original analysis likely was, in light of the sources of uncertainty identified using PBPK. This would tend to broaden the confidence limits of the traditional risk assessments, almost certainly beyond the limits generated in a thoughtful PBPK-based assessment. For example, many analyses using delivered dose assume that in the interspecies scaling factor, BW(x), x is known with perfect certainty (e.g., it is known to equal 2/3 or 1.0). An analysis that uses an empirically-derived probability distribution for x, which might reasonably extend from approximately 0.6 to approximately 1.0, would yield a rather broad distribution of uncertainty in the resulting estimate of risk.

The Agency also agrees that the primary uncertainties lie in the choice of the dose surrogate and assumptions regarding cross-species scaling. Clewell [Ex. 23-14] investigated the uncertainty of the PBPK parameters using Monte Carlo analyses of the assumed distributions of uncertainty of each parameter. The resulting estimates of dose surrogate values were characterized by a mean of the distribution and an upper 95th percentile estimate. Mr. Clewell stated [Ex. 19-59]:

[T]he use of the 95th percentile of the distribution of estimates accounts for additional uncertainty concerning the true values of the PBPK parameters for the bioassay animals and humans.

Mr. Clewell recommended that OSHA use the upper 95th percentile of the Monte Carlo distribution of GST metabolites (from PBPK modeling) as an input to the multistage model to generate risk estimates, and then use of the MLE from the multistage model in those risk estimates, in accordance with previous OSHA risk assessments. He remarked that use of the upper 95th percentile of the PBPK output would be a reasonable mechanism to account for the uncertainty quantified in these analyses. Using the upper 95th percentile of the distribution of GST metabolites, Mr. Clewell's risk estimate for lifetime occupational exposure to 25 ppm MC was 0.9 deaths per 1000 using the MLE of the multistage model, and 1.1 per 1000 using the 95th percentile upper confidence limit (UCL) from the multistage model. Using the mean of the distribtution of GST metabolites, his MLE risk estimate was 0.28 deaths per 1000 at the same exposure level, with an UCL of 0.35/1000.

The HSIA disagreed with using the upper 95th percentile for estimating risks, and stated [Ex. 105]:

[T]he analyses conducted by Clewell et al. indicate that consideration of model parameter variability does not contribute orders of magnitude to the uncertainty associated with PB-PK risk assessments. Further, the uncertainty associated with PB-PK risk assessments is significantly less than that associated with risk assessments that fail to consider pharmacokinetics. The uncertainty in PB-PK based procedures is simply more readily available for calculation.

OSHA disagrees with the HSIA that the uncertainty and variability associated with PBPK risk assessments is significantly less than that associated with risk assessments that fail to consider pharmacokinetics. Quantification of uncertainty does not equate with reducing uncertainty in an analysis. In fact, at a different level, the assumptions made regarding mechanism of action of MC and extrapolation of lung metabolic rates from one human in vitro sample may serve to underestimate the uncertainty inherent in the PBPK-based risk assessment if the underlying assumptions are wrong. Also, as stated above, identification of uncertainty may lead us to recalibrate the uncertainty associated with traditional risk assessment methods. In any event, the possibility that using PBPK significantly reduces uncertainty does not affect the need to account for whatever uncertainty remains.

In addition, OSHA agrees with Clewell that using the upper 95th percentile of the Monte Carlo distribution of GST metabolites as input to the multistage model is a reasonable way to incorporate the quantifiable uncertainty and variability into a risk assessment. In its final risk estimates, OSHA has used the upper 95th percentile on the distribution of GST metabolites from the Bayesian analysis as the input to the multistage model, as described later in this document.

E. Other Risk Estimates Based on PBPK Models Prior to OSHA's Final Analysis.

A PBPK model can produce estimates of target tissue doses (or dose surrogates) for different hypotheses of action of a chemical. The appropriate choice of target tissue dose can greatly influence risk estimates based on that dose. For MC, the dose surrogate that has been used most frequently to estimate cancer risks is the amount of GST metabolite produced. The amount of GST metabolite can then be used to extrapolate from a high bioassay dose of MC to a low occupational (or environmental) dose of MC and from mouse MC metabolic rates to human metabolic rates.

In the NPRM, OSHA reviewed available risk assessments for MC that used PBPK modeling in a variety of ways. The Food and Drug Administration risk assessment [Ex. 6-1] was not adjusted to account for pharmacokinetic information. The Consumer Product Safety Commission, in its "Updated risk assessment for methylene chloride" [Ex. 7-126], used pharmacokinetic data to adjust for differences in metabolism in extrapolating from high dose (4000 ppm mouse bioassay) to low dose (1 ppm) exposures, but did not adjust for interspecies differences in the metabolism of MC. The resulting risk estimate was approximately 2-fold lower than a risk estimate using applied dose.

The U.S. EPA analyzed the MC pharmacokinetic data in its documents, "Technical analysis of new methods and data regarding dichloromethane hazard assessment" [Ex. 7-129] and "Update to the Health Assessment Document and Addendum for dichloromethane (methylene chloride): pharmacokinetics, mechanism of action, and epidemiology" [Ex. 7-128]. The EPA used the PBPK data to adjust its risk estimates in its Integrated Risk Information System (IRIS) database. Adjustments were made for high-to-low dose and cross-species extrapolation. EPA's risk estimates for low human exposures to MC were decreased by approximately a factor of 9 from its risk estimates made without consideration of PBPK data.

The HSIA [Ex. 105] and ECETOC [Ex. 14] also submitted risk assessments based on PBPK data. The primary difference between the HSIA and the EPA risk estimates was that the HSIA did not use a surface area correction to account for interspecies differences other than pharmacokinetics (e.g., pharmacodynamic differences) while the EPA did. Also, HSIA's risk estimates used OSHA's preferred breathing rates and an occupational exposure scenario. ECETOC based its risk estimates on different measures of human MC metabolism. In a pre-hearing submission, "Using PB-PK Models for Risk Assessment with Methylene Chloride (Comparison of U.S. and U.K. procedures)" [Ex. 19-83A], scientists from the U.S. and the U.K. compared methodologies for using PBPK data in the MC risk assessment and presented a consensus opinion that OSHA should use the methodology developed by Dr. Richard Reitz [Ex. 7-225] for the U.S. For this reason, OSHA evaluated Dr. Reitz's analysis, as presented by the HSIA, and did not separately consider the ECETOC risk assessment.

As described previously, Clewell [Ex. 96] and NIOSH [Ex. 94] have submitted analyses of the PBPK data and risk assessments based on those analyses. Both of these analyses used PBPK modeling of the amount of GST metabolites produced in their estimates of carcinogenic risks.

OSHA has evaluated the data in the rulemaking record and has concluded that, if PBPK modeling is used to adjust estimates of risk, the weight of evidence supports using the amount of GST metabolites as the preferred surrogate for target tissue dose. The amount of GST metabolites predicted by the PBPK model varies depending upon the values or distributions chosen for the parameters in the model.

Of the risk assessments described above, OSHA has chosen to compare risks estimated using PBPK models submitted by Reitz et al., Clewell et al. and NIOSH with applied dose methodology using either of two scaling assumptions: the inhaled dose in mg/kg/day (the estimates of risk presented in the NPRM) and ppm-to-ppm extrapolation. OSHA evaluated the methodologies used in developing these risk estimates before developing its final risk estimates, which are presented in the next section.

The risk estimates derived from using PBPK with the multistage dose-response model submitted to the Agency by Reitz et al., Clewell et al., and NIOSH, and the risk estimates derived from applied dose methodologies, are shown in Table VI-4.

  Table VI-4. -- Lifetime Excess Risk Estimates (per 1000) From
                Occupational Exposure Based on Female Mouse Lung
                                  Tumor Data


| MLE (UCL)(**)
Model |______________________________________________
| | | | 25 ppm | 50 ppm | 500 ppm


OSHA NPRM Risk | | |

Assessment (mg/kg/d, | | | BW extrapolation) | | | without PBPK | | | Adjustment...........| 2.32 (2.97)..| 4.64 (5.92).| 45.5 (57.7) PPM to PPM | | | extrapolation | | | without PBPK | | | Adjustment...........| 11.3 (14.4)..| 22.4 (28.5).| 203 (251) PBPK Reitz female | | | mouse lung -- Reitz | | | human (HSIA | | | assumptions).........| 0.43 (0.53)..| 0.93 (1.17).| 14.3 (17.9) PBPK Reitz female | | | mouse lung -- | | | Dankovic average | | | human (NIOSH | | | assumptions).........| 0.81 (1.02)..| 1.69 (2.12).| 15.0 (18.7) PBPK Clewell female | | | mouse lung -- Clewell| | | human (Navy | | | assumptions)(*)......| 0.91 (1.14)..| 1.88 (2.36).| 27.5 (34.2) OSHA Final Risk | | | Assessment (female | | | mouse lung with PBPK)| 3.62.........| 7.47........| 125.8


Footnote(*) Upper 95th percentile of the GST metabolites distribution was used as input in the multistage model.

Footnote(**) Maximum likelihood estimates and 95th percentile upper confidence limit (in parentheses) of the multistage dose-response function.

Of those risk estimates considered by OSHA prior to its final risk assessment, the risk estimates for lifetime occupational exposure to the 8-hour TWA PEL of 25 ppm ranged from 0.43 per 1000 to 11.3 per 1000. The risk assessment presented in the NPRM was based on a body weight extrapolation from mice to humans of a mg/kg/day dose of MC. Mr. Harvey Clewell [Ex. 19-59] stated that this dose was not a useful dose for estimating risks from volatile solvents such as MC. He suggested that, if PBPK modeling was not used to estimate target tissue dose (his preferred method of estimating risk), then a ppm-to-ppm extrapolation would be more appropriate. The ppm-to-ppm extrapolation resulted in an estimated risk of 11.3 deaths per 1000 after lifetime occupational exposure to 25 ppm. However, the ppm-to-ppm extrapolation is generally preferred for site-of-contact tumors. Although it is possible that the MC lung tumors were the result of a site-of-contact mechanism of action, the data are more supportive of a systemic, genotoxic mechanism mediated through metabolites of MC. In addition, the liver tumors are clearly not the result of a site-of-contact carcinogen because the liver is not a site of contact during inhalation bioassays.

Several commenters [Exs. 19-26, 19-28, 19-29, 19-45, 19-48, 19-57, 19-59, 25-E, 25-I] suggested using PBPK modeling to estimate target tissue dose and to account for differences in metabolism at high and low doses and differences in metabolism of MC across species. OSHA compared three sets of parameters in the PBPK models submitted by interested parties to adjust the dose across species and across doses. The risk estimates for those models (using the MLE of the multistage model parameters) ranged from 0.43 to 0.91 deaths per 1000 after lifetime occupational exposure to 25 ppm. Mr. Clewell's risk estimate (0.91/1000 MLE), unlike the other PBPK analyses, represent the upper 95th percentile of the Monte Carlo distribution of GST metabolites as input into the multistage model. The Monte Carlo simulation takes into account the assumed distribution of values for each parameter, including the parameters used to estimate human metabolism of MC. The other PBPK models used point estimates instead of distributions for the PBPK parameters, and therefore it is not known whether these are central estimates or upper bounds. OSHA agrees that the distributional approach used by Clewell is a reasonable way to account for the uncertainty and variability inherent in PBPK modeling, and that uncertainty and variability must be considered in any useful risk assessment. The Agency has used the upper 95th percentile on the distribution of GST metabolites from the Bayesian modeling, coupled with the MLEs of the multistage model parameters, for its final estimates of MC risk.

OSHA has concluded that all the risk estimates presented above support an 8-hour TWA PEL of 25 ppm or lower. The risks estimated from the PBPK models were less than an order of magnitude different from estimates of risk based on applied dose methodology. Either with or without PBPK modeling, the estimates of risk at 25 ppm clearly indicate a significant risk.

The risks estimated from these PBPK models and ppm-to-ppm extrapolation offer a range of risks which might be expected after lifetime occupational exposure to MC. OSHA has assessed these models and has decided to modify and expand on the submitted PBPK and uncertainty analyses in its final estimates of cancer risk, in order to give full consideration to all of the available data. This analysis is presented in the next section.

F. OSHA's PBPK Analysis and Final Risk Estimates

In developing an approach to PBPK modeling for MC, OSHA wished to use all of the available, appropriate and quantifiable biochemical and physiological data in its PBPK modeling and in assessing the uncertainty and variability in model parameters. The Agency determined that this approach would provide the best characterization of the variability and uncertainty in the data and the model. In addition, incorporation of as much of the available data as possible should give the most realistic PBPK model, and in turn, the most realistic risk estimate. Before development of OSHA's PBPK model, Clewell's approach (described above) was the most comprehensive pharmacokinetic approach submitted to the Agency. It addressed many of the issues of concern to the Agency, and OSHA believes that Clewell's approach was a reasonable template for using PBPK in risk assessment. However, since Clewell's work was done, PBPK modeling has continued to advance. Therefore OSHA modified Clewell's model to accommodate these advances and to allow incorporation of additional biochemical and physiological data that had been added to the rulemaking record. The following is a summary of OSHA's final (revised) PBPK analysis. A more detailed discussion can be found in the reports submitted to the Agency, reflecting OSHA's analysis in which the Agency was assisted by contractors [Ex. 128].

1. Review of Clewell's PBPK Analysis

a. Clewell's analytical approach. Clewell et al. [Ex. 96] employed Monte Carlo techniques to investigate imprecision in estimates of human health risk from occupational exposure to MC, as a function of imprecision in parameter values of the PBPK and dose-response models. (As described below, OSHA and its contractors believe that Clewell et al. did not correctly parse out uncertainty and variability, so their analysis is described as accounting for "imprecision" rather than uncertainty or variability). In the Clewell et al. analysis, probability distributions were specified for each PBPK model parameter in an attempt to characterize imprecision. Computer-based techniques were used to obtain pseudo-random samples from these statistical distributions, generating multiple sets of model parameter values. These sets of parameter values were then used to obtain a corresponding distribution of PBPK model predictions of various measures of internal dose for a simulated animal bioassay (e.g., GST metabolism in lungs of mice exposed to 2000 ppm and 4000 ppm for 6 hrs/day, 5 days/wk). The mean of the mouse internal dose distribution was used as the dose input to obtain the MLE and UCL on the multistage model parameters, using the tumor incidence data from the NTP bioassay. The multistage model was run a second time using the upper 95th percentile of the mouse internal dose distribution as the dose input to obtain the MLE and UCL on the multistage model parameters. This yielded a total of four estimates of the parameters (q(o), q(1), and q(2)) of the mouse dose-response function: 1) Mean of internal dose distribution/MLE of multistage model parameters; 2) Mean of internal dose distribution/UCL of multistage model parameters; 3) Upper 95th percentile of internal dose distribution/MLE of multistage model parameters; and 4) Upper 95th percentile of internal dose distribution/UCL of multistage model parameters.

Each set of dose-response parameters obtained from the analysis of the mouse data was then used to calculate human risk estimates. The upper 95th percentile of the human internal dose distribution was used to calculate the dose surrogate at 25 ppm, 8 hr/d exposure and then substituted into the MLE and UCL of the multistage parameters to obtain the MLE and UCL estimates of risk. Similarly the mean of the human internal dose distribution was used in conjunction with the MLE and UCL of the multistage model parameters. Therefore, four human risk estimates were generated, based on the distribution of human internal doses and the dose-response function derived from the multistage analysis of the NTP mouse bioassay. The four human risk estimates are: 1) upper 95th percentile of the human internal dose distribution/MLE of the multistage model parameters; 2) mean of human internal dose distribution/MLE of the multistage model parameters; 3) upper 95th percentile of the human internal dose distribution/UCL of the multistage model parameters; and 4) mean of the human internal dose distribution/UCL of the multistage model parameters.

A major finding of that analysis was that the mean estimate of added cancer risk for occupational exposure at the proposed PEL of 25 ppm based on the PBPK-derived GST-lung dose surrogate (PBPK(mean)/potency(MLE) = 0.39 x 10(-3)) was 6-fold lower than the corresponding OSHA estimate (MLE = 2.32 x 10(-3)) based on administered dose scaled to body weight. The 95 percentile upper bound estimate of risk using the same PBPK distributions and the distribution of 95% UCLs on carcinogenic potency (PBPK(95%)/potency(95%) = 1.56 x 10(-3)), was nearly 2-fold less than OSHA's 95% UCL on risk (2.97 x 10(-3)).

b. Clewell's PBPK model. The PBPK model used by Clewell et al. in performing their Monte Carlo analysis was slightly modified from the PBPK model developed by Andersen et al. and submitted to OSHA by HSIA [Ex. 328]. The primary modification was the addition of a separate compartment for the GI-tract. The general structure of this model has received considerable use by PBPK modelers. Nevertheless, there were several deficiencies in this model and in the subsequent statistical analysis that the Agency believed warranted major modification. These are described in the following section.

c. Prior distributions for model parameters.Truncated normals were used as the form for all probability distributions except for metabolic constants, which were described by truncated lognormals. All distributions were truncated to prevent sampling of nonsensical values (e.g., negative values). Truncation in some instances was 2 standard deviations (SDs) from mean values, in others more than 4 SDs.

A variety of sources of information were used as a basis for the probability distributions of the PBPK parameters in Clewell's model: literature summaries for most physiologic and anatomic parameters, direct laboratory measurement of partition coefficients based on vial equilibration studies, and statistical regression analyses of experimental data for fitted metabolic constants.

Clewell et al. stated that the focus of their analysis was on characterizing the effect of "uncertainty" in parameter values on uncertainty in PBPK model predictions, uncertainty being defined as the possible error in estimating the "true" value of a parameter for a representative "average" animal. To maintain consistency with a focus on investigating effects of parameter uncertainty, a logical choice would have been to center their probability distributions using estimates of mean values for all model parameters and to use the standard error of the mean (SEM) to characterize dispersion. It it unclear whether this was done for blood flows, tissue volumes, inhalation rates or cardiac output, since Clewell et al. appear to have relied extensively on an unpublished review of scientific literature performed by S. Lindstedt for the ILSI Risk Science Institute Physiological Parameter Working Group.

Based on Clewell's comments accompanying his PBPK model, it appears that standard errors were not used to characterize variability among individual replicates of measured equilibrium partition coefficients; instead, standard deviations were used. Nor does it appear that Clewell et al. consistently made use of standard errors in characterizing imprecision in their fitted metabolic constants. Inspection of the joint confidence region for their fitted estimates of mouse VmaxC and Km (for the MFO pathway), shown in Figure 6 of Ex. 399, suggest coefficients of variation (%CVs) for VmaxC of about 2%. Similarly, for KfC, the %CV in the fitted MLE appears to be about 3%. These %CVs are considerably smaller than the assumed values of 20% and 30%, respectively, used by Clewell et al. in their Monte Carlo analysis. On the other hand, their %CV for Km does coincide with that indicated by the joint confidence regions. One should also note the high degree of correlation among the fitted values for VmaxC and Km.

In assessing variability in the ratio of in vitro MFO and GST metabolism in lung versus liver tissue (i.e., the A1 and A2 parameters), Clewell et al. used the in vitro MC metabolism data of Reitz et al. (1989). Yet it appears that the %CV for these data is 24% when one uses SDs among replicates for MFO metabolism in lung and liver of mice. This is substantially less than the 50% assumed by Clewell. One obtains a %CV of 9% when using SEMs.

It appears then, that some of the probability distributions used by Clewell et al. reflect variability beyond that readily identifiable as uncertainty in estimates of sample means. It may be that Clewell made a subjective inflation of variances. Though ad hoc, inflating variances would not be unreasonable given the sparse data on certain model parameters. Another possibility is that the distributions reflect variability due to both uncertainty and intersubject heterogeneity -- another reason to inflate variances, or alternately, use SDs rather than SEMs to describe the distributions of the parameters. If so, then it might be more appropriate to view the proportion of simulated estimates of risk that fall within a specified interval as the probability that the true risk for a randomly selected individual is in that interval. Yet strictly speaking this would require that the probability distributions reflect both the full range of uncertainty and heterogeneity in the population of interest, with the latter being unlikely based on inspection. If the analysis only considered imprecision due to uncertainty, as suggested in Clewell et al., then the resulting distribution should instead be viewed as describing the uncertainty in risk for a hypothetical "average" individual.

2. OSHA's Modifications to PBPK Analysis

a. Basis for modifying approach of Clewell et al. In addition to the likelihood that Clewell et al. used broader distributions than those necessary to model uncertainty in the PBPK analysis (as opposed to modeling some hybrid of uncertainty and variability), the analytical approach they used (1992 and 1993) also has two well-known methodological limitations. Their representation of imprecision in fitted parameters (e.g., VmaxC, Km, KfC) is problematic because they estimated the variability in these parameters by optimizing the model fit to in vivo data, while assuming nominal values for all other model parameters. However, the organ volumes, blood flows, and partition coefficients for the mice used in the gas uptake studies and the humans used in the open chamber studies are clearly not known with exact precision, and are not, therefore, accurately represented by nominal values. Consequently, the variances of the fitted parameters will be underestimated with this approach, since full acknowledgment of variability in other model parameters will have been ignored. Furthermore, it is quite likely that the joint parameter space for fitted PBPK model parameters will exhibit a considerable degree of correlation. Importantly, failure to account for such covariances when performing Monte Carlo sampling may overstate variance in some model predictions by assuming independence where it does not exist. The implications of these methodological limitations on predicted risk are unclear, since they would seem to exert countervailing effects on estimating uncertainty. Thus, OSHA decided that it was important to perform an analysis that addressed these limitations. The use of a Bayesian statistical framework provided a means of overcoming the above limitations.

b. Bayesian Approach. A Bayesian analysis allows the logical combination of two forms of information: "prior knowledge" about parameter values drawn from the scientific literature, and data from experimental studies (e.g., the mouse gas uptake studies, or, for humans, the open chamber experiments performed by Dow Chemical company), all within the context of a PBPK model. Clearly, neither prior information about parameter values nor experimental data alone are capable of precisely determining all parameter values in the PBPK model. If prior information were sufficient, the additional experiments performed by Clewell et al. and Dow Chemical Co. would not have had to be done. But the available experimental data alone are insufficient to pin down all parameters of the model to reasonable values (which is why no attempt was made to simultaneously optimize all PBPK parameters to data). Fitting only two or three parameters while holding others constant so as to reduce dimensionality leads to the biases and underestimation of variance mentioned above.

A second feature of this Bayesian approach is that it yields distributions for all of the PBPK model parameters together with information about their entire joint covariance structure. Thus, the Bayesian analysis outputs distributions of parameter values that are consistent with both all the available data as well as the prior information. It is then possible to use samples from the joint posterior distribution of the parameters to simulate formation of GST metabolites in lung tissue from different species and cancer risk, therefore producing posterior distributions for these endpoints. It should be noted that if no data are available (or if the data are not informative as to the likely value of the parameter), the posterior distribution is equivalent to the prior distribution and this approach is then equivalent to the standard Monte Carlo sampling from the prior distribution, as in Clewell et al. Alternately, Bayesian updating with a uniform prior distribution (i.e., complete ignorance about plausible values) used in conjunction with data leads to a posterior distribution proportional to the distribution of the data. The most important applications of the Bayesian approach arise when informative (e.g., physiological, anatomical) prior distributions exist, in parallel with experimental metabolic data. This is now the case with PBPK modeling of MC. In this case, Bayesian modeling results in all the information content of both prior distributions of parameter values and metabolic data being incorporated in the posterior distribution of parameter values, which will have reduced variance compared to the prior distribution. Distributions of parameter values for both human and mouse PBPK models, and the multistage cancer model, were determined with this technique.

c. PBPK Model Modifications. OSHA's final risk estimates were based on the Bayesian analysis described here. The Clewell model formed the structural core of the analysis, although five additional structural modifications were made as described below. These modifications were necessary to make the PBPK model more physiologically realistic:

(1) Bone marrow was treated as a separate compartment. In the Clewell model (as in many PBPK models), bone marrow tissue was combined with other tissues into a (presumably) kinetically homologous compartment. Based on blood perfusion rates, a reasonable choice would be to place marrow in the well-perfused tissue compartment. However, if the physicochemical affinity of the compartment is considered, it makes more sense to place marrow in the adipose tissue compartment, since red marrow (at least in humans) has a fat content of about 40% and yellow marrow has a fat content of 80%. In comparison, liver, brain, kidney and heart all have fat contents (in humans) well under 20%. In addition, bone marrow accounts for a significant percentage of body weight and receives a substantial fraction of cardiac output. Therefore, a strong argument can be made for treating bone marrow as a separate compartment, as OSHA has done here.

(2) Partitioning MFO and GST metabolism between the lung and liver. Clewell made the MFO and GST metabolic constants for lung dependent on the fitted constants for the liver, so as to reduce the number of fitted parameters to be simultaneously estimated from rodent and human in vivo data. For example, A1 is defined as the ratio of lung to liver in vitro MFO enzymatic activity, normalized to microsomal protein,

                nmol DCM oxidized/min/mg lung microsomal protein
          A1 = __________________________________________________
               noml DCM oxidized/min/mg liver microsomal protein

Similarly, A2 is the ratio of lung to liver in vitro GST enzymatic
activity, normalized to cytosolic protein,

               nmol DCM oxidized/min/mg lung cytosolic protein
          A2 = __________________________________________________
               noml DCM oxidized/min/mg liver cytosolic protein

This assumes that lung and liver have equivalent mg protein per mg tissue contents. Yet the data of Litterst et al. (1973) argue against such an assumption. Litterst et al. measured microsomal protein and soluble protein in lung and liver tissues of mice, rats, hamsters, guinea pigs and rabbits. These data indicated ratios of mg microsomal protein content of lung versus liver tissue of less than 0.3, and a similar ratio for soluble protein of about 0.7. Thus, some adjustment of the constants A1 and A2 are required.

The equations used to compute a lung Vmax for the MFO pathway and a lung Kf for the GST pathway from a liver Vmax and Kf were thus modified to include an additional proportionality factor to account for differences in microsomal and cytosolic protein content of lung and liver tissue. Specifically,

   Vmax(lung.MFO) = Vmax(liver,MFO) x [V(lung)/V(liver)] x A1 x B1

where B1 is the ratio of [mg microsomal protein per mg of lung tissue] to the same measure for liver tissue. A geometric mean and geometric standard deviation for B1 were derived from the data of Litterst et al. (1973) to use as input in the Bayesian prior distribution for this parameter. Notably, accounting for this difference in protein content leads to a proportionality factor approximately four-fold less than that used by the Clewell et al. (i.e., A1 x B1 = 0.41 x 0.27 = 0.11).

Similarly, for Kf(lung.GST),

               Kf(lung.GST) = Kf(liver.GST) x A2 x B2

Here too, the data of Litterst et al. (1973) were used to compute a ratio of mg soluble protein per mg lung to the same measure for liver, yielding a mean value of 0.68 for B2. For a human B2, the average of the ratios computed for mice, rats, hamsters, guinea pigs, and rabbits as per Litterst et al. (1973) was used.

(3) Linkage of alveolar ventilation to cardiac output. In recognition of OSHA's interest in occupational exposures, Clewell used values of cardiac output and alveolar ventilation rates consistent with the performance of light work. However, they did not account for the altered distribution of regional blood flows known to occur in response to increases in work intensity [Exs. 7-115, 7-120, 21-81], as was done in subsequent MC PBPK work by Dankovic and Bailer [Ex. 23-18] (1994). In the latter analysis, alveolar ventilation (QP) was made dependent on cardiac output (QC) by making QP = QC x VPR, where VPR is the ventilation-perfusion ratio. VPR was treated as a random variable with an assigned prior probability distribution.

(4) Linkage of work intensity to changes in physiology. Cardiac output, ventilation perfusion ratio, and percent of cardiac output delivered to tissues were made dependent on work intensity. Using the data of Astrand (1983) [Ex. 21-81] -- and similar to what was done by Dankovic and Bailer (1994) [Ex. 23-18] -- slope factors were derived to describe change in flows per change in work intensity as measured in watts. These slope factors were then used to modify resting flows for varying levels of work intensity. This approach was taken so that the influence of variability in work load (i.e, work load was treated as a random variable) -- with concomitant adjustments to regional blood flows and ventilation rate -- on delivered dose could be modeled.

(5) Maintaining mass balance in sampling of fractional blood flows and compartment volumes. Monte Carlo sampling of fractional quantities such as the proportion of cardiac output delivered to different compartments, or the proportion of body weight represented by a given compartment, requires the imposition of some type of constraint to prevent random sampling leading to summed proportions greater than the whole (and thus causing nonsensical departures from mass balance). The following constraint was imposed: VppC = 0.82 -- delta ViC's (0.82 is a nominal value for the fraction of body weight absent bone, blood, and stomach and intestinal contents), QwpC = 1 -- delta QiC's (in the mouse model), and QppC = 1 -- delta QiC's (in the human model). The use of either QwpC or QppC as the quantity to be made dependent on the other fractional flows has biological appeal -- one expects that higher fractional blood flow to the poorly-perfused compartment (i.e., muscle and skin) should be accompanied by a lower fractional flow to the well-perfused compartment, and vice versa. The choice of QwpC versus QppC as the one to be made dependent on others appeared to be unimportant in work with the mouse model. The choice was important in work with the human model. Here it was necessary to choose QppC, because of its large variance relative to QwpC (i.e., since QppC cannot be estimated precisely, it makes sense to let our greater knowledge of the other fractional flows inform us about plausible values of QppC).

The above approach modifies the approach taken by Clewell et al. [Ex. 96]. Their approach was to randomly draw from the distributions for cardiac output and all fractional flows, use the random draws to compute the absolute flows to the individual compartments, and then to sum the individual flows to make a new cardiac output value for use in the simulation. On the other hand, OSHA's final analysis avoided arbitrarily modifying the prior distribution for cardiac output (which happens to be one of the relatively well-known parameters). Furthermore, Clewell did not make the fractional flows dependent on one another.

d. Prior Probability Distributions. A skewed, lognormal-like distribution is generally observed for biological parameters. However, most, if not all, parameters are also positive and have physiological bounds. Thus, truncated lognormal distributions of the parameter values were used in this analysis. They do not differ appreciably from normal distributions for small values of the variance.

In specifying prior distributions an attempt was made to characterize the variability of the mean parameter values for small groups of rodents and humans. This focus was adopted to make the prior distribution congruent with the data sets available for Bayesian analysis. For example, the rodent gas uptake data represent the aggregate pharmacokinetic behavior of groups of 5 mice. Prior distributions were therefore constructed to reflect the degree of variability in mean physiological and anatomical PBPK parameters for small groups of mice. A similar approach was taken in defining prior distributions for human physiologic and anatomic parameters, since the available experimental data reflected the averaged pharmacokinetic behavior of 6 subjects. In practice, this meant amassing studies reporting mean values for certain PBPK parameters (e.g., tissue weights, blood flows, cardiac output, minute ventilation), and then using these means as data for computing a geometric mean (GM) and geometric standard deviation (GSD) with which to estimate the parameter values for the truncated lognormal distributions. Sampling of all lognormal distributions was truncated at 2 GSDs, with one exception. Truncation of the blood:air partition coefficient was extended to 3 GSDs based on results from preliminary runs.

Table VI-5 presents a summary of the prior probability distributions used in the Bayesian fitting of the mouse and human data sets. The prior distributions for metabolic constants to be estimated from in vivo data were made very broad (i.e., assigned a GSD of 10) to reflect our ignorance of these values before examining the data. Similarly, the prior distributions for parameters of the multistage cancer model were broad uniform distributions, constrained to be positive, as required by the standard model.

   Table VI-5. -- Prior Distributions Used in Bayesian Analysis of
                           Mouse and Human In-Vivo Data
                |                                |    Mouse priors
                |           Parameter            |____________________
                |                                |    GM    |  GSD
Flows:          |                                |          |
    QCC         | Cardiac Output (l/hr/kg--BW).. |(a)34.8   |   1.14
    VPR         | Alveolar Ventilation Perfusion |          |
                | Rate...........................| (b)1.22  |   1.95
Tissue Blood    |                                |          |
 Flows (fraction|                                |          |
 of cardiac     |                                |          |
 output):       |                                |          |
    QgiC        | GI Tract...................... |    0.165 |   1.30
    QliC        | Liver......................... |    0.017 |   1.20
    QfatC       | Fat........................... |    0.047 |   1.60
    QppC        | Poorly Perfused Tissues....... |    0.276 |   1.25
    QwpC        | Well Perfused Tissues......... | (c)0.369 |   1.10
    QmarC       | Bone Marrow................... |    0.089 |   1.60
Tissue Volumes  |                                |          |
 (fraction of   |                                |          |
 body weight):  |                                |          |
    VgiC        | GI Tract...................... |    0.035 |   1.30
    VliC        | Liver......................... |    0.045 |   1.20
    VfatC       | Fat........................... |    0.077 |   1.40
    VppC        | Poorly Perfused Tissues....... | (c)0.556 |   1.10
    VwpC        | Well Perfused Tissues......... |    0.065 |   1.15
    VluC        | Lung.......................... |    0.008 |   1.30
    VmarC       | Bone Marrow................... |    0.033 |   1.50
Equilibrium     |                                |          |
 Partition      |                                |          |
 Coefficients:  |                                |          |
    Pblo        | Blood:Air..................... |   13.7   |   1.80
    Pgi         | GI Tract:Air.................. |   10.5   |   1.20
    Pli         | Liver:Air..................... |   22.9   |   2.00
    Pfat        | Fat:Air....................... |   98.2   |   1.40
    Ppp         | Poorly Perfused Tissues:Air... |    9.5   |   1.30
    Pwp         | Well Perfused Tissues:Air..... |   10.2   |   1.20
    Plu         | Lung:Air...................... |   10.0   |   1.30
    Pmar        | Bone Marrow:Air............... |   62.0   |   1.60
Metabolic       |                                |          |
 Parameters:    |                                |          |
    VmaxC       | Maximum metabolic velocity of  |  750     |  10.00
                |  MFO saturable pathway (mg/hr/ |          |
                |  kg -- liver).                 |          |
    KM          | Affinity of MFO saturable      |    1.35  |  10.00
                |  pathway (mg/l)............... |          |
    KFC         | First order rate constant for  |    1.5   |  10.00
                |  GST pathway (l/hr/kg-0.25).   |          |
    A1          | Ratio of lung to liver         |    0.405 |   1.67
                |  in-vitro MFO metabolic        |          |
                |  velocities (nmol/min/gm --    |          |
                |  lung -- micros.Prot)/ (nmol/  |          |
                |  min/gm -- liver --            |          |
                |  micros.Prot).                 |          |
    A2          | Ratio of lung to liver         |    0.282 |   1.67
                |  in-vitro GST metabolic        |          |
                |  velocities (nmol/min/gm --    |          |
                |  lung -- cytos.Prot)/ (nmol/   |          |
                |  min/gm -- liver --            |          |
                |  cytos.Prot).                  |          |
    B1          | Ratio of lung and liver tissue |    0.271 |    1.25
                |  content of microsomal protein.|          |
    B2          | Ratio of lung and liver tissue |    0.721 |    1.25
                |  content of cytosolic protein. |          |
    Sp -- Kf    | Allometric scaling power for   | ........ |  .......
                |  body weight scaling of KFC    |          |
                |  from mice to humans.          |          |

   Table VI-5. -- Prior Distributions Used in Bayesian Analysis of
                    Mouse and Human In-Vivo Data - Continued
                |                                |    Human priors
                |           Parameter            |____________________
                |                                |    GM    |  GSD
Flows:          |                                |          |
    QCC         | Cardiac Output (l/hr/kg--BW).. | 4.2......| 1.10
    VPR         | Alveolar Ventilation Perfusion |          |
                | Rate.......................... | 1.35.....| 1.15
Tissue Blood    |                                |          |
 Flows (fraction|                                |          |
 of cardiac     |                                |          |
 output):       |                                |          |
    QgiC        | GI Tract...................... | 0.191....| 1.25
    QliC        | Liver......................... | 0.067....| 1.20
    QfatC       | Fat........................... | 0.057....| 1.45
    QppC        | Poorly Perfused Tissues....... | 0.198(c) | 1.55
    QwpC        | Well Perfused Tissues......... | 0.443....| 1.25
    QmarC       | Bone Marrow................... | 0.044....| 1.70
Tissue Volumes  |                                |          |
 (fraction of   |                                |          |
 body weight):  |                                |          |
    VgiC        | GI Tract...................... | 0.017....| 1.10
    VliC        | Liver......................... | 0.026....| 1.10
    VfatC       | Fat........................... | 0.204....| 1.20
    VppC        | Poorly Perfused Tissues....... | 0.470(c) | 1.15
    VwpC        | Well Perfused Tissues......... | 0.044....| 1.10
    VluC        | Lung.......................... | 0.008....| 1.15
    VmarC       | Bone Marrow................... | 0.050....| 1.10
Equilibrium     |                                |          |
 Partition      |                                |          |
 Coefficients:  |                                |          |
    Pblo        | Blood:Air..................... | 8.4......| 1.30
    Pgi         | GI Tract:Air.................. | 8.1......| 1.60
    Pli         | Liver:Air..................... | 9.9......| 1.60
    Pfat        | Fat:Air....................... | 97.6.....| 1.25
    Ppp         | Poorly Perfused Tissues:Air... | 6.8......| 1.60
    Pwp         | Well Perfused Tissues:Air..... | 7.6......| 1.40
    Plu         | Lung:Air...................... | 7.6......| 1.50
    Pmar        | Bone Marrow:Air............... | 48.8.....| 1.60
Metabolic       |                                |          |
 Parameters:    |                                |          |
    VmaxC       | Maximum metabolic velocity of  | 75.......| 10.00
                |  MFO saturable pathway (mg/hr/ |          |
                |  kg -- liver).                 |          |
    KM          | Affinity of MFO saturable      | 0.6......| 10.00
                |  pathway (mg/l)............... |          |
    KFC         | First order rate constant for  | Mouse    | Mouse
                |  GST pathway (l/hr/kg-0.25).   | post.(d).| post.(d)
    A1          | Ratio of lung to liver         | 0.0045...| 4.50
                |  in-vitro MFO metabolic        |          |
                |  velocities (nmol/min/gm --    |          |
                |  lung -- micros.Prot)/ (nmol/  |          |
                |  min/gm -- liver --            |          |
                |  micros.Prot).                 |          |
    A2          | Ratio of lung to liver         | 0.122....| 3.60
                |  in-vitro GST metabolic        |          |
                |  velocities (nmol/min/gm --    |          |
                |  lung -- cytos.Prot)/ (nmol/   |          |
                |  min/gm -- liver --            |          |
                |  cytos.Prot).                  |          |
    B1          | Ratio of lung and liver tissue | 0.297....| 1.10
                |  content of microsomal protein.|          |
    B2          | Ratio of lung and liver tissue | 0.807....| 1.20
                |  content of cytosolic protein. |          |
    Sp -- Kf    | Allometric scaling power for   | -0.272(e)| 0.08(e)
                |  body weight scaling of KFC    |          |
                |  from mice to humans.          |          |
  Notes: (a) value computed for 0.025 kg mouse, 70 kg human; (b)
unitless; (c) prior distribution not used, fractional flow made
functionally dependent on others (see text); (d) human prior set
equal to mouse posterior; (e) mean and standard deviation of a
truncated normal distribution.

While it is desirable to separate variability into components reflecting pure uncertainty (e.g., measurement error) versus interindividual heterogeneity and to propagate them separately, it is necessary to build from the start an adequate statistical model. The problem here is complicated by the fact that both the rodent and human in vivo data used for estimating metabolic constants reflected either aggregated or averaged pharmacokinetic behavior. Thus the prior distributions and the statistical model used here aggregate variability due to both finite precision in measured values and heterogeneity among average values for small groups of rodents or humans; they do not, it must be emphasized, reflect heterogeneity among the individual humans in a large, representative population.

e. In Vivo Rodent and Human data. Bayesian updating of the distributions was performed using the same data sets used by Clewell et al. to obtain fitted estimates of mouse and human metabolic constants; namely, gas uptake studies with mice with or without pretreatment with a MFO inhibitor and the human open chamber inhalation studies. All mouse gas uptake studies were conducted with 5 female mice in a single chamber. Thus, measured observations of decline in chamber concentration of MC represent the aggregate pharmacokinetic behavior of groups of 5 animals.

The human in vivo data were obtained from Tables 2 and 3 in Andersen et al. (1991) [Ex. 21-94]. Briefly, these data represent exhaled breath and venous blood concentrations of MC for 6 male human volunteers exposed to MC concentrations of 100 or 350 ppm for a period of 6 hours. These data have only been reported as means and standard deviations of the six subjects, which is unfortunate. Thus, the available data reflect the average pharmacokinetic behavior of the 6 subjects. When simulating the human data reported in Andersen et al. (1991), the work load was assumed to be zero watts (rest) and the averaged body weight of the 6 subjects was assumed to be known without error (86 kg).

f. Simulating the Rodent Bioassay and Human Occupational Exposure. Distributions for GST metabolism in the lungs of mice exposed to 2000 ppm or 4000 ppm exposures, for 6 hrs/day and 5 days/week, were obtained by simulating these two exposures (the ones used in the NTP bioassay) with 5000 realizations drawn from the joint posterior distribution of the mouse PBPK parameters.

The quantity of metabolites formed during the 4th week (dynamic equilibrium reached) was divided by 7 to give an average measure per day. For use as an input dose to the multistage model, these posterior distributions were approximated by truncated lognormals.

The same set of 5000 parameter vectors was used to simulate both 2000 and 4000 ppm MC exposures. The control dose was always assumed to be 0. Thus, a 5000-by-3 matrix of doses was generated, where the three column vectors represent different realizations of a particular dose group (0, 2000 and 4000 ppm MC) and the row vectors represent different realizations of bioassay doses.

This method of using the joint posterior distributions for the two doses in the mouse bioassay implies certain assumptions about the uncertainties. Most importantly, this approach (referred to in this document as the "dependence case") assumes that the posterior distributions primarily reflect uncertainty about a single average value equally applicable to all groups of approximately 50 mice (i.e., it assumes groups of 50 mice will have the same "average" physiological, anatomical, physicochemical and metabolic attributes, and that these average values are simply known to us with uncertainty). An alternative would be to model the "independence case" by using a different random draw from the vector of PBPK parameters for one dose group than for the other. This approach assumes that the posterior distributions primarily reflect heterogeneity in the average attributes of groups of 50 rodents. Under the dependence case, estimates of metabolized dose for the two exposures would tend to move in tandem for a given simulation (i.e., when one dose is estimated to be low relative to its average, so is the other; likewise, when one is high, so is the other), and in principle would therefore exhibit less variability in dose-response shape (e.g., linear, sublinear, supralinear).

It appears that the dependence case is more reasonable than the independence case, by appealing to biological theory and by examining the results of the sensitivity analysis conducted as part of this risk assessment. The sensitivity analysis showed that predicted mouse GST metabolism at 2000 ppm was most sensitive to variation in the model parameter A2. Variability in A2 was primarily a consequence of uncertainty in using an in vitro ratio of enzymatic activity to make inferences about an in vivo ratio. Therefore, uncertainty rather than heterogeneity seems to dominate the distribution of mouse GST metabolism estimates. Besides, laboratory rodents have a carefully controlled genetic makeup, primarily so that they will differ little from each other physiologically; thus, groups of 50 rodents should have extremely similar average characteristics (the variance of the mean of a characteristic within a group of 50 rodents will be approximately 50 times smaller than the (already small) inter-individual variance). OSHA has determined that this reasoning supports use of the dependence case in this analysis. (Note that the excess risk estimates using the dependence case are only about a factor of 1.5 higher than those using the independence case).

Five human occupational exposures were simulated: constant exposure to 10, 25, 50, 100 or 500 ppm MC for 8-hrs per day and 5 days per week. Simulations were made up to 4 weeks of work, at which a dynamic equilibrium was reached, and as with mice, were performed using 5000 parameter human vectors drawn from their joint posterior distribution, augmented by allowing for additional variability in human body weight and work intensity (the latter linked to changes in cardiac, ventilation-perfusion and regional blood flow as described above).

g. Sensitivity Analysis. The influence of variability in mouse and human PBPK model parameters on variability in predicted mouse and human GST lung metabolism was assessed by computing pairwise correlation coefficients using each parameter vector (i.e., the marginal posterior distribution) and the corresponding vector of model predictions. For mice, the sensitivity to predicted GST -- lung metabolism in the simulated 2000 ppm bioassay dose group was evaluated. For humans, predicted GST -- lung metabolism for an occupational exposure to 25 ppm was considered. Pairwise correlation coefficients were computed using 5000 parameter vectors drawn from the joint posterior distribution and the associated model output vector.

Table VI-6 presents the results from the sensitivity analysis. The strongest pairwise correlation between predicted lung GST metabolism and any input parameter, for either mouse or human simulations, was A2. For the mouse simulation of a 2000 ppm exposure, B2 gave the next strongest pairwise correlation. The mouse parameters QlivC, VlivC, VmaxC, Pfat and QppC all exhibited more moderate (though not negligible) correlations. For the human occupational simulation, the parameters KfC, VmaxC, Sp__Kf, and B2 all exhibited moderate pairwise correlations with human lung GST metabolism. For both mice and human sensitivity analyses, there were a half-dozen or more parameters exhibiting weak (r between 0.1 and 0.2) correlations. It is important to note that all parameters are further correlated via their posterior joint distribution function. This explains why the sum of the regression coefficients (i.e., squares of the correlation coefficients) is greater than 1. Thus considerable care should be exercised in quantitatively estimating the ability of variability in any input parameter to explain variability in predicted GST metabolism, especially among parameters with similar pairwise correlation coefficients.

  Table VI-6. -- Correlation Coefficients for Total GST Lung
             Metabolism From Monte Carlo Analysis Using Mouse and
                         Human Posterior Distributions
         Mouse 2000 PPM       |         Human 25 PPM
            |    Correlation  |               |   Correlation
  Parameter |    coefficient  |   Parameter   |   coefficient
            |       (r)       |               |      (r)
A2..........|           0.860 | A2            |              0.850
B2..........|           0.530 | KfC           |              0.315
QliC........|           0.335 | VmaxC         |             -0.291
VliC........|          -0.248 | Sp -- Kf      |              0.232
VmaxC.......|          -0.229 | B2            |              0.221
Pfat........|          -0.203 | Pmar          |             -0.183
QppC........|          -0.202 | QfatC         |              0.180
VPR.........|           0.193 | B1            |              0.179
Pli.........|          -0.173 | VliC          |              0.161
A1..........|          -0.149 | VmarC         |              0.146
QgiC........|          -0.145 | Work          |              0.142
Pmar........|           0.144 | QwpC          |              0.141
VwpC........|          -0.121 | VfatC         |              0.136
KfC.........|           0.120 | QmarC         |              0.136
Pwp.........|          -0.106 | Km            |             -0.095
VluC........|          -0.120 | QC            |             -0.083
B1..........|          -0.093 | QliC          |             -0.083
QmarC.......|          -0.083 | A1            |             -0.071
Ppp.........|          -0.076 | QgiC          |             -0.065
VgiC........|           0.074 | Pfat          |             -0.061
Pgi.........|           0.054 | Pwp           |             -0.058
QC..........|          -0.049 | VluC          |             -0.052
BW..........|          -0.042 | Pgi           |             -0.050
Plu.........|           0.039 | VwpC          |              0.041
Km..........|          -0.035 | Pblood        |              0.039
tVmaxC......|           0.024 | dVPR/dW       |              0.039
QfatC.......|           0.020 | BW            |             -0.038
Pblood......|           0.019 | dQli/dW       |             -0.033
VfatC.......|          -0.013 | Plu           |              0.023
Vmar........|          -0.007 | Ppp           |              0.021
            |                 | dQfat/dW      |              0.016
            |                 | VgiC          |             -0.012
            |                 | Pli           |             -0.010
            |                 | dQgi/dW       |             -0.010
            |                 | dQmar/dW      |             -0.009
            |                 | VPR           |              0.006
            |                 | dQC/dW        |             -0.000
            |                 | dQwp/dW       |             -0.000

h. Posterior PBPK Parameter Distributions. Table VI-7 lists the posterior distributions for mouse PBPK parameters obtained by Bayesian updating of the prior distributions using the available gas uptake data. Comparison of the prior and posterior probability distributions reveals that the gas uptake data retain considerable influence on the distributions of many of the important PBPK model parameters. Medians of the posterior distributions for VPR, Qfat, Pblood, Pmar, Km, A1, and A2 were all appreciably different than the medians for their corresponding prior distributions. Percent CVs for nearly all posterior distributions were considerably smaller than those of their prior distributions. As expected, the marginal variances for the metabolic constants were considerably greater than what was obtained under nonlinear maximum likelihood regression analysis with all other model parameters fixed at nominal values.

   Table VI-7. Prior and Posteriot (Fitted) Distributions of the
                            Mouse Model Parameters

         |              |                  |         |
         |              | Central tendency |         |  Variability
         |              |__________________| Maximum |_______________
         |  Parameter   | Prior  |Posterior|posterior|Prior|Posterior
         |              | median | median  |         | %CV |  %CV
         |              |        |         |         |     |
Flows:   |              |        |         |         |     |
  QCC    |Cardiac Output|        |         |         |     |
         | (1/hr/kg_BW).| 34.8   |  34.4   |  37.6   |  18 |    9
  VPR    |Aveolar       |        |         |         |     |
         | Ventilation  |        |         |         |     |
         | Perfusion    |        |         |         |     |
         | Ratio........|  1.22  |   1.59  |   1.49  |  75 |   14
         |              |        |         |         |     |
Tissue   |              |        |         |         |     |
 Blood   |              |        |         |         |     |
 Flows   |              |        |         |         |     |
 (frac-  |              |        |         |         |     |
 tion    |              |        |         |         |     |
 of      |              |        |         |         |     |
 cardiac |              |        |         |         |     |
 output):|              |        |         |         |     |
  QgiC   | GI Tract.....|  0.165 |   0.140 |   0.175 |  26 |   16
  QliC   | Liver........|  0.017 |   0.020 |   0.017 |  19 |   16
  QfatC  | Fat..........|  0.047 |   0.090 |   0.098 |  43 |   19
  QppC   | Poorly       |        |         |         |     |
         |  Perfused    |        |         |         |     |
         |  Tissues.....|  0.276 |   0.290 |   0.243 |  22 |   18
  QwpC   | Well Perfused|        |         |         |     |
         |  Tissues.....|  0.369 |(a)0.360 |   0.378 |     |   (a)
  QmarC  | Bone Marrow..|  0.089 |   0.100 |   0.090 |  51 |   27
         |              |        |         |         |     |
Tissue   |              |        |         |         |     |
 Volumes |              |        |         |         |     |
 (frac-  |              |        |         |         |     |
 tion    |              |        |         |         |     |
 of      |              |        |         |         |     |
 body    |              |        |         |         |     |
 weight):|              |        |         |         |     |
  VgiC   | GI Tract.....|  0.035 |   0.040 |   0.038 |  26 |   22
  VliC   | Liver........|  0.045 |   0.050 |   0.050 |  18 |   12
  VfatC  | Fat..........|  0.077 |   0.070 |   0.055 |  35 |   24
  VppC   | Poorly       |        |         |         |     |
         |  Perfused    |        |         |         |     |
         |  Tissues.....|  0.556 |(b)0.540 |   0.569 |     |   (b)
  VwpC   | Well Perfused|        |         |         |     |
         |  Tissues.....|  0.065 |   0.070 |   0.065 |  14 |   12
  VluC   | Lung.........|  0.008 |   0.010 |   0.007 |  27 |   22
  VmarC  | Bone Marrow..|  0.033 |   0.040 |   0.037 |  42 |   29
         |              |        |         |         |     |
Equili-  |              |        |         |         |     |
 brium   |              |        |         |         |     |
 Parti-  |              |        |         |         |     |
 tion    |              |        |         |         |     |
 Coeffi- |              |        |         |         |     |
 cients: |              |        |         |         |     |
  Pblo   | Blood:Air....| 13.7   |  18.5   |  13.1   |  66 |   18
  Pgi    | GI Tract:Air.| 10.5   |  11.3   |   9.5   |  19 |   17
  Pli    | Liver:Air....| 22.9   |  28.2   |  23.9   |  79 |   32
  Pfat   | Fat:Air......| 98.2   | 100.5   | 106.7   |  35 |   21
  Ppp    | Poorly       |        |         |         |     |
         |  Perfused    |        |         |         |     |
         |  Tissues:Air.|  9.5   |  12.1   |  13.1   |  27 |   17
  Pwp    | Well Perfused|        |         |         |     |
         |  Tissues:Air.| 10.2   |  10.4   |  10.3   |  19 |   16
  Plu    | Lung:Air.....| 10.0   |  11.3   |  12.5   |  27 |   22
  Pmar   | Bone         |        |         |         |     |
         |  Marrow:Air..| 62.0   |  70.4   |  89.2   |  50 |   25
         |              |        |         |         |     |
Metabolic|              |        |         |         |     |
 Para-   |              |        |         |         |     |
 meters: |              |        |         |         |     |
  VmaxC  | Maximum      |750     | 718     | 661     |1413 |   12
         |  metabolic   |        |         |         |     |
         |  velocity of |        |         |         |     |
         |  MFO         |        |         |         |     |
         |  saturable   |        |         |         |     |
         |  pathway     |        |         |         |     |
         |  (mg/hr/kg_  |        |         |         |     |
         |  liver).     |        |         |         |     |
  tVmaxC | Maximum      |  8.4   |   7.2   |  11.3   |  58 |   50
         |  metabolic   |        |         |         |     |
         |  velocity of |        |         |         |     |
         |  MFO         |        |         |         |     |
         |  saturable   |        |         |         |     |
         |  pathway in  |        |         |         |     |
         |  t-DCE       |        |         |         |     |
         |  pretreated  |        |         |         |     |
         |  mice.       |        |         |         |     |
  Km     | Affinity of  |        |         |         |     |
         |  MFO         |        |         |         |     |
         |  saturable   |        |         |         |     |
         |  pathway     |        |         |         |     |
         |  (mg/l)......|  1.35  |   0.04  |   0.03  |1413 |   97
  KfC    | First order  |  1.5   |   1.77  |   2.47  |1413 |   24
         |  rate        |        |         |         |     |
         |  consistant  |        |         |         |     |
         |  for GST     |        |         |         |     |
         |  pathway     |        |         |         |     |
         |(l/hr/kg^0.25)|        |         |         |     |
  A1     | Ratio of lung|  0.405 |   0.28  |   0.30  |  54 |   31
         |  to liver    |        |         |         |     |
         |  in-vitro MFO|        |         |         |     |
         |  metabolic   |        |         |         |     |
         |  velocities  |        |         |         |     |
         |(nmol/min/    |        |         |         |     |
         |  gm_lung_    |        |         |         |     |
         |  micros.Prot)|        |         |         |     |
         |  /(nmol/min/ |        |         |         |     |
         |  gm_liver_   |        |         |         |     |
         |  micros.Prot)|        |         |         |     |
  A2     | Ratio of lung|  0.282 |   0.37  |   0.30  |  55 |   41
         |  to liver    |        |         |         |     |
         |  in-vitro GST|        |         |         |     |
         |  metabolic   |        |         |         |     |
         |  velocities  |        |         |         |     |
         |  (nmol/min/  |        |         |         |     |
         |  gm_lung_    |        |         |         |     |
         |  cytos.Prot) |        |         |         |     |
         |  /(nmol/min/ |        |         |         |     |
         |  gm_liver_   |        |         |         |     |
         |  cytos.Prot).|        |         |         |     |
  B1     | Ratio of lung|  0.271 |   0.26  |   0.29  |  23 |   18
         |  and liver   |        |         |         |     |
         |  tissue      |        |         |         |     |
         |  content of  |        |         |         |     |
         |  microsomal  |        |         |         |     |
         |  protein.    |        |         |         |     |
  B2     | Ratio of lung|  0.721 |   0.70  |   0.84  |  22 |   17
         |  and liver   |        |         |         |     |
         |  tissue      |        |         |         |     |
         |  content of  |        |         |         |     |
         |  cytosolic   |        |         |         |     |
         |  protein.    |        |         |         |     |
  Notes:(a) functionally defined as 1__sum (other fractional flows);
(b) functionally defined as 0.82__sum (other fractional volumes).

Table VI-8 presents the corresponding set of results for human PBPK parameters. The human in vivo data also appeared to contain considerable information about many of the model parameters, as evidenced by shifts in medians and tightening of posterior distributions relative to priors. Fitted estimates of the metabolic constants were fairly precise, even for Km (Table VI-8); indeed, the fits were markedly superior to those shown in Andersen et al. [Ex. 21-94] and Clewell et al. [Ex. 96].

   Table VI-8. -- Prior and Posterior (Fitted) Distributions of the
                              Human Model Parameters
           |                   |         Prior distribution
           |                   |______________________________________
           |    Parameter      |          |          |
           |                   |   GM     |    GSD   |     %CV
Flows:     |                   |          |          |
  QCC      |Cardiac Output     |    4.2   |    1.10  |         10
           | (1/hr/kg_BW).     |          |          |
  VPR      |Aveolar Ventilation|    1.35  |    1.15  |         15
           | Perfusion Ratio.  |          |          |
           |                   |          |          |
Tissue     |...................|..........|..........|...............
 Blood     |                   |          |          |
 Flows     |                   |          |          |
 (fraction |                   |          |          |
 of cardiac|                   |          |          |
 output):  |                   |          |          |
  QgiC     | GI Tract..........|    0.191 |    1.25  |         23
  QliC     | Liver.............|    0.067 |    1.20  |         19
  QfatC    | Fat...............|     .057 |    1.45  |         38
  QppC     | Poorly Perfused   |          |          |
           |  Tissues.         |    0.198 |    1.55  |         (a)
  QwpC     | Well Perfused     |          |          |
           |  Tissues.         |    0.443 |    1.25  |         23
  QmarC    | Bone Marrow.......|    0.044 |    1.70  |         57
           |                   |          |          |
Tissue     |...................|..........|..........|.............
 Volumes   |                   |          |          |
 (fraction |                   |          |          |
 of body   |                   |          |          |
 weight):  |                   |          |          |
  VgiC     | GI Tract..........|    0.017 |    1.10  |         10
  VliC     | Liver.............|    0.026 |    1.10  |         10
  VfatC    | Fat...............|    0.204 |    1.20  |         18
  VppC     | Poorly            |          |          |
           |  Perfused         |          |          |
           |  Tissues.         |    0.470 |    1.15  |         (b)
  VwpC     | Well Perfused     |          |          |
           |  Tissues.         |    0.044 |    1.10  |          9
  VluC     | Lung..............|    0.008 |    1.15  |         14
  VmarC    | Bone Marrow.......|    0.050 |    1.10  |         10
           |                   |          |          |
 Partition |                   |          |          |
 Coeffi-   |                   |          |          |
 cients:   |                   |          |          |
           |                   |          |          |
 PC.blood  | Blood:Air.........|    8.4   |    1.30  |         26
 PC.gi     | GI Tract:Air......|    8.1   |    1.60  |         50
 PC.li     | Liver:Air.........|    9.9   |    1.60  |         50
 PC.fat    | Fat:Air...........|   97.6   |    1.25  |         22
 PC.pp     | Poorly Perfused   |          |          |
           |  Tissues:Air......|    6.8   |    1.60  |         48
 PC.wp     | Well Perfused     |          |          |
           |  Tissues:Air.     |    7.6   |    1.40  |         35
 PC.lu     | Lung:Air..........|    7.6   |    1.50  |         43
 PC.mar    | Bone Marrow:Air...|   48.8   |    1.60  |         49
           |                   |          |          |
Metabolic  |...................|..........|..........|.............
 Parameters|                   |          |          |
  VmaxC    | Maximum MFO       |          |          |
           |  metabolic        |          |          |
           |  rate (mg/mg/hr/  |          |          |
           |  kg_liver)........|   75.0   |   10.00  |       1413
  Km       | MFO Michaelis     |          |          |
           |  Menton constant  |          |          |
           |  (mg/l)...........|    0.60  |   10.00  |       1413
  Kf       | 1st order rate    |          |          |
           |  consistant for   |          |          |
           |  GST pathway      |          |          |
           |  (l/hr).          |    0.12  |    2.07  |         81
  A1       | [V/S]_lung/       |          |          |
           |  [V/S]_MFO_liver. |    0.0045|    4.50  |        226
  A2       | [V/S]_lung/       |          |          |
           |  [V/S]_GST_liver. |    0.236 |    2.04  |         83
  B1       | [mg micr. Prot/gm |          |          |
           |  lung]/[mg micr.  |          |          |
           |  Prot/gm liver]   |    0.297 |    1.10  |         10
  B2       | [mg cyt. Prot/gm  |          |          |
           |  lung]/[mg cyt.   |          |          |
           |  Prot/gm liver].  |    0.807 |    1.20  |         18

   Table VI-8. -- Prior and Posterior (Fitted) Distributions of the
                       Human Model Parameters - Continued
           |                   |          Posterior distribution
           |                   |_____________________________________
           |                   |                |
           |     Parameter     | Posteriors for |    Modified by
           |                   |   Bayesian fit |      exercise
           |                   |________________|____________________
           |                   |        |       |         |
           |                   | Median |  %CV  | Median  |  %CV
Flows:     |                   |        |       |         |
  QCC      |Cardiac Output     |        |       |         |
           | (1/hr/kg_BW).     |  4.0   |     6 |   6.2   |    17
  VPR      |Aveolar Ventilation|        |       |         |
           | Perfusion Ratio.  |  1.03  |     1 |   1.37  |     9
           |                   |        |       |         |
Tissue     |                   |        |       |         |
 Blood     |                   |        |       |         |
 Flows     |                   |        |       |         |
 (fraction |                   |        |       |         |
 of cardiac|                   |        |       |         |
 output):  |                   |        |       |         |
  QgiC     | GI Tract..........|  0.149 |    12 |   0.122 |    14
  QliC     | Liver.............|  0.063 |    15 |   0.041 |    24
  QfatC    | Fat...............|  0.045 |    10 |   0.052 |    11
  QppC     | Poorly Perfused   |        |       |         |
           |  Tissues.         |  0.378 |  (a)9 |(a)0.453 |    10
  QwpC     | Well Perfused     |        |       |         |
           |  Tissues.         |  0.294 |     3 |   0.258 |     7
  QmarC    | Bone Marrow.......|  0.071 |    38 |   0.072 |    38
           |                   |        |       |         |
Tissue     |...................|........|.......|.........|........
 Volumes   |                   |        |       |         |
 (fraction |                   |        |       |         |
 of body   |                   |        |       |         |
 weight):  |                   |        |       |         |
  VgiC     | GI Tract..........|  0.018 |     8 |   0.018 |     8
  VliC     | Liver.............|  0.026 |     8 |   0.026 |     8
  VfatC    | Fat...............|  0.183 |    11 |   0.183 |    11
  VppC     | Poorly Perfused   |        |       |         |
           |  Tissues.         |  0.489 |  (b)5 |(b)0.489 |     5
  VwpC     | Well Perfused     |        |       |         |
           |  Tissues.         |  0.47  |     7 |   0.047 |     7
  VluC     | Lung..............|  0.008 |    11 |   0.008 |    11
  VmarC    | Bone Marrow.......|  0.049 |     8 |   0.049 |     8
           |                   |        |       |         |
 Partition |                   |        |       |         |
 Coeffi-   |                   |        |       |         |
 cients:   |                   |        |       |         |
           |                   |        |       |         |
 PC.blood  | Blood:Air.........| 16.5   |     2 |  16.5   |     2
 PC.gi     | GI Tract:Air......| 10.7   |    36 |  10.7   |    36
 PC.li     | Liver:Air.........| 13.7   |    33 |  13.7   |    33
 PC.fat    | Fat:Air...........| 84.4   |    12 |  84.4   |    12
 PC.pp     | Poorly Perfused   |        |       |         |
           |  Tissues:Air......| 13.3   |    13 |  13.3   |    13
 PC.wp     | Well Perfused     |        |       |         |
           |  Tissues:Air......| 13.1   |    14 |  13.1   |    14
 PC.lu     | Lung:Air..........|  9.4   |    33 |   9.4   |    33
 PC.mar    | Bone Marrow:Air...| 47.8   |    27 |  47.8   |    27
           |                   |        |       |         |
Metabolic  |                   |        |       |         |
  VmaxC    | Maximum MFO       |        |       |         |
           |  metabolic rate   |        |       |         |
           |  (mg/mg/hr/       |        |       |         |
           |  kg_liver).       | 97.2   |    11 |  97.2   |    11
  Km       | MFO Michaelis     |        |       |         |
           |  Menton constant  |        |       |         |
           |  (mg/l)...........|  0.52  |    39 |   0.52  |    39
  Kf       | 1st order rate for|        |       |         |
           |  consistant GST   |        |       |         |
           |  pathway (l/hr).  |  0.23  |    63 |   0.23  |    63
  A1       | [V/S]_lung/       |        |       |         |
           |  [V/S]_MFO_liver  |  0.24  |    77 |   0.24  |    77
  A2       | [V/S]_lung/       |        |       |         |
           |  [V/S]_GST_liver. |  0.364 |    49 |   0.364 |    49
  B1       | [mg micr.Prot/gm  |        |       |         |
           |  lung]/[mgmicr.   |        |       |         |
           |  Prot/gm liver]   |  0.300 |     8 |   0.300 |     8
  B2       | [mg cyt.Prot/gm   |        |       |         |
           |  lung]/[mg cyt.   |        |       |         |
           |  Prot/gm liver].  |  0.845 |    15 |   0.845 |    15
  Notes (a) operationally defined as 1 -- sum (other fractional
flows); (b) functionally defined as 0.82 -- sum (other fractional

Tables VI-9 and VI-10 compare the posterior distributions for mice and human PBPK parameters with the distributions used by Clewell. For mice, there were appreciable differences in the median values for QCC, VPR, QfatC, QwpC, VwpC, VmaxC, Km, KfC, and the apparent A1 (i.e., A1 x B1). With the exception of VliC, Pblood, Pliv, Ppp and Km, the posterior distributions for all other parameters were tighter than the distributions used by Clewell. The human posterior distributions in Table VI-10 are somewhat different than those in Table VI-8, in that they reflect the influence of modeling variable work intensity on QC, VPR, and all regional blood flows. In comparing these modified posterior distributions to the distributions used by Clewell, one finds appreciable differences in median values for VPR, many of the fractional blood flows (QgiC, QliC, QppC, QwpC), VgiC, PCblood, PCliv, PCfat, VmaxC, KfC, and the apparent A2 (i.e., A2 x B2). All human posterior distributions except for VliC, Pli, and Sp__Kf, had appreciably tighter distributions than those used by Clewell et al. [Ex. 96].

Table VI-9. -- Comparison of Mouse Probability Distributions Used by
                 Clewell et al. With OSHA's Posterior Probability
                |                           |     Central tendency
                |                           |_________________________
                |       Parameter           | Clewell et  | OSHA
                |                           | al. median  | median
Flows:          |                           |             |
    QCC         | Cardiac Output (1/hr/     |(a)41.5      |   34.4
                |  kg_BW)...............    |             |
    VPR         | Alveolar Ventilation      | (b)1.76     |    1.59
                |  Perfusion Ratio.....     |             |
Tissue Blood    |                           |             |
 Flows (fraction|                           |             |
 of cardiac     |                           |             |
 output):       |                           |             |
    QgiC        | GI Tract................. |    0.165    |    0.14
    QliC        | Liver.................... |    0.035    |    0.02
    QfatC       | Fat...................... |    0.030    |    0.09
    QppC        | Poorly Perfused Tissues.. |    0.250    |    0.29
    QwpC        | Well Perfused Tissues.... |    0.520    | (c)0.36
    QmarC       | Bone Marrow.............. |   NA        |    0.10
Tissue Volumes  |                           |             |
 (fraction of   |                           |             |
 body weight):  |                           |             |
    VgiC        | GI Tract................. |    0.031    |    0.04
    VliC        | Liver.................... |    0.046    |    0.05
    VfatC       | Fat...................... |    0.100    |    0.07
    VppC        | Poorly Perfused Tissues.. |    0.513    | (d)0.54
    VwpC        | Well Perfused Tissues.... |    0.041    |    0.07
    VluC        | Lung..................... |    0.008    |    0.01
    VmarC       | Bone Marrow.............. |   NA        |    0.04
Equilibrium     |                           |             |
 Partition      |                           |             |
 Coefficients:  |                           |             |
    Pblo        | Blood:Air................ |   23.0      |   18.5
    Pgi         | GI Tract:Air............. |   11.4      |   11.3
    Pli         | Liver:Air................ |   38.7      |   28.2
    Pfat        | Fat:Air.................. |  107.0      |  100.5
    Ppp         | Poorly Perfused           |    8.5      |   12.1
                | Tissues:Air.              |             |
    Pwp         | Well Perfused Tissues:Air |   11.4      |   10.4
    Plu         | Lung:Air................. |   10.0      |   11.3
    Pmar        | Bone Marrow:Air.......... |   NA        |   70.4
Metabolic       |                           |             |
 Parameters:    |                           |             |
    VmaxC       | Maximum metabolic         |  970        |  718
                |  velocity of MFO          |             |
                |  saturable pathway (mg/hr/|             |
                |  kg__liver).              |             |
    Km          | Affinity of MFO saturable |    1.35     |    0.04
                |  pathway (mg/l).          |             |
    KfC         | First order rate constant |    1.5      |    1.77
                |  for GST pathway (l/hr/   |             |
                |  kg__0.25).               |             |
    A1          | Ratio of lung to liver    |    0.405    |    0.28
                |  in-vitor MFO metabolic   |             |
                |  velocities (nmol/min/    |             |
                |  gm__lung__micros.Prot)/  |             |
                |  (nmol/min/               |             |
                |  gm__liver__micros.Prot). |             |
    A2          | Ratio of lung to liver    |   0.282     |   0.37
                |  in-vitro GST metabolic   |             |
                |  velocities (nmol/min/    |             |
                |  gm__lung__cytos.Prot)/   |             |
                |  (nmol/min/               |             |
                |  gm__liver__cytos.Prot).  |             |
    B1          | Ratio of lung and liver   |    1        |    0.25
                |  tissue content of        |             |
                |  microsomal protein.      |             |
    B2          | Ratio of lung and liver   |    1        |    0.69
                |  tissue content of        |             |
                |  cytosolic protein.       |             |

Table VI-9. -- Comparison of Mouse Probability Distributions Used by
                 Clewell et al. With OSHA's Posterior Probability
                           Distributions - Continued
                |                           |         Variability
                |                           |_________________________
                |       Parameter           | Clewell et  |
                |                           |   al. %CV   | OSHA %CV
Flows:          |                           |             |
    QCC         | Cardiac Output (1/hr/     |           9 |        9
                |  kg_BW)..............     |             |
    VPR         | Alveolar Ventilation      |          58 |       14
                |  Perfusion Ratio.....     |             |
Tissue Blood    |                           |             |
 Flows (fraction|                           |             |
 of cardiac     |                           |             |
 output):       |                           |             |
    QgiC        | GI Tract................. |          25 |       16
    QliC        | Liver.................... |          96 |       16
    QfatC       | Fat...................... |          60 |       19
    QppC        | Poorly Perfused Tissues.. |          40 |       18
    QwpC        | Well Perfused Tissues.... |          50 |       (c)
    QmarC       | Bone Marrow.............. |          NA |       27
Tissue Volumes  |                           |             |
 (fraction of   |                           |             |
 body weight):  |                           |             |
    VgiC        | GI Tract................. |          30 |       22
    VliC        | Liver.................... |           6 |       12
    VfatC       | Fat...................... |          30 |       24
    VppC        | Poorly Perfused Tissues.. |          30 |       (d)
    VwpC        | Well Perfused Tissues.... |          30 |       12
    VluC        | Lung..................... |          30 |       22
    VmarC       | Bone Marrow.............. |          NA |       29
Equilibrium     |                           |             |
 Partition      |                           |             |
 Coefficients:  |                           |             |
    Pblo        | Blood:Air................ |          15 |       18
    Pgi         | GI Tract:Air............. |          30 |       17
    Pli         | Liver:Air................ |          20 |       32
    Pfat        | Fat:Air.................. |          30 |       21
    Ppp         | Poorly Perfused           |          10 |       17
                | Tissues:Air.              |             |
    Pwp         | Well Perfused Tissues:Air |          20 |       16
    Plu         | Lung:Air................. |          30 |       22
    Pmar        | Bone Marrow:Air.......... |          NA |       25
Metabolic       |                           |             |
 Parameters:    |                           |             |
    VmaxC       | Maximum metabolic         |          20 |       12
                |  velocity of MFO          |             |
                |  saturable pathway (mg/hr/|             |
                |  kg__liver).              |             |
    Km          | Affinity of MFO saturable |          30 |       97
                |  pathway (mg/l).          |             |
    KfC         | First order rate constant |          30 |       24
                |  for GST pathway (1/hr/   |             |
                |  kg__0.25).               |             |
    A1          | Ratio of lung to liver    |          50 |       31
                |  in-vitor MFO metabolic   |             |
                |  velocities (nmol/min/    |             |
                |  gm__lung__micros.Prot)/  |             |
                |  (nmol/min/               |             |
                |  gm__liver__micros.Prot). |             |
    A2          | Ratio of lung to liver    |          50 |       41
                |  in-vitro GST metabolic   |             |
                |  velocities (nmol/min/    |             |
                |  gm__lung__cytos.Prot)/   |             |
                |  (nmol/min/               |             |
                |  gm__liver__cytos.Prot).  |             |
    B1          | Ratio of lung and liver   |           0 |       18
                |  tissue content of        |             |
                |  microsomal protein.      |             |
    B2          | Ratio of lung and liver   |           0 |       17
                |  tissue content of        |             |
                |  cytosolic protein.       |             |
  Notes:(a) value computed for 0.025 kg mouse; (b) unitless; (c)
functionally defined as 1 -- sum (other fractional flows); (d)
functionally defined as 0.82 -- sum(other fractional volumes); (na)
not applicable.

 Table VI-10. Comparison of Human Probability Distributions Used by
                      Clewell et al. With OSHA's Posterior
                        Probability Distributions
              |                           |      Central tendency
              |                           |___________________________
              |       Parameter           | Clewell et |    OSHA
              |                           | al. median |   median
Flows:        |                           |            |
    QCC       | Cardiac Output (1/hr/     | (a)6.2     |     (c)6.3
              |  kg_BW................... |            |
    VPR       | Alveolar Ventilation      | (b)1.95    |     (c)1.36
              |  Perfusion Ratio......... |            |
Tissue Blood  |                           |            |
 Flows        |                           |            |
 (fraction)   |                           |            |
 of cardiac   |                           |            |
 output):     |                           |            |
    QgiC      | GI Tract................. |    0.195   |     (c)0.12
    QliC      | Liver.................... |    0.070   |     (c)0.04
    QfatC     | Fat...................... |    0.050   |     (c)0.05
    QppC      | Poorly Perfused Tissues.. |    0.240   |     (c)0.46
    QwpC      | Well Perfused Tissues.... |    0.445   | (c),(d)0.26
    QmarC     | Bone Marrow.............. |   NA       |     (c)0.07
Tissue Volumes|                           |            |
 (fraction of |                           |            |
 body weight):|                           |            |
    VgiC      | GI Tract................. |    0.045   |        0.017
    VliC      | Liver.................... |    0.023   |        0.026
    VfatC     | Fat...................... |    0.160   |        0.187
    VppC      | Poorly Perfused Tissues.. |    0.480   |     (e)0.483
    VwpC      | Well Perfused Tissues.... |    0.033   |        0.047
    VluC      | Lung..................... |    0.006   |        0.008
    VmarC     | Bone Marrow.............. |   NA       |        0.050
Equilibrium   |                           |            |
 Partition    |                           |            |
 Coefficients:|                           |            |
    Pblo      | Blood:Air................ |   12.9     |       16.5
    Pgi       | GI Tract:Air............. |   12.0     |       13.5
    Pli       | Liver:Air................ |   37.4     |       13.6
    Pfat      | Fat:Air.................. |  117.0     |       81.2
    Ppp       | Poorly Perfused           |   10.0     |       13.3
              | Tissues:Air.............. |            |
    Pwp       | Well Perfused Tissues:Air |   12.0     |       13.0
    Plu       | Lung:Air................. |   10.6     |        9.1
    Pmar      | Bone Marrow:Air.......... |   NA       |       51.2
Metabolic     |                           |            |
 Parameters:  |                           |            |
    VmaxC     | Maximum metabolic         |   75.2     |       94.2
              |  velocity of MFO          |            |
              |  saturable pathway (mg/hr/|            |
              |  kg__liver).              |            |
    Km        | Affinity of MFO saturable |    0.4     |        0.49
              |  pathway (mg/l).          |            |
    KfC       | First order rate constant |    1.5     |        1.82
              |  for GST pathway (l/hr/   |            |
              |  kg__0.25).               |            |
    A1        | Ratio of lung to liver    |    0.015   |        0.03
              |  in-vitor MFO metabolic   |            |
              |  velocities (nmol/min/    |            |
              |  gm__lung__micros.Prot)/  |            |
              |  (nmol/min/               |            |
              |  gm__liver__micros.Prot). |            |
    A2        | Ratio of lung to liver    |    0.18    |        0.45
              |  in-vitro GST metabolic   |            |
              |  velocities (nmol/min/    |            |
              |  gm__lung__cytos.Prot)/   |            |
              |  (nmol/min/               |            |
              |  gm__liver__cytos.Prot).  |            |
    B1        | Ratio of lung and liver   |    1.0     |        0.31
              |  tissue content of        |            |
              |  microsomal protein.      |            |
    B2        | Ratio of lung and liver   |    1.0     |        0.84
              |  tissue content of        |            |
              |  cytosolic protein.       |            |
    Sp_Kf     | Allometric scaling        |   -0.25    |       -0.267
              |  for body weight scaling  |            |
              |  of KFC from mice to      |            |
              |  humans.                  |            |

 Table VI-10. Comparison of Human Probability Distributions Used by
                   Clewell et al. With OSHA's Posterior
                  Probability Distributions - Continued
              |                           |        Variability
              |                           |___________________________
              |       Parameter           | Clewell et |
              |                           |   al. %CV  | OSHA %CV
Flows:        |                           |            |
    QCC       | Cardiac Output (1/hr/     |          9 |      (c)17
              |  kg_BW...............     |            |
    VPR       | Alveolar Ventilation      |         18 |       (c)9
              |  Perfusion Ratio.....     |            |
Tissue Blood  |                           |            |
 Flows        |                           |            |
 (fraction    |                           |            |
 of cardiac   |                           |            |
 output):     |                           |            |
    QgiC      | GI Tract................. |         10 |      (c)13
    QliC      | Liver.................... |          5 |      (c)23
    QfatC     | Fat...................... |         30 |      (c)15
    QppC      | Poorly Perfused Tissues.. |         15 |      (c)10
    QwpC      | Well Perfused Tissues.... |         20 |   (c),(d)7
    QmarC     | Bone Marrow.............. |         NA |      (c)45
Tissue Volumes|                           |            |
 (fraction of |                           |            |
 body weight):|                           |            |
    VgiC      | GI Tract................. |         10 |          8
    VliC      | Liver.................... |          5 |          8
    VfatC     | Fat...................... |         30 |         12
    VppC      | Poorly Perfused Tissues.. |         30 |       (e)5
    VwpC      | Well Perfused Tissues.... |         10 |          7
    VluC      | Lung..................... |         10 |         12
    VmarC     | Bone Marrow.............. |         NA |          8
Equilibrium   |                           |            |
 Partition    |                           |            |
 Coefficients:|                           |            |
    Pblo      | Blood:Air................ |         15 |          2
    Pgi       | GI Tract:Air............. |         30 |         31
    Pli       | Liver:Air................ |         20 |         34
    Pfat      | Fat:Air.................. |         30 |         13
    Ppp       | Poorly Perfused           |         10 |         14
              | Tissues:Air.              |            |
    Pwp       | Well Perfused Tissues:Air |         20 |         14
    Plu       | Lung:Air................. |         30 |         32
    Pmar      | Bone Marrow:Air.......... |         NA |         35
Metabolic     |                           |            |
 Parameters:  |                           |            |
    VmaxC     | Maximum metabolic         |         30 |         12
              |  velocity of MFO          |            |
              |  saturable pathway (mg/hr/|            |
              |  kg__liver).              |            |
    Km        | Affinity of MFO saturable |         50 |         35
              |  pathway (mg/l).          |            |
    KfC       | First order rate constant |         50 |         24
              |  for GST pathway (1/hr/   |            |
              |  kg__0.25).               |            |
    A1        | Ratio of lung to liver    |         70 |         69
              |  in-vitor MFO metabolic   |            |
              |  velocities (nmol/min/    |            |
              |  gm__lung__micros.Prot)/  |            |
              |  (nmol/min/               |            |
              |  gm__liver__micros.Prot). |            |
    A2        | Ratio of lung to liver    |         70 |         71
              |  in-vitro GST metabolic   |            |
              |  velocities (nmol/min/    |            |
              |  gm__lung__cytos.Prot)/   |            |
              |  (nmol/min/               |            |
              |  gm__liver__cytos.Prot).  |            |
    B1        | Ratio of lung and liver   |          0 |          8
              |  tissue content of        |            |
              |  microsomal protein.      |            |
    B2        | Ratio of lung and liver   |          0 |         14
              |  tissue content of        |            |
              |  cytosolic protein.       |            |
    Sp_Kf     | Allometric scaling power  |          0 |         22
              |  for body weight scaling  |            |
              |  of KFC from mice to      |            |
              |  humans.                  |            |
  Notes:(a) value computed for 70 kg human; (b) unitless; (c)
posterior distributions adjusted for effects of light activity; (d)
functionally defined as 1 -- sum(other fractional flows); (e)
functionally defined as 0.82 -- sum(other fractional volumes); (NA)
not applicable.

i. Alternative analysis using the "parallelogram" approach. Andersen et al. [Ex. 21-94] estimated a human first order rate constant (Kf) for glutathione (GST) metabolism of MC in the liver by allometric scaling of a fitted estimate of an in vivo mouse rate constant (KfC(mouse)). Specifically,

                 Kf(human)=KfC(mouse) X BW(spKf)

where spKf was the allometric scaling power with value -0.25. In their Monte Carlo analysis, Clewell et al. followed the approach of Andersen et al., treating KfC(mouse) as a lognormally distributed random variable and spKf as a known constant. The Bayesian analysis summarized above also made use of the allometric scaling given by the equation above, but prior probability distributions were specified for both KfC(mouse) and spKf.

Reitz et al. (1988, 1989) [Exs. 7-225 and 21-53] proposed an alternative approach for estimating an apparent in vivo human Kf. The approach, referred to as the "parallelogram method," assumes there is a constant proportionality across species between in vitro and apparent in vivo metabolic rates when normalized for substrate concentration ([S]). For example, the equation modeling the apparent in vivo rate of GSH conjugation (dM(GST)/dt) is given by:

              ------- =Kf x [S] x Vol(liver)

The constant proportionality between apparent in vivo rates of
metabolism and in vitro rates implies

        ----------  =k(p) x [V/S](GST)=Kf x Vol(liver)

where [V/S](GST) denotes an in vitro enzymatic rate normalized to [S] and k(p) the in vivo -- in vitro proportionality constant. This approach assumes a common value of k(p) across species, such that knowledge of a [V/S](GST)-rodent and Kf(rodent) (sufficient to estimate a value for k(p) as the ratio of Kf(rodent) to [V/S](GST-rodent)) and knowledge of [V/S](GST-human) is sufficient to estimate the remaining corner of a parallelogram, namely Kf(human). Therefore, this approach assumes,

                [V/S](GST(human))   Kf(human)
                _________________ = __________
                [V/S](GST(human))   Kf(rodent)


       Kf(human) = [V/S](GST(human)) x  _________________

Reitz et al. [Ex. 21-53] obtained an estimate for Kf(human) using the parallelogram method that was very similar to the estimate obtained by Andersen et al. [Ex. 21-94] using allometric scaling. However, Reitz and coworkers estimated a mean [V/S](GST-human) based on liver samples from only four human subjects -- three of which had appreciable enzymatic activity and one with no detectable activity. More recent publications (Bogaards et al., 1993 [Ex. 127-16]; Graves et al., 1995 [Ex. 122]) and unpublished data (Green et al., 1987 [Ex. 14]) provide measured values of [V/S](GST) on another 35 human subjects. These additional data demonstrate considerable variation in rates of GST metabolism among human subjects, consistent with a known human polymorphism for GST, described earlier in this Quantitative Risk Assessment. Moreover, these data indicated that, putting aside questions of interlaboratory comparability of measurements, three of the four human samples used by Reitz et al. had GST metabolic rates among the highest reported to date. Consequently, the mean [V/S](GST-human) used by Reitz and coworkers was greater than the mean estimable from the full complement of data on human GST activity.

Since OSHA was interested in assessing the effect of accounting for the full complement of data on human GST activity on estimates of cancer risk, this additional analysis was performed, despite the Agency's reservations concerning the appropriateness of using the parallelogram approach in the MC risk assessment. Although this approach allows the use of all of the available data, the uncertainties in the ratio of in vitro to in vivo metabolic constants raise serious questions for the utility of this analysis. OSHA is presenting this analysis for purposes of comparison and notes that HSIA and Clewell used allometric adjustments in their final PBPK models.

The use of a Kf(human) derived by the parallelogram method required:

(1) modification of the human PBPK model; (2) specification of a prior probability distribution for Kf(human); (3) replication of the Bayesian analysis of the human in vivo open chamber data using the new prior for Kf(human); (4) simulation of the occupational exposure scenario using the joint posterior distributions from the new Bayesian analysis to obtain a posterior distribution for human GST lung metabolism; and (5) re-estimation of the extra cancer risk.

(1) PBPK Model Modifications. The only structural modification to the PBPK models was to replace the parameter for allometric scaling of Kf(mouse) with a direct insert of a model parameter Kf(human), having its own prior probability distribution.

(2) Prior Probability Distributions. Mouse prior probability distributions were unchanged. Prior probability distributions for human model parameters were also unchanged, with the exception of prior distributions for KfC, spKf and A2. Prior probability distributions for KfC and spKf were replaced with a prior probability distribution for Kf(human). The prior probability distribution for A2 was modified to account for additional data on human lung GST activity submitted to OSHA by HSIA [Ex. 122].

The prior probability distribution for Kf(human) was derived using the equation:

      Kf(human) =  Kf(rodent) x __________________   x err(k(p))

where err(kp) is an error term added to account for uncertainty in estimating the proportionality constant k(p), as k(mouse). Thus, to derive a prior probability distribution for Kf(human), it was necessary to derive prior distributions for Kf(rodent), [V/S](GST-rodent), [V/S](GST-human) and err(kp), which in turn were propagated using Monte Carlo techniques in accordance with the relationships specified by the equation above.

(i) Prior distribution for rodent Kf. The posterior probability distribution used in the main analysis for the apparent in vivo rodent KfC parameter was used as the basis for a prior probability distribution for Kf(rodent). The posterior distribution was well described by a truncated lognormal distribution with a mean and standard deviation of 1.8 and 0.43 l/hr/bw/(-0.25), and lower and upper truncations at 0.84 and 3.07 l/hr/bw/(-0.25), respectively. The posterior distribution was converted to units of (hour)(-1) by using Monte Carlo techniques to multiply the truncated lognormal by the scalar, (rodent body weight)(-0.25).

(ii) Prior for rodent liver GST [V/S]. A prior probability distribution for a low dose mouse [V/S](GST) was obtained as the ratio of the fitted estimates of in vitro V(max) and K(m) reported by Reitz et al. for liver glutathione conjugation of MC. The fitted estimates of V(max) and K(m) and their associated standard errors were used to set the parameters for normal distributions. Monte Carlo techniques were used to obtain the ratio of these two distributions (i.e., V(max)/K(m)), under the assumption that the joint sample space for V(max) and K(m) was correlated with a Pi = 0.9. Correlation was induced because a reanalysis of the mouse in vitro reported in Reitz et al. showed that the joint parameter space for these two fitted parameters was highly correlated.

(iii) Prior distribution for human GST [V/S]. There were four data sets reporting measured values of in vitro GST activity in liver samples from 39 human subjects. These data reflect work from different laboratories using (in some cases) different assay methods and different substrate concentrations. In order to make use of all the data to estimate central tendencies and population variability, it was necessary that all measurements be placed on a common scale.

With respect to assay methods, two of the studies [Exs. 21-53 and 122] reported measured values of [V/S](GST) based on detection of [36]Cl from labelled MC. The other two studies [Exs. 14 and 127-16] reported values of [V/S](GST) based on detection of formaldehyde, a known decomposition product from GSH conjugation with MC. In a comparison of these two methods, Green et al. [Ex. 14] reported results indicating a systematic difference in measured values of [V/S](GST), with the [36]Cl detection method appearing to give estimates approximately 1.7-fold higher than the formaldehyde detection method. In this analysis, the [36]Cl method was chosen as the common scale, since the mouse [V/S](GST) data used above were based on this method. Thus, the formaldehyde assay results were multiplied by 1.7 to put them on the [36]Cl scale.

Adjustments for both substrate concentration and nonlinear metabolism were made by converting all the reported in vitro velocity data, [V](GST), to V(max)/K(m) ratios (i.e., low dose metabolic velocity), by the equation:

               V(max)   ([V](GST) x (K(m) + [S]))/[S]
               ______ = _____________________________
                K(m)                K(m)

The above equation follows from assuming in vitro kinetics can be reasonably modeled as a single-substrate Michaelis-Menton process (i.e., [V](GST) = {V(max) x [S]}/{K(m) + [S]}). In making adjustments, assay specific substrate concentrations were used (i.e., [S], which ranged from 35 to 94 mM) along with the average estimate of an in vitro Km reported by Reitz et al. [Ex. 21-53] in analysis of data from two human subjects (44 mM). It is noteworthy that none of the human in vitro [V/S](gst) data reported in Reitz et al. were truly reflective of linear kinetics, whereas the mice data were.

After the two above adjustments were made, a lognormal distribution was fit to the transformed data yielding a GM of 0.031 l/min/mg protein, and a GSD of 2.72. This distribution models intersubject variability in in vitro metabolic activity. However, the prior probability distribution for [V/S](gst-human) should reflect variation in means of six subjects, because the in vivo human data from Dow Chemical Company reflect the averaged pharmacokinetic behavior of tissue from six subjects. Thus, dispersion in the above distribution was adjusted to give the corresponding sampling distribution for means of n = 6.

(iv) Prior distribution for error term. The in vivo and in vitro metabolic data on the MFO metabolic pathway, reported by Reitz et al. [Ex. 21-53], were used to estimate the uncertainty in assuming a constant k(p) across species. These were the only data for which both in vivo and in vitro information was available on several species and which was directly relevant to MC. To avoid artifacts due to the very imprecise fitted estimates of apparent in vivo Km's, in vivo/in vitro comparisons were constructed based on estimates of Vmax alone. These estimates were then normalized by the ratio obtained for mice, providing a measure of the error in using a mouse ratio to estimate ratios in three other species: rats (1.42), hamsters (0.64), and humans (0.41). The GM (0.72) and GSD (1.89) of these three values were used to set parameters for a lognormal distribution used as the prior probability distribution for err(kp). Note that the human value of 0.41 reflected an average of separate estimates on four human subjects, with ratios ranging from 0.1 to 1.0.

(v) Monte Carlo simulation to obtain a prior for human Kf. The above prior probability distributions for Kf(mouse), [V/S](gst-mouse), [V/S](GST) and err(kp) were independently sampled by Monte Carlo techniques (n = 5000) and combined to give a prior distribution for Kf(human) for use in Bayesian analysis of the human open chamber data.

(vi) Revised prior distribution for A2. A2 is the ratio of in vitro GST enzymatic activity in lung tissue to the same activity in liver tissue. In the main analysis, the prior probability distribution for A2 was derived according to the equation:

             A2 =  _________________  x err(vivo/vitro)

where err(vivo/vitro) is an error term to account for uncertainty in using a ratio of in vitro activity to make inferences about in vivo activity, and the data of Reitz et al. [Ex. 21-53] were used to estimate prior distributions for [V/S](GST-lung) and [V/S](GST-liver). This prior distribution was revised to account for additional human [V/S](GST-lung) and [V/S](GST-liver) data.

(vii) Prior for human lung GST [V/S]. Previously, only a single measured value for [V](GST-lung) from a pooled lung sample from two human subjects was available for estimating A2. Mainwaring et al. [Ex. 124] recently submitted additional [V](GST-lung) data to OSHA, consisting of measured values on three additional human subjects (0.00, 0.06 and 0.21 nmol/min/mg protein). The value reported as 0.00 was assumed equal to one-half the detection limit for the assay. Since these new [V](GST-lung) data were obtained using the formaldehyde detection assay, it was necessary to transform the values to the [36]Cl scale. Lacking direct information, it was assumed that the same HCOOH ---> [36]Cl correction factor derived for the liver data held for the lung data. A correction for substrate concentration was also made, under the assumption of equivalency in lung and liver in vitro Km's. The resulting transformed [V](GST-lung) data were used to construct a prior probability distribution describing uncertainty in the mean of five(1) observations (GM = 0.00082, GSD = 1.61). Note that, in this case, an attempt was made to model pure uncertainty in a low dose [V/S](GST-lung), without information indicating appreciable heterogeneity in the ratio of lung and liver enzymatic activity within an individual.


Footnote(1) Since the single observation of [V](GST-lung) reported by Reitz et al. (1988) was from a pooled sample of lung tissue from two human subjects, the data point was treated as two observations with the same value.

(viii) Prior probability distribution for uncertainty in human liver GST [V/S]. Because of the focus on uncertainty in A2, the prior probability distribution for [V/S](GST-liver) derived above was modified to describe uncertainty about the mean, given a sample size of 39 subjects.

(ix) Uncertainty in using an in vitro ratio of lung and liver GST activity to make an inference about the corresponding ratio for apparent in vivo GST activity. A prior probability distribution for err(vivo/vitro) was derived using data on in vivo and in vitro ratios of liver MFO enzymatic activity for different species, as a surrogate for intra-species lung versus liver GST enzymatic activity. Thus, two key assumptions are made: (i) That relative enzymatic activity for liver tissue from two species is a reasonable surrogate for relative activities of lung versus liver tissue within a single species, and (ii) that the degree of consistency in ratios of in vivo versus in vitro enzymatic activity will be the same for either MFO or GST mediated processes.

If the apparent in vivo Vmax for the MFO pathway in the lung was modeled as:

                                     [V/S](MFO(lung))    Vol(lung)
V(max(MFO(lung)))=V(max(MFO(liver)))x_________________ x __________
                                     [V/S](MFO(liver))   Vol(liver)

    it follows that,

              V(max)A(MFO(lung))     [V/S](MFO(lung))
              __________________  x  ________________
              V(max)A(MFO(liver))    [V/S](MFO(liver))

where VmaxA denotes normalization of Vmax to unit tissue volume. Although there were insufficient data to allow for a direct evaluation of the above equation, the data tabulated by Reitz et al. [Ex. 7-225] for MFO enzymatic activity in mice, rats and hamsters did allow an evaluation of the equality,

          V(max)A(MFO(liver(sp1)))     [V/S}(MFO(liver(sp1)))
          ________________________  =  ______________________
          V(max)A(MFO(liver(sp2)))     [V/S](MFO9liver9sp2)))

where the subscripts sp1 and sp2 denote species 1 and 2 (e.g., mouse and rat). Using the apparent in vivo Vmax and in vitro [V/S] data reported in Reitz et al. [Exs. 7-225 and 21-53], it was possible to compute mouse:rat, hamster:mouse and rat:hamster ratios for in vivo Vmax and in vitro [V/S] as shown in table VI-11, below.

      Table VI-11. -- Interspecies Comparison of MFO Activity
                                 |     Ratios of MFO enzymatic
                                 |             activity
       Species ratio             |         |    in  |
                                 | in vivo |  vitro |   Fold-
                                 |   Vmax  |  [V/S] | Difference(*)
Rat: mouse.......................|   0.49  |   0.36 |    1.36
Mouse: hamster...................|   1.20  |   0.79 |    1.53
Hamster: rat.....................|   0.59  |   0.28 |    2.06
  Footnote(*) Ratio of values in in vivo Vmax column to values in in
vitro [V/S] column.

The assumption was made that the use of an in vitro ratio as a surrogate for an in vivo ratio is unbiased (i.e., err(vivo/vitro) should be centered on a value of 1). The mean of the three estimates of fold-difference (1.65) is our best estimate of a GSD for err(vivo/vitro). Thus, the prior probability distribution for err(vivo/vitro) was modeled as a lognormal variate with expected value 1.0 and GSD of 1.65.

(x) Monte Carlo simulation to obtain a prior probability distribution for A2. The above prior probability distributions for [V/S]GST-lung, [V/S]GST-liver and err(vivo/vitro) were independently sampled by Monte Carlo techniques (n = 5000) and combined to give a prior probability distribution for A2 for use in Bayesian analysis with the human open chamber data. The resulting distribution was well described as a lognormal variate with a GM of 0.236 and a GSD of 2.0.

(3) Human in vivo data and simulating occupational exposure. Bayesian updating was performed with the same human in vivo data used in the main analysis. These data consisted of time serial measurements of exhaled breath and venous blood concentrations of MC for 6 human volunteers exposed to 100 and 350 ppm MC for 6 hours. Unfortunately, the data have only been reported as averages of the 6 subject-specific observations at each time point. When simulating the human data, subjects were assumed to be at rest (i.e., work load set equal to 0), and the reported average body weight for the six subjects (86 kg) was assumed to be known without error.

A single human occupational exposure was simulated: constant exposure to 25 ppm MC for 8-hours per day and 5 days per week.

(4) Distribution of human metabolized dose and sensitivity analysis. The distribution for GST metabolism in the human lung resulting from simulated occupational exposure to 25 ppm MC had a median and mean of 0.139 and 0.192 mg/day/liter lung, about 3-fold less than values obtained using the allometrically scaled Kf.

From the sensitivity analysis, Kf and A2' exhibited the strongest pairwise correlations with predicted lung GST metabolism, with all other parameters having considerably smaller correlation coefficients. Indeed, other than PC.mar (partition coefficient air:marrow), all other parameters were only weakly correlated with GST lung metabolism. These results differ somewhat from those obtained when using an allometrically scaled Kf, and reflect the effect of greater variability in a Kf based on the parallelogram method.

(5) Posterior distributions in the "parallelogram method" analysis. The posterior distributions for many model parameters were considerably tighter than their corresponding prior distributions, most notably for fractional blood flow and partition coefficient parameters. Similar results were obtained in the main analysis. In general, medians and %CVs of the posterior distributions were similar to those in the main analysis, with the exception of Kf, which was expected, given its revised prior distribution. However, differences among the posterior distributions for Kf were less than expected due to an appreciable shift toward larger values (and some tightening) in the posterior distribution for the parallelogram-based Kf relative to its prior distribution. Thus, it would appear that the data had some information about plausible values of Kf.

The results of the covariance analysis indicated that the covariance structure was fairly similar to the results from the main analysis, with moderate to high pairwise correlations among 15 pairs of parameters.

G. Results of OSHA's PBPK Risk Assessments; Discussion

Summary statistics for OSHA's main analysis modifying the other analysis and the alternative (parallelogram) analysis are reported in Table VI-12. From the main analysis, the MLE of excess cancer risk obtained using the upper 95th percentile of the human internal dose distribution was 3.62/1000, for an occupational lifetime exposure to 25 ppm MC. The MLE of cancer risk obtained using the mean of the human internal dose distribution was 1.24/1000. The alternative (parallelogram) analysis yielded slightly lower estimates of risk. In that analysis, the MLE of cancer risk using the upper 95th percentile of the human internal dose distribution was 1.23/1000. The MLE of cancer risk for the alternative analysis using the mean of the human internal dose distribution was 0.40/1000. After evaluating the methodologies and uncertainties in the two analyses, OSHA determined that the main analysis was most appropriate for the Agency's final risk assessment and the MLE of cancer risk using the upper 95th percentile of the human internal dose distribution was best supported as OSHA's final MC risk estimate. Therefore, OSHA's final risk estimate for occupational lifetime exposure to MC at 25 ppm is 3.62/1000.

  Table VI-12. -- Summary Statistics on Estimates of Extra Cancer
                Risk From Occupational Exposure to 25 ppm MC for 8
                         hrs/day, 5 days/wk for 45 years
                   |          Summary statistics for distributions
                   |                     of extra risk
 Computational     |_________________________________________________
   approach        |  95%(**) |  Mean  | %CV(*) | Skewness | Kurtosis
Maximum likelihood |          |        |        |          |
 fitting:          | 3.62(***)| 1.24   |  103   |   2.2    |   10.2
 Dependence case...| per 1000 |per 1000|        |          |
                   |          |        |        |          |
Maximum likelihood | 2.43     | 0.79   |  113   |   2.3    |   11.3
 fitting:          | per 1000 |per 1000|        |          |
 Independence case.|          |        |        |          |
  Footnote(*) %CV denotes coefficient of variation ([standard
deviation/mean] x 100).
  Footnote(**) 95% denotes the 95th percentile value of the
distribution of GST matabolites for extra cancer risk.
  Footnote(***) OSHA's final risk estimate.

Figure VI-1 shows the end result of the main PBPK analysis: the cumulative distribution function of excess lifetime cancer risk (log(10) scale) from exposure to 25 ppm MC, 8 hours per day, 5 days per week for 45 years, when estimated using the MLE of the dose-response parameters, GST lung metabolism as the dose surrogate, and a human Kf based on allometric scaling and Bayesian prior information. As described in the main analysis, the "dependence case" was used. Several summary statistics can be discerned from this cumulative distribution function: (1) the 95th percentile of this hybrid distribution of uncertainty and heterogeneity gives a risk estimate of 3.62 x 10(-3) (point "A" in the figure); (2) the mean value of the distribution (point "B" in the figure) gives a risk estimate of 1.24 x 10(-3).

(For Figure VI-1, Click Here)

Figure VI-1 Dependence case; Estimated cumulative distribution of human cancer risk linked to a 45 year occupational exposure to methylene chloride, at 25 ppm in the air, 8 hours/day, 5 days per year. Generated using the results described in Figure 3.

OSHA conducted the alternative analysis in order to determine the impact of basing the human GST metabolite distribution on allometry (human GST metabolic rates estimated based on the relative size of animals and humans) versus the parallelogram approach (human GST metabolic rates based on ratio of various rodent in vitro: in vivo metabolic rates applied to human in vitro rates) on risk estimates. As discussed in greater detail above, allometry predicts that one would expect that humans have approximately seven-fold less GST activity than mice. The parallelogram approach, on the other hand, predicts approximately 18-fold less GST activity in humans than in mice. After analyzing the available data, OSHA has determined that the allometric assumptions are best supported by the scientific literature, primarily because of the lack of human in vivo GST data and the lack of validation of the parallelogram approach. The Agency has therefore used that approach in its final (main) estimate of risk, but has also presented an alternative analysis using the parallelogram methodology.

During the rulemaking, studies were submitted to the Agency by HSIA challenging the relevance of the mouse data for estimating human cancer risks. However, as described in detail previously, if one examines the HSIA data critically, it is clear that the studies most likely could not detect differences in metabolic activity (and hence in risk) between mice and humans of the magnitude predicted by allometry. For example, the lack of detection of an increase in DNA ss breaks in human cells compared to mouse cells could be explained because the methodology used could not detect an increase in ss breaks 7-fold smaller than that observed in mice. Clearly, an 18-fold difference, as predicted by the parallelogram method, would be even harder to detect.

Moreover, if the human in vitro data are examined more closely, it becomes apparent that the in vitro: in vivo ratios calculated for the 35 individual humans who have been studied were as low as 4.6 (the median value in this series was 24). Therefore, the use of allometry (ratio = 7) or the parallelogram approach (ratio = 18) would lead to risk estimates that clearly underestimate the risks for some individuals. In addition, RNA adduct data [Ex. 126-25] indicate that exposure of human cells to MC results in only a 3-fold lower amount of RNA adducts than formed in mouse cells. This ratio may not be a close surrogate for the GST ratio, but it does heighten concern that both PBPK approaches may be underestimating cancer risks from occupational exposure to MC, because humans may be appreciably less sensitive than mice.

The distribution of risk presented in either the main or the alternative analysis most closely reflects uncertainty about risk for some randomly chosen worker (with respect to work intensity and body weight), chosen among the population of workers with physiologic, anatomic, and metabolic attributes similar to those of the average subject from the Dow human study group. The Dow pharmacokinetic data did not contain individual data on the 6 subjects, so the results obtained and the predictions made are conditioned by the use of averages. This means that the model is truly only applicable to people who physiologically and biochemically resemble the Dow group of six subjects. Although six subjects do not represent a large data base from which to draw a representative PBPK sample, this is much more human data than is usually available to base a risk assessment on. In fact, in OSHA's preliminary quantitative risk assessment, point estimates were used for body weight, breathing rates, etc. to represent the entire working population with a single "average" number. Therefore, this sample, although small, represents a significant improvement over the point estimates of human parameter values for PBPK modeling. Although these are the best data available, the small number of individuals upon which the human parameter values are based increases concern that the Agency may be underestimating risks for a significant portion of the working population by relying upon these values and using PBPK modeling to estimate human internal doses. OSHA considered making an ad hoc inflation of the variance of the distributions of human GST enzyme kinetics parameters in order to account for some of this unmeasured heterogeneity (as recommended by the NAS Committee report discussed above), but decided not to make this "conservative" choice but instead to rely on the unadjusted analyses.

OSHA has chosen for its final risk estimate to couple one measure of central tendency (the MLE of the dose-response parameters) with a somewhat "conservative" measure (the 95th percentile of the distribution of human GST metabolites (internal dose)). Congress and the courts have permitted -- indeed, encouraged -- OSHA to consider "conservative" responses to both uncertainty and human variability. The OSH Act addresses the latter when it refers, for example, to OSHA's responsibility to set standards such that "no employee shall suffer material impairment of health * * *;" a standard that only considered risk to the average employee clearly would not be responsive to the statute. Similarly, the 1980 "Benzene decision" affirmed that "the Agency is free to use conservative assumptions in interpreting the data with respect to carcinogens, risking error on the side of over-protection rather than under-protection."

In past rulemakings, OSHA has frequently estimated carcinogenic potency via the MLE of the multistage model parameters. The Agency has recently received comments, particularly in a public meeting in February 1996 on risk assessment issues surrounding the first phase of its "PEL Update" process, critical of the MLE on the grounds that this estimator can be highly unstable with respect to small fluctuations in the observed bioassay response rates. Although OSHA may in the future move to a different estimator, such as the mean value of the likelihood function of the multistage model parameters, such a change would have neglible practical impact in the case of MC. The observed data in the NTP mouse bioassay follow a nearly precisely linear trend, so the MLE, mean and UCL estimates are all very nearly equivalent to each other.

However, OSHA needs to take particular care not to underestimate risk when it departs from a relatively simple methodology (in this case, the assumption that administered dose is the most relevant measure of exposure) in favor of a relatively more complex and computationally-intensive methodology (in this case, that the human lung GST metabolite, calculated via a PBPK model, is the most relevant measure of exposure). This is even more important in this particular PBPK analysis, because the variance of the output distributions represents an unknown hybrid of uncertainty in the various parameters and true heterogeneity among the humans exposed to MC. As Clewell stated with respect to his own PBPK analysis (see discussion above), the 95th percentile estimator provides a modicum of assurance that the risk to the average human -- and hence the population risk -- is not underestimated.

Moreover, it is critical to use an estimator other than the central tendency here so that it will not be inevitable that the risk to a human of above-average susceptibility (due to enzyme kinetics that produce relatively more reactive metabolite per unit of administered dose, or due to other attributes related to body weight, organ volumes, partition coefficients, etc.) is not underestimated, potentially by a substantial amount. Any "conservatism" introduced by using the 95th percentile of the PBPK output distribution is further attenuated by the unmeasured model uncertainty inherent in this more complex model structure. Several aspects of the model itself are known to be oversimplifications (e.g., assuming the lung is the only tissue at risk); therefore, the resulting risk distributions could be biased downward.

Finally, it is important to note that there is no risk of "cascading conservatism" with this 95th percentile estimator; the individual model parameters are permitted to vary over their entire ranges, and the selected percentile is only applied to the distribution resulting from the combined influence of all parameters. Furthermore, the newest refinements to the model ensure that the 95th percentile is not affected by any probability assigned to impossible combinations of parameters. The attention paid to issues of mass balance, covariance structure and truncation ensures that this percentile represents a fully plausible set of input parameters. In sum, the combination of the MLE of the multistage parameters and the 95th percentile of the PBPK output distribution represents a reasonable attempt to account for uncertainty and variability without unduly exacerbating the magnitude or the probability of underestimation of errors.

H. Comparison of Animal-Based Risk Estimates With "Non-Positive" Epidemiology Data

Direct comparisons between animal bioassays and human epidemiological studies are difficult to make because experimental protocols between animal and human studies differ substantially. Animals are generally exposed to a fixed dose of a chemical, for several hours per day, from approximately 6-8 weeks of age until study termination, which is usually at 2 years. This would be chronologically equivalent to a human exposure that starts when a human is approximately 4-5 years old and continuing until the human is approximately 74 years old (assuming a 74 year average life-span for humans) [Ex. 89]. This clearly differs from the typical pattern of occupational exposure encountered in epidemiological studies of worker populations. For example, in the Kodak cohort, the workers were never exposed to a constant level of MC; exposure to MC for these workers did not start until their adult life; and most of them were exposed to the chemical for less than one third of their life-span.

Exposure to MC has been found to induce lung and liver cancer in mice and mammary tumors in rats. As discussed above, there are positive epidemiology studies which suggest an association between MC exposure and cancer risk. Because exposure data are inadequate or unavailable, it is not possible to quantify the risks in these studies. OSHA acknowledges that there are also non-positive epidemiology studies.

In 1986, Crump analyzed the preliminary results from the 1964-70 Kodak cohort followed through 1984 and compared them to the rodent bioassay results. The results from the Kodak epidemiological study have also been used by Tollefson et al. [Ex. 7-249], Hearne [Ex. 91-D], and NIOSH to compare the predictions of excess cancer risk from the animal risk assessment models. In addition, Hearne used data from the cellulose triacetate fiber study in Cumberland, Maryland, and a different analytical approach, to validate the excess cancer risk predicted by the animal data [Ex. 91-D]. The details of these analyses can be found in the cited exhibits. OSHA has analyzed the different approaches to assessing the mouse bioassay in light of the epidemiology data and has determined that the approach taken by NIOSH (summarized below) represents the most comprehensive and clearest way to examine those data. OSHA also agrees with the conclusions reached by NIOSH, that the epidemiology results and the mouse bioassay data are not inconsistent with each other.

NIOSH compared the confidence intervals for the standardized mortality ratios (SMRs) from the Kodak study with the predicted confidence intervals derived from OSHA's risk assessment models from the NPRM [Ex. 89]. To estimate predicted SMRs using the multistage model, NIOSH used the following approach:

1. The expected excess number of deaths in each of the exposure groups was derived by multiplying the number of workers in each exposure group by the excess risk as determined by the multistage model (after correcting for dose equivalence between animals and humans, and differences in length of follow-up).

2. This number of expected deaths, derived from the animal data, was then added to the expected (denoted E(p)) number of deaths which were derived from the Kodak study, after correcting for the HWE, (this can be viewed as the background risk) to estimate the number of "observed" deaths that would have been predicted by the multistage model assuming it was valid for humans (denoted O(p)).

3. O(p) was then divided by E(p) to calculate predicted SMRs and 95% confidence intervals, where calculated.

NIOSH's results indicated that the non-positive findings from the Kodak study were not inconsistent with the predicted risk estimates in OSHA's risk assessment. The predicted confidence intervals from the animal multistage model were completely nested within the observed confidence intervals from the Kodak study. This is not to suggest that results from this non-positive epidemiology study are equivalent to the positive results from the animal inhalation study. Rather, based on these findings, one can conclude that the non-positive results from the Kodak epidemiologic study were not of sufficient power to contradict risk predictions of the multistage model developed from the animal bioassay data (when appropriate adjustments for differences in study protocol were taken into account).

Basically, the Kodak study examined approximately 1000 workers whose average MC exposure was 26 ppm. Therefore, the animal-based potency estimates would predict only about 3 excess cancer deaths in that cohort (the risk at 26 ppm is approximately 3 per 1000), even if they were followed for many decades after exposure ceased. This small predicted excess is clearly too small an increment to be observable with statistical confidence, considering the much larger background of cancer present in the human population. The differences between the NIOSH and Hearne analyses essentially represent different ways to estimate the "signal-to-noise" ratio for the Kodak study; OSHA believes that any reasonable method of estimating this ratio would conclude that the Kodak study has insufficient power to rule out a "signal" of significant human risk.

NIOSH's approach for adjusting for the healthy worker effect (HWE) was criticized in the comments to the record submitted by Hearne. Hearne stated that the HWE is unlikely to be present in long term cancer studies and therefore an adjustment for the HWE is not necessary [Ex. 91-D]. Hearne argued that since the HWE diminishes with time, the healthy worker effect would have been minimal in the 1946-70 Kodak cohort because the median follow-up period was 32 years and that only 20% of the cohort members were still actively employed [Tr. 10/15/92].

There is evidence in the literature showing that the HWE can be weaker for some types of cancer than for other causes of death; however, in this case NIOSH believed and OSHA agreed that the difference between control and exposed populations reflected an HWE for cancer. In addition, results from a similar analysis done by NIOSH without the HWE adjustment did not contradict the results including the HWE adjustment. NIOSH testified [Tr. 985-6, 9/21/92] that there would be a difference in the results obtained when adjusting for HWE and the unadjusted results. However, the conclusions reached would not be different. In other words, the analysis still supported the conclusion that the epidemiologic and mouse bioassay results were not inconsistent with each other. OSHA supports NIOSH's position on the use of an adjustment factor for HWE in this cohort. Other criticisms of NIOSH's approach can be found in the hearing transcripts and post-hearing comments. OSHA has evaluated these methodological criticisms and has determined that NIOSH used the best available methodology in analyzing this issue and that their conclusions are supported by those arrived at independently by Crump and by Tollefson et al.

Specifically, NIOSH predicted 23.25 deaths from cancers (at all sites) in the full cohort, after adjusting for the HWE. This value is closer to the observed number (22) than is the unadjusted expected number of deaths (29.61). Looking at lung cancer deaths separately, NIOSH predicted 22.36 deaths for the entire cohort (adjusted for HWE) compared with 22 observed and 28.67 expected by Hearne. Hearne observed no deaths from liver cancer in the entire cohort (1.14 deaths were expected). NIOSH predicted 0.88 deaths from liver cancer when they adjusted for the HWE.

OSHA believes that NIOSH's approach in comparing results from an animal bioassay to those of an epidemiological study is the most reasonable comparison between data sets because it is more accurate and better addresses computational and experimental issues inherent in the data sets. The Agency has evaluated the extent to which the cancer risk calculated using the human data is consistent with the cancer risk calculated using animal data. Based on its review of those studies, OSHA concluded that the human epidemiology results are not inconsistent with the animal bioassays and has determined that the bioassays are the appropriate basis for its quantitative risk assessment.

I. Conclusions

OSHA has determined that MC is a potential occupational carcinogen and has conducted a quantitative risk assessment in order to estimate human risks of cancer after occupational exposure to MC. The Agency reviewed all of the human and animal data on MC and determined that MC is carcinogenic in mice and in rats, causing tumors at multiple sites, in both species, and in both sexes of animals. Some epidemiologic data also indicate an association between MC exposure and excess cancer in exposed workers (statistically significant increases in biliary cancers in textile workers and astrocytic brain cancer in workers exposed to MC in solvent applications). Mechanistic data indicate that MC is likely to be metabolized to a genotoxic carcinogen. MC has been clearly shown to be metabolized by similar enzymatic pathways in rodents and humans, indicating that the metabolic processes which produce cancer in mice and rats are also present in humans. Finally, no data have been presented which demonstrate that the mouse is an inappropriate model for humans because of a physiological or biochemical component or process. Therefore, the Agency has determined that it is appropriate to assess the carcinogenic risks of MC using the NTP mouse bioassay dose-response.

The NTP mouse MC bioassays demonstrated a clear dose-tumor response relationship. OSHA determined that the NTP female mouse lung tumor response was the best data set on which to base a quantitative analysis because there was a clear dose-response, low background tumor incidence and it represented the most sensitive tumor site/sex combination.

After examining the PBPK models submitted to the Agency, OSHA concluded that PBPK modeling estimates of the amount of GST metabolites produced are reasonable dose surrogates for MC and are supported by substantial scientific evidence in the record. For that reason, OSHA has used PBPK modeling in its final risk assessment. OSHA reviewed methodologies used in PBPK models submitted to the Agency and decided to modify and expand an existing model. Specifically, a Bayesian analysis was conducted as described above. Use of the Bayesian model analysis was a logical next step in development and use of pharmacokinetic models for MC. It has great advantages in accounting for the covariance of the PBPK parameters and incorporating distributions of physiological parameters obtained from the scientific literature. OSHA's final estimates of risk use the PBPK analysis described above and are based on the MLE of the dose-response parameters using the upper 95th percentile of the human internal dose distribution. For an occupational lifetime exposure to 25 ppm MC, OSHA estimates an excess risk of 3.6 MC-induced cancer deaths per 1000 workers.

[62 FR 1494, January 10, 1997]

Regulations (Preambles to Final Rules) - Table of Contents

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