Regulations (Preambles to Final Rules) - Table of Contents|
| Record Type:||Occupational Exposure to Methylene Chloride|
| Title:||Section 5 - V. Health Effects|
V. Health Effects
The toxicology of MC is summarized below. A more detailed review of MC toxicology can be found in the NPRM [56 FR 57036].
B. Absorption and Disposition of Methylene Chloride
Inhalation is the most significant route of entry for MC in occupational settings. The quantity of MC taken into the body depends on the concentration of MC in inspired air, the breathing rate, the duration of exposure to MC, and the solubility of MC in blood and tissues. Because MC is volatile, inhalation exposures to MC can be quite high, especially in poorly ventilated spaces.
Dermal absorption of MC is a slow process relative to inhalation. In the NPRM, OSHA described the rate of skin absorption of pure MC as insignificant relative to inhalation. In contrast, Mr. Harvey Clewell, in comments prepared for the U.S. Navy [Ex. 19-59], stated that substantial occupational exposure could occur through the dermal route when the employee is exposed to high concentrations of MC vapor and protective clothing is not worn [Ex. 19-59]. Mr. Clewell provided a physiologically-based pharmacokinetic (PBPK) model to describe the potential absorption through skin exposed to high vapor concentrations of MC. Where the employee is protected from inhalation exposure by use of an air-supplied respirator and the skin (exposed surface area = two hands) is unprotected in high MC-vapor concentrations, the primary route of exposure in this case will be dermal exposure. Mr. Clewell has determined that sufficient MC may be absorbed by the dermal route over an 8-hour shift to give an internal concentration which would exceed that experienced by workers exposed to MC through inhalation of 25 ppm for 8 hours.
In the NPRM, OSHA also indicated that the burning sensation associated with dermal exposure to liquid MC would likely lead employers and employees to limit skin absorption. However, exposure to high concentrations of vapor may not be associated with a burning sensation, and there is evidence in the record [Tr. 2468-70, 10/15/92] to suggest that employees are exposed to liquid MC without protective clothing. OSHA believes that dermal exposure to liquid and high vapor concentrations of MC should be limited to the extent feasible to protect the employee from overexposure. For this reason, in this standard OSHA has required that employers provide personal protective clothing and equipment appropriate to the hazard. For example, if an employee will be at risk of hand contact with liquid MC, impermeable gloves must be provided.
C. Metabolism of MC
Once MC is absorbed into the body, it is widely distributed in the body fluids and in various tissues. The uptake and elimination of MC has been well described in human and animal studies [Exs. 7-156, 7-157, 7-174].
The carcinogenic mechanism of action for MC has not been clearly established. Although it has not been proven whether MC is carcinogenic through a genotoxic or non-genotoxic mechanism, current evidence supports the hypothesis that MC is a genotoxic carcinogen. Genotoxic carcinogens typically are reactive compounds or metabolized to reactive compounds. MC is unreactive in the body until it is metabolized. Therefore, many investigators believe that one or more of the metabolites of MC, and not MC itself, is the ultimate carcinogen.
It has been established by Kubic and Anders [Ex. 7-167] and Ahmed and Anders [Ex. 7-25] that MC is metabolized by rat liver enzymes in vitro by two distinct pathways. The first pathway is the mixed function oxidase system (MFO pathway) associated with the microsomal cell fraction and the second is the glutathione dependent pathway localized primarily in the cytoplasm and mediated by glutathione-S-transferase (GST pathway). The metabolism of MC is illustrated in Figure 1.
(For Figure V-1, Click Here)
Figure V-1. Proposed metabolic pathways for methylene chloride metabolism. (Adapted from Andersen et al.) (1987) [Ex. 7-125]
The MFO pathway metabolizes MC via a cytochrome-P450 dependent oxidative dehalogenation [Ex. 7-167] which produces formyl chloride. The formyl chloride decomposes to give chloride ion and carbon monoxide. It has been postulated that if the MFO pathway contributes to the carcinogenicity of MC, it is through the production of the reactive compound, formyl chloride. The end product of the MFO pathway, carbon monoxide, can be detected in the blood and breath of humans and animals exposed to MC, and has been used as a surrogate measure of MC exposure in humans.
The GST pathway metabolizes MC to formaldehyde and chloride ions via a postulated S-chloromethylglutathione conjugate [Ex. 7-25]. Formaldehyde is further metabolized to carbon dioxide in mammalian systems. Potential reactive metabolites in this pathway are the S-chloromethylglutathione conjugate and formaldehyde (known to react with protein, RNA and DNA).
Animal data indicate that the MFO pathway is saturated at ambient concentrations less than 500 ppm, while the GST pathway remains linear throughout the exposure levels examined [Exs. 7-161, 7-171]. Saturation of the MFO pathway in humans has been estimated to occur at a level which is within the range of the animal data (estimates range from 200 to 1000 ppm MC) [Exs. 7-114, 7-115, 8-32]. The GST pathway is not thought to be saturated for any of the species investigated at doses up to 4000 ppm.
The evidence for the carcinogenicity of MC has been derived from mutagenicity studies, animal bioassays and human epidemiological studies. OSHA analyzed data from each of these sources in determining that MC is carcinogenic to test animals and a potential occupational carcinogen. The evidence that OSHA evaluated in making this determination is summarized below. Additional evidence pertaining to the hazard identification of MC is discussed in the Quantitative Risk Assessment, Section VI, below.
1. Mutagenicity Studies
Mutagenicity and genotoxicity studies are useful in describing the possible carcinogenic mechanism of action of MC. Evidence for the interaction of MC or MC metabolites with DNA (producing mutations or toxicity) is consistent with a genotoxic mechanism for the carcinogenic action of MC, rather than a non-genotoxic action (i.e., by acting as a promoter, increasing cell turnover). The EPA reviewed the literature on the mutagenic potential of MC in their "Health Assessment Document for Dichloromethane (Methylene Chloride)" (HAD) [Ex. 4-5] and studies conducted by ECETOC in the "Technical Analysis of New Methods and Data Regarding Dichloromethane Hazard Assessments" [Ex. 7-129].
As described in the MC Notice of Proposed Rulemaking (56 FR 57036), the documentation of positive responses in the production of mutations in bacteria, yeast and Drosophila, chromosomal aberrations in CHO cells and sister chromatid exchanges (SCE) in CHO and V79 cells and equivocal responses in other systems indicated the potential genotoxicity of MC.
A paper submitted to the record by Dr. Trevor Green [Ex. L-107], for the Halogenated Solvents Industry Alliance (HSIA), investigated the role of metabolites of the GST pathway in the bacterial mutagenicity of MC. The authors of this study found that in glutathione-deficient strains of Salmonella typhimurium there was approximately a two-fold decrease in mutations. Mutation rates returned to normal when bacteria were supplemented with exogenous glutathione. They also investigated whether individual metabolites in the GST pathway were likely to be responsible for mutagenesis. Experiments in S. typhimurium strains were consistent with the S-chloromethylglutathione conjugate as the mutagenic moiety. Experiments in Escherichia coli strains implicated formaldehyde as the active mutagen. Overall, these results support the hypothesis that MC may act as a genotoxic carcinogen, but the ultimate reactive species still remains to be identified.
Dillon et al. [Ex. 21-89] also conducted experiments on the mechanism of MC mutagenicity in bacterial cells, using wild type and glutathione-deficient Salmonella typhimurium TA100. Dose-related increases in mutagenicity were observed with and without metabolic (cytosolic or microsomal) activation. The authors characterized the mutagenicity as marginally highest in the presence of cytosol at the highest MC concentrations. The glutathione-deficient strain was slightly less responsive to MC-induced mutation than the wild type. In contrast to the study by Green, Dillon et al. found that MC mutagenicity was not appreciably enhance by the addition of microsomal or cytosolic liver fractions or exogenous glutathione. They concluded that it was not clear to what extent, if any, glutathione was involved in MC mutagenicity, and noted that "* * * the residual glutathione present in the glutathione-deficient strain may have been sufficient to facilitate the mutagenic responses observed."
The differing results in these studies suggest that the exact mechanism of MC mutagenicity, even in bacterial cells, has not been determined with certainty. However, OSHA has concluded that the evidence that MC is genotoxic is compelling. Additional studies supporting classification of MC as a genotoxin were submitted to the Agency in late 1995 and are discussed in the Quantitative Risk Assessment, Section VI, below.
2. Animal Studies
The evidence for the carcinogenicity of MC has been derived primarily from data obtained in chronic toxicity studies in rodents. Table V-1 contains a summary of the major bioassays. These bioassays have been conducted in three rodent species (rat, mouse and hamster) using two routes of administration (oral and inhalation) and a wide range of doses (from 5 mg/kg/d, oral to 4000 ppm inhaled for 6 hr/d, 5 d/wk).
The National Toxicology Program conducted two 2-year inhalation bioassays [Ex. 7-8] using B6C3F1 mice and Fischer 344 rats. In the NTP mouse study [Ex. 7-8], groups of 50 male and 50 female B6C3F1 mice were exposed to 0, 2000 or 4000 ppm MC, 6 hr/day, 5 d/wk for 102 weeks. All animals were necropsied and examined histopathologically.
Treated male and female mice had increased incidences of alveolar or bronchiolar adenomas and carcinomas as compared with control animals. In addition, there was an increased number of lung tumors per tumor-bearing animal (multiplicity of tumors) with increasing dose of MC.
In the liver, the toxic effects of MC were expressed as cytologic degeneration in male and female mice which was not present in the controls. An increased incidence of hepatocellular adenomas and carcinomas (combined) was observed in male mice. The incidence of hepatocellular carcinomas in male mice was statistically significantly increased at 4000 ppm. Female mice also experienced dose-related increases in the incidences of hepatocellular adenomas and carcinomas. An increased multiplicity of liver tumors was also found in both male and female mice.
Table V-1. -- Methylene Chloride Lifetime Bioassays _____________________________________________________________________ | | Route and | Dosage | Reference | Species/ | dosing | (No. of | Comments | Strain | schedule | animals) | ____________|__________|____________|_____________|__________________ NTP (1985)..| B6C3F(1) | Inhalation | 0, 2000, | Lung and liver | mouse.. | 6 hr/day, | 4000 ppm | tumors both sexes, | |5 days/week.| (50 mice/ | both doses. | | | sex/dose). | | | | | Serota (NCA)| B6C3F(1) | Daily in | 0 (125M, | (1986)....| mouse.. | water... | 100F), 60 | No tumors | | | (200M, 100F)| observed. | | | 125 (100M, | | | | 50F), 185 | | | | (100M, 50F),| | | | and 250 | | | | (125M, 50F) | | | | mg/kg/d. | | | | | NTP (1985)..| Fischer | Inhalation |0, 1000, 2000| Mammary and | 344 rat. | 6 hr/day, | and 4000 ppm| integumentary | |5 days/week.| (50 rats/ | fibromas and | | | sex/dose). | fibrosarcomas in | | | | both sexes. | | | | Burek (DOW) | Sprague- | Inhalation |0, 500, 1500 | Malignant salivary (1980)....| Dawley | 6 hr/day, | and 3500 ppm| gland tumors at | rat. |5 days/week.|(95 rats/sex | 3500 ppm, | | | dose). | dose-related | | | | increase in mammary | | | | tumors. | | | | Nitschke | Sprague | Inhalation |0, 50, 200 | No tumors observed. (DOW) (1982)| Dawley | 6 hr/day, | and 500 ppm | | rat. |5 days/week.|(70 rats/sex | | | | dose). | | | | | Serota (NCA)| Fischer | Daily in |0, 5, 50, 125| No tumors observed. (1986).....| 344 rat. | water....|and 250 mg/ | | | |kg/d (135/sex| | | |at 0, 85/sex/| | | | dose). | | | | | Burek (DOW) | Syrian | Inhalation |0, 500, 1500,| No tumors observed. (1980).....| Golden | 6 hr day, |3500 ppm (90| | hamster. |5 days/week.|hamsters/sex | | | | dose). | ____________|__________|____________|_____________|__________________
The dose-related increase in the incidence of lung and liver tumors in mice, and the increased multiplicity of these tumors, present the strongest evidence for the carcinogenicity of MC. NTP concluded that, based on the evidence from these lung and liver tumors, there was clear evidence of the carcinogenicity of MC in both male and female mice.
In a second two-year bioassay, the NTP examined the effects of inhalation of MC at 0, 1000, 2000 and 4000 ppm in F344 rats [Ex. 7-8]. Body weights of all exposure groups were comparable. The highest dose female rats experienced reduced survival after 100 weeks of exposure.
The incidence of mammary tumors in the high dose group in both sexes was statistically significantly higher than in control animals (concurrent and historical). The incidence of mammary fibroadenomas alone and the combined incidence of fibroadenomas and adenomas in male and female rats occurred with statistically significant positive trends. When subcutaneous fibromas or sarcomas in the male rat, which were believed to have originated in the mammary chain, were included in comparisons, differences between control and exposed animals were even greater.
MC-exposed male and female rats also showed increased incidence of liver effects, characterized by hemosiderosis, hepatocytomegaly, cytoplasmic vacuolization and necrosis. Neoplastic nodules alone and combined incidence of neoplastic nodules and hepatocellular carcinomas in female rats occurred with significant positive trends by the life table test. Pair-wise comparisons did not indicate statistically significant effects at any one dose. Although this is suggestive of a carcinogenic response in the female rat liver, NTP did not use this response in their determination of the carcinogenicity of MC.
NTP based its determination of the carcinogenicity of MC in the rat on the mammary tumor incidence data. NTP has concluded that the increased incidences of mammary gland tumors in the female rats provided clear evidence of carcinogenicity and, in the male rats, some evidence of carcinogenicity.
The Dow Chemical Company [Ex. 7-151] conducted experiments in which Sprague-Dawley rats and Syrian Golden hamsters were exposed to 0, 50, 1500 or 3500 ppm MC, 6 hr/d, 5 d/wk for 2 years. A dose-related statistically-significant increase in the number of mammary tumors per tumor-bearing female rat was observed. These results support the NTP findings of increased mammary tumors in F344 rats. The background mammary tumor response in the Sprague-Dawley rat is higher than in F344 rats, so a quantitative analysis of risk is easier to perform on the data from the NTP study.
A statistically significant increase in male rat salivary tumors was also observed in this study, although the authors believed that this response should be discounted because of the presence of sialodacryoadenitis virus in the rats. OSHA believes that the presence of this virus in the rats would complicate the interpretation of the data, and so has relied on the NTP studies for its quantitative risk assessments.
No statistically significant excess incidence of tumors was observed in either sex of hamsters at any exposure level. This suggests that hamsters are less sensitive to the carcinogenic effects of MC than either mice or rats. Metabolism data gathered in hamsters indicate that hamsters have less capability to metabolize MC by the GST pathway than rats or hamsters (or humans). This correlation between lack of GST metabolism capacity and lack of tumor response supports the hypothesis that GST metabolism is important in MC carcinogenesis and also indicates that it would not be protective to use the hamster response to MC as the basis for a carcinogenic risk assessment.
A second inhalation study in Sprague-Dawley rats conducted by investigators at Dow Chemical [Ex. 7-173], with exposures up to 500 ppm, showed an increase in the number of mammary tumors per tumor-bearing animal in female rats at the highest dose level only. This study extended the finding of excess mammary tumors in rats to the 500 ppm level. However, because of the high background rates of mammary tumors in Sprague-Dawley rats, the NTP study showed a clearer dose-response relationship between MC exposure and incidence of mammary tumors.
In a study conducted for the National Coffee Association [Ex. 7-180], no statistically significant increased incidence of tumors was observed in B6C3F1 mice or F344 rats exposed to up to 250 mg/kg/d MC in drinking water. These studies used the drinking water route of exposure instead of inhalation and exposed animals to lower doses (on an mg/kg/d basis) than the NTP and high-dose Dow studies. These factors most likely accounted for the lack of a positive tumor response. The NCA studies were used by Reitz et al. in the development of the physiologically-based pharmacokinetic models for MC. Specifically, these studies helped to determine that the lack of tumor development was consistent with model predictions of the amount of GST metabolites in lung and liver of mice and that the MFO pathway was most likely not primarily responsible for the mouse tumor response.
The Agency believes that the NTP studies show the clearest evidence of a carcinogenic effect of MC and has used these studies as the basis of its risk assessment for the following reasons: (1) The studies were well conducted and underwent extensive peer review. (2) The inhalation route of exposure was used, which is the most appropriate route for extrapolation to occupational exposures. (3) Dose-related, statistically significant increases in tumor incidence were observed in both sexes in mice and in female rats. OSHA believes that because of the clear tumor response, and quality of the studies, the NTP studies provide the best data for quantitative cancer risk assessment. OSHA concludes from these studies that MC causes cancer in two species of test animals by the inhalation route, and that a clear dose-response has been demonstrated.
3. Epidemiological Studies
Epidemiological studies of occupational exposure to MC have been conducted in the manufacturing of triacetate fibers, photographic film production, and the manufacturing of paint and varnish. Those studies were reviewed by OSHA in the preamble to the proposed rule [56 FR 57075] and are summarized and updated in this document. In addition, an epidemiological study of MC exposure and astrocytic brain cancer is reviewed in this text.
a. Studies of triacetate fiber production workers. Ott et al. [Ex. 7-76] performed a retrospective cohort study using a cellulose diacetate and triacetate plant in Rock Hill, South Carolina to examine the effects of MC on a working population. In particular, Ott et al. evaluated the effects that were possibly mediated through the metabolism of MC to carboxyhemoglobin. Employees at this plant had MC exposures close to OSHA's time weighted average (TWA) permissible exposure limit (PEL) of 500 ppm. Ott et al. used workers in a plant in Narrows, Virginia as a comparison population because it had operations similar to those at the Rock Hill plant, but did not use MC. In this study, Ott et al. compared the number of deaths within the exposed cohort with the United States population and the Narrows, Virginia referent group. Ott et al. observed that the overall mortality of the cohort was comparable to that of the age, sex, and race-matched U.S. population. Comparing exposed and referent cohorts, statistical differences in risk were observed in white men for "all causes" (risk ratio=2.2, p< 0.01), "diseases of the circulatory system" (risk ratio=2.2, p< 0.5), and "ischemic heart disease" (risk ratio=3.1, p< 0.05).
In interpreting the results of this study, Ott noted that there may have been differences in hiring practices in the two plants which could have contributed to the observed differences in mortality. In their conclusion, Ott et al. stated that a healthy worker effect (HWE) and the low power of their study did not permit them to dismiss the possibility of increased health risks within the working population exposed to MC.
Dr. Mirer of UAW testified [Tr. 1896-6, 9/24/92] that there is some evidence that there is excess work-related heart disease mortality in epidemiological studies that have observed SMRs greater than 80% for ischemic heart disease or any other cardiovascular disease. Furthermore, when the MC epidemiological studies are looked at together, there is evidence, although limited, that MC exposure has an effect on cardiovascular mortality.
On the other hand, Kodak [Ex. 91D] questioned the appropriateness of the referent population in the Rock Hill study, alleging that the SMR for ischemic heart disease in the referent population was unusually low, and that this fact, rather than an effect of MC exposure, caused the observed differences in ischemic heart disease rates.
In contrast, NIOSH considered the Rock Hill study to be suggestive of an effect of MC on risk of cardiac disease. According to NIOSH [Tr. 879, 9/21/92] the Ott study did not use appropriate analytic techniques that would allow the acute effects of MC on cardiac disease risk to be examined. Furthermore, NIOSH suggested [Tr. 969, 9/21/92] that future epidemiological studies should examine risks from MC exposure during the period when employees are actively working.
In an update to the Rock Hill study, Lanes et al. followed the Ott et al. cohort through September 1986 [Ex. 7-260] and December 1990 [Ex. 106]. Lanes et al. used the population of York County, South Carolina as the comparison group. Statistically significant excess mortality was observed for cancer of the liver and biliary passages (SMR=5.75, CI:1.82-13.78) in the study group. Excess mortality was also observed for buccal cavity and pharynx cancer (SMR=2.31, 95% CI:0.39-7.60) and melanoma (SMR=2.28, CI:0.38-7.51), although mortality from these causes did not reach statistical significance. No excess mortality was observed for ischemic heart disease (SMR=0.90, CI:0.62-1.27).
Examination of the liver and biliary cancers indicated that the workers had ten or more years of employment and at least 20 years since first employment (4 observed v. 0.35 expected). Three of the four employees who died from liver/biliary cancer had tumor sites in the intrahepatic and common bile duct, common bile duct, and ampulla of Vater. Approximate durations of employment for these three cases were 28 years, 20 years, and less than one year. No medical record for the third case could be obtained. However, an autopsy report indicated adenocarcinoma of the liver for this case. To estimate the expected number of biliary cancer deaths, Lanes et al. used Surveillance, Epidemiology, and End Results (SEER) mortality rates of the continental United States. The computed risk estimate, based on 0.15 cases expected, was SMR=20 (95% CI:5.2-56.0).
The authors hypothesized that the biliary duct cancer cases may have been due to factors such as oral contraceptive use, gallstones, or ulcerative colitis. However, it appeared that medical records showed no indication of gallstones or ulcerative colitis in workers who died of biliary cancer. Moreover, although these factors were not specifically controlled for, there is no reason to believe the rates of these factors would be different in the exposed cohort compared to the general U.S. population.
Lanes et al. updated their study through December 31, 1990 [Ex. 106] using the National Death Index and focused on mortality from pancreatic cancer, biliary and liver cancer, and ischemic heart disease. Lanes et al. ascertained fifty more death certificates from the end of the last follow-up period on September 1, 1986. As before, York County, South Carolina was used as the comparison population.
The overall SMR from all causes of death was 0.90, and for malignant neoplasms, the SMR was 0.82. In this follow-up, the SMR for liver and biliary cancer dropped from 5.75 to 2.98 (95% CI:0.81-7.63). No additional deaths from biliary or liver cancer were observed. In the original and updated studies combined, four deaths from biliary/liver cancer were observed and 0.64 were expected. Using a Poisson distribution, Lanes et al. calculated the probability of failing to observe any liver/biliary cancer deaths in this update if the "true" value of the SMR for liver/biliary cancer was 5.75 (from the previous study) and then expecting 3.68 deaths in this follow-up (0.64 x 5.75). They estimated the probability that this update would have no observed biliary/liver cancer deaths if the true SMR were 5.75, as e(-3.68)=0.025. On the other hand, if MC had no effect on liver and biliary cancer mortality, Lanes et al. estimated that the probability of observing zero deaths would have been 0.527 (e(-0.64)). Lanes et al. used the likelihood ratio (0.527/0.025=21.08) to compare these two hypotheses. The authors concluded that the null hypothesis that the SMR=1.0 was 21 times more probable than the hypothesis that the SMR=5.75.
Because of the small number of cases involved and the instability of the numbers generated in this type of statistical analysis, OSHA believes that this study, overall, is suggestive (but not definitive) of an association between occupational exposure to MC and elevation of human cancer risk. Furthermore, the Agency has determined that the study results are not inconsistent with the results of the NTP cancer bioassay.
Hoechst-Celanese [Ex. 19-65, pp. 6-8; Ex. 19-19] was concerned that OSHA considered the incidence of biliary cancer as evidence of a positive effect. They argued that the reported excess in biliary tract cancer did not support the conclusion that MC exposure is associated with an increased risk of cancer. Specifically, they noted that,
(1) Biliary cancers have not been reported in any of the animal cancer studies of MC; (2) no statistically significant increase in biliary cancers was seen in the Cumberland study (described below); (3) no statistically significant excess in biliary cancers was reported in the Kodak studies (described below); (4) It was unlikely that MC could have been responsible for the biliary tract cancer observed in one employee who had been exposed to MC for less than one year; and (5) the Rock Hill study did not control for other chemical exposures.
Comments by the Halogenated Solvents Industry Alliance (HSIA) [Ex. 19-45, p. 47] were in accord with those of Hoechst-Celanese.
Dr. Shy, on behalf of Kodak, asserted [Tr. 1303, 9/22/92; Ex. 91F] that MC exposure failed to meet Bradford Hill's criteria for causality (e.g., biological plausibility, dose-response, and consistency) for producing biliary tract cancer. Dr. Shy acknowledged that animal bioassays have demonstrated liver tumors from MC exposure, but he noted that there is no evidence in humans that liver and biliary tract cancers have the same etiology. Furthermore, Dr. Shy argued that,
(1) the results from the Lanes study is not supported by in vitro or pharmacokinetic studies.
(2) a dose-response relationship could not be determined from the Lanes study because there were no direct measurements of worker exposure to MC.
(3) the observed association between MC exposure and liver/biliary cancer was an isolated finding and the existence of a causal relationship could not be concluded.
(4) the excess biliary tract cancer in the Lanes study was not consistent with the other three epidemiological studies (Hearne, 1987, 1990, 1992; Hearne, 1992; Gibbs, 1992).
Dr. Shy did recognize that there was a strong association between MC exposure and biliary tract cancer in the Lanes study (SMR=20). Moreover, the 20 year time interval between first exposure and death from biliary tract cancer provided evidence that "exposure preceded cancer with an appropriate interval for induction of the tumor [Ex. 91F]."
OSHA disagrees with the conclusions reached by Dr. Shy. The Agency believes that the risks of biliary cancer observed in these studies is consistent with risks derived from its pharmacokinetic analysis (see the Quantitative Risk Assessment, Section VI). Since the occupational exposures in these studies are likely to have been among the highest in any of the epidemiologic cohorts, there is no evidence that the increased biliary/liver cancer result is inconsistent with other reported epidemiological findings. Regarding the biological plausibility, the Agency notes that human biliary cells appear to contain high concentrations of the mRNA for GST (the enzyme many investigators believe to be responsible for MC-induced carcinogenesis) [Exs. 124 and 124A]. Although this requires more investigation to determine if there is a direct relationship, OSHA believes there is a plausible mechanistic argument for MC causality in human biliary tract cancers. The Agency agrees with Dr. Shy, however, that the lack of dose-response data and the small number of cases in this cohort limit the strength of conclusions that can be drawn from this study. After weighing these considerations, the Agency has determined that there is suggestive evidence of a causal role for MC in these cases of biliary cancer.
Gibbs et al. conducted a study of another cellulose acetate and triacetate fibers plant in Cumberland, Maryland [Ex. 54] to evaluate the possible relationship between MC exposure and biliary/liver cancer. This plant, which ceased to operate in 1982, had operations similar to the plant in Rock Hill, and it was assumed to have had similar MC exposure levels as well. However, exposure measurements were not submitted for the Cumberland plant and it is unknown whether the Cumberland employees experienced the same exposures as their Rock Hill counterparts.
The Gibbs study investigated the mortality of 3,211 workers who were employed at this plant on or after January 1970. There were 2,187 men and 1,024 women in the cohort. Most of the workers in the cohort were hired prior to 1979 (2,566 total). The study population was divided into three subcohorts based on their estimated exposure to MC: 1) 834 men and 146 women in the "high exposure" group (estimated to be 350-700 ppm), 2) 1095 men and 832 women in the "low but never high exposure" group (estimated to be 50-100 ppm), and 3) 256 men and 46 women in the "no exposure" group. This cohort was followed through December 1989. The observed mortality was compared to expected death rates for Allegany County, Maryland (where the plant was located and where most of the cohort deaths occurred), the State of Maryland, and the United States.
The author of this study believed that the county rates were the most appropriate to use because the city of Cumberland is located in a rural area of Maryland and the state rates may have been influenced by rates in large urban areas such as Baltimore. In addition, local rates tend to adjust for social, economic, ethnic, and cultural factors which may be related to disease risk, access to medical care, etc. However, if the fiber plant was the major employer in this rural area, then county rates may reflect the cohort's mortality rather than the background risk, in which case, state rates or U.S. population rates would be more appropriate. The overall mortality rate for the high MC-exposed group was below the expected rates for Allegany County, Maryland, and the U.S. population.
As in the Rock Hill study, mortality from biliary tract cancer was observed in the Cumberland study, although no statistically significant elevated incidence of biliary cancer was found (two cases of biliary tract cancer were observed). In the high exposure group, there was one death (1.24 expected with Allegany rates (SMR=80.5) and 1.42 expected with Maryland rates (SMR=70.4)). In the low MC-exposed group, there was also one death from biliary/liver cancer. For the high MC-exposed subcohort, Gibbs et al. estimated SMRs of 80.4, 70.3, and 75.1 when comparisons were made with Allegany County, Maryland, and U.S. rates, respectively. In the low MC-exposed subcohort, the SMRs using Allegany and Maryland rates were 75.4 and 76.4, respectively. This cohort should be followed for a longer period of time to help clarify the suggested association between MC exposure and biliary cancer observed in the Rock Hill cohort.
Statistically significant excess mortality was also observed from prostate, uterine, and cervical cancers, although these also represented small numbers of cases: 13, 2, and 1, respectively.
The excess of prostate cancer in the Gibbs et al. study suggested an exposure-response relationship (3 deaths in no MC-exposure group, 9 in low MC-exposure group, and 13 in high MC-exposure group). According to Gibbs et al. and Shy [Tr. 1303, 9/22/92; Exs. 19-64, 91F], this response may have been related to other chemical exposures (occupational or non-occupational). In support of this hypothesis, no other epidemiological or animal studies of MC exposure have suggested a relationship between prostate cancer and MC. Hoechst-Celanese [Ex. 19-65, pp. 10-12; Ex. 91D, p. 12] cautioned OSHA not to overinterpret the excess of prostate cancer in the Cumberland study for the following reasons:
(1) of all the epidemiological studies, only the Cumberland study has shown an excess of prostate cancer; (2) of the thirteen high subcohort men who died of prostate cancer, twelve worked in the extrusion area of the Cumberland plant before methylene chloride was used as a solvent in cellulose triacetate fiber production. Thus, these men may have had longer exposure to other chemicals; (3) the study did not control for other personal risk factors; (4) Gibbs reported an increased incidence of prostate cancer elsewhere in the textile industry; and (5) the large number of statistical tests may have increased the probability of finding the death rate of a specific cause to be elevated or depressed.
OSHA believes that the increased risk of prostate cancer should be noted as a possible positive effect of MC exposure on cancer risk, particularly considering the exposure-response relationship. However, because of potential confounding factors and lack of corroborating findings in other studies, OSHA believes this is suggestive rather than conclusive evidence of a human carcinogenic effect.
b. Studies of film production workers. In their original study of film production workers, Friedlander et al. [Ex. 4-27] conducted both a proportionate mortality study and a retrospective mortality cohort study to determine if workers exposed to MC experienced an increased risk for specific causes of mortality. The cohort in these studies consisted of workers who worked in any department in film production that used MC as its primary solvent for approximately thirty years. The cohort was followed through 1976.
Proportionate mortality analysis for those workers ever employed in the study area versus a comparison group of workers in other Kodak Park departments produced a proportionate mortality ratio (PMR) of 143.88 for liver (intrahepatic ducts-primary) cancer. For ischemic heart disease, Friedlander et al. calculated a PMR of 94.74. No statistically significant differences were observed at p < /= 0.05.
For the cohort mortality study, Friedlander et al. used rates from the 1964-70 hourly males age group exposed to MC in the film department and the other Kodak Park departments for internal comparison. Mortality rates for New York State, excluding New York City, males age group were used for external comparisons.
Forty-five deaths from circulatory diseases were observed in the MC-exposed cohort versus 38.5 expected in the Kodak Park referent group. Also, 6 deaths from respiratory diseases were reported in the MC-exposed group versus 3.2 expected for the Kodak Park comparison group. No liver deaths were observed in this cohort. Thirty-three deaths from ischemic heart disease were observed in this cohort compared with 28.7 expected in the Kodak Park population. None of these observed differences in mortality reached statistical significance.
Hearne et al. conducted several updates to the cohort study involving MC exposure and mortality among workers in film production areas at the Kodak plant in Rochester, New York [Exs. 7-122, 7-163, 49 A-1]. In the first update, the study cohort was followed through 1983. Two referent groups were utilized in this study: the general population of upstate New York men, excluding New York City, and Kodak Park employees.
No statistically significant findings were observed for any cause of death. However, Hearne et al. did find a relatively large number (8 observed) of pancreatic cancer deaths compared with the New York State (3.2 expected) and Kodak (3.1 expected) populations. This observation did not achieve statistical significance and a dose-response relationship was not observed when Hearne et al. considered latency and dose.
Hearne et al. then updated this study through 1988 [Ex. 7-163] and 1990 [Ex. 49 A-2]. In the 1988 update, nonsignificant deficits in observed-expected ratios for lung and liver cancer were found. Also, overall mortality from 1964 to 1988 was significantly less than in both referent groups. Since 1986, the number of pancreatic cancer deaths remained the same. As before, dose-response analysis showed no statistically significant pattern when latency or dose were considered.
The 1990 update showed that deaths due to liver cancer, lung cancer, and ischemic heart disease were below the expected numbers in both referent groups. Also, no additional pancreatic cancer deaths were observed in this second update. Since the start of the follow-up, Hearne et al. observed 8 deaths from pancreatic cancer compared with 4.5 expected (SMR = 1.78, p = 0.17).
Hearne et al. [Ex. 49 A-1] conducted a second Kodak cohort study involving workers in cellulose triacetate preparation and film base manufacturing between 1946 and 1970. Hearne et al. addressed the potential selection bias in the 1964-70 Kodak cohort by including only workers exposed primarily to MC after it was introduced in these areas and making the study more complete by adding workers in the Dope Department, which prepares the viscous cellulose triacetate mixture used in the film base coating, and the Distilling Department, which redistills and reblends solvents recovered from the coating operations.
The 1,311 men in the cohort were followed through 1990. An occupational control group could not be formed because death rates for Kodak employees before 1964 were unavailable. Instead, male residents of upstate New York living outside of the five New York City counties were used.
Hearne et al. combined exposures by job and time period with occupational history information to produce a career exposure estimate for each individual in the study for dose-response analyses. The mean career individual exposure was approximately 40 ppm for 17 years and the average interval between first exposure and end of follow-up was about 32 years.
Total mortality for this cohort was 22% below the expected mortality (statistically significant). Circulatory diseases and ischemic heart disease mortality were also statistically significantly below expectation. For lung cancer there were 22 deaths (28.7 expected) and for liver/biliary cancer there was one death (1.5 expected). Hearne et al. found that the number of pancreatic cancer deaths observed (4) was similar to the expected number (4.4). In this cohort, the number of observed deaths was greater than expected for diseases of the colon/rectum (13 observed v. 10.8 expected), brain (5 v. 2.3), and for leukemia (7 v. 3.4), but were not statistically significant.
Hearne et al. concluded that the findings in the 1964-70 cohort were consistent with the 1946-70 cohort: mortality from all causes, cancer (including lung and liver malignancies), and ischemic heart disease was lower than expected. Also, since the number of observed pancreatic cancer deaths in this cohort was similar to the expected number, Hearne et al. believed that this provided further evidence that the earlier finding of an excess of pancreatic cancer in the 1964-70 cohort was due to chance or to factors other than MC exposure.
Kodak [Tr. 1287-88, 9/22/92] also investigated the risk of adverse health effects during active occupational exposure to MC, as suggested by NIOSH [Tr. 970, 9/21/92]. Using person-years of active employment only in their analysis, Hearne observed 27 deaths (36 were expected in the internal Kodak reference group) from ischemic heart disease in the 1964-70 Kodak cohort; in the 1946-70 cohort, Kodak recorded 33 deaths compared with 43 expected in the New York State comparison population.
NIOSH testified [Tr. 877-83, 9/21/92] that the healthy worker effect (HWE) could have obscured any excess mortality from ischemic heart disease caused by MC exposure. NIOSH has stated that the HWE may be particularly strong for cardiovascular diseases.
The HWE is likely to be less of a factor when occupational comparison groups are used. Kodak's use of the Kodak Park employees as a comparison group should reduce the HWE in its studies. However, there are two potential problems with using occupational comparison groups in this instance:
(1) Cancer rates are more stable in larger populations, so comparison with state and national rates may be more appropriate.
(2) Due to the volume of MC used in the Kodak plant, the occupational comparison group may be exposed to air- or water-borne environmental concentrations of MC which could obscure the impact of occupational exposure to MC on cancer incidence.
c. Study of workers in paint and varnish manufacturing. The NPCA submitted to the record an epidemiological study of employees who worked for at least one year in the manufacture of paint or varnish [Ex. 10-29B]. OSHA's review of this study was published in the proposed rule [56 FR 57077]. Although no statistically significant excess of mortality was reported, OSHA noted that there were 4 pancreatic cancers (1.93 expected) and 15 cancers of digestive organs and peritoneum (10.66 expected) among MC-exposed workers.
d. Astrocytic brain cancer among workers in electronic equipment production and repair. In its March 11, 1994 Notice of Limited Reopening of the Rulemaking Record, OSHA solicited comments on a case-control study submitted to the Agency by the National Cancer Institute (NCI) [Exs. 112 and 113].
Heineman et al. conducted a case-control study to examine the potential association between brain cancer and exposure to organic solvents as a group and six chlorinated aliphatic hydrocarbons (CAHs) including MC. Cases were defined as white males who died from brain or other central nervous system tumors in southern Louisiana, northern New Jersey, and Philadelphia, Pennsylvania. Controls were randomly selected from death certificates and included white males who died of causes other than brain tumors, cerebrovascular diseases, epilepsy, suicide, and homicide. Controls were frequency-matched to cases by age, year of death, and geographic area.
Four-digit Standard Industrial Classification (SIC) and 4-digit Standard Occupational Classification (SOC) codes were employed to code occupational histories of study subjects. These codes linked work histories to job-exposure matrices which "characterized likely exposure to the six CAHs and to organic solvents" [Ex. 112]. Gomez et al. [Ex. 112] used an algorithm to assign estimates of probability and intensity of exposure to each industry/occupation combination in subjects' work histories. As noted by Gomez et al., these estimates were based on "occupation alone, industry alone, or both occupation and industry, depending on the specificity of the exposure environment that could be inferred from the occupational (SOC) code."
The following surrogate measures of dose, for each substance, were used to summarize "likely" exposure histories for each study subject: duration of employment in occupation/industry combinations considered exposed, a cumulative exposure score, and "average" intensity of exposure. Odds ratios were calculated for exposure intensity categories to refrain from using weights. These categories did not include duration in jobs with lower intensity for subjects with high or medium intensity jobs. In their statistical analyses, Heineman et al. controlled for age, geographic area, and employment in electronics-related occupations/industries.
- Astrocytic brain cancer was not found to be associated with "ever"
being exposed to organic solvents as a group or to any of the six CAHs examined in this study. However, as probability of exposure to organic solvents as a group, and MC in particular, increased, the risk of brain cancer increased (chi-squared statistics for trend for organic solvents and MC were 1.93 and 2.29 (p< 0.05), respectively). For MC there was a 2.4-fold increase in risk for subjects with a high probability of exposure (confidence interval=1.0-5.9).
Risk of brain cancer significantly increased with duration of exposure for subjects with high probabilities of MC exposure (OR=6.1; CI=1.1-43.8). Heineman et al. found that, in the high probability of MC exposure category, risk significantly increased with duration (chi for trend=2.58, p< 0.01). Similar results were seen for organic solvents and methyl chloroform for all probabilities combined (chi-squared statistics for trend were 2.35 (p< 0.01) and 1.87 (p< 0.05), respectively).
Lagging exposure by 10 years produced findings analogous to those noted above. Higher risks and a sharper increase with duration was observed for organic solvents when exposure was lagged by 20 years (all probabilities: 2-20 years, OR=1.3 (95% CI=0.9-2.0); 21+ years, OR=2.8 (1.1-3.7); p for trend=0.006; high probability: 2-20 years, OR=1.2 (95% CI=0.7-1.9); 21+ years, OR=3.1 (1.3-7.4), p=0.009).
Subjects with a high probability of MC exposure experienced a statistically significant increased risk as the cumulative exposure score increased (chi-squared statistics for trend=2.18, p< 0.05). However, risk did not increase monotonically with cumulative exposure.
Lagging exposure 20 years supported the odds ratios and the trends for organic solvents, particularly in men with a high probability of exposure (low cumulative score: OR=1.1 (95% CI=0.5-2.3); medium: OR=1.4 (0.8-2.5); high: OR=2.2 (1.0-4.5); p for trend=0.02). Few individuals had high cumulative scores when exposure was lagged 20 years for the individual CAHs.
Compared with jobs with medium or low intensity exposures to organic solvents and all six CAHs, risk of brain cancer was higher for subjects who worked in jobs with high intensity exposures. Brain cancer was associated most strongly, and increased with probability of exposure, among subjects who worked 20 or more years with high intensity exposure to MC (all probabilities: OR=6.7, CI=1.3-47.4; high probability: OR=8.8, CI=1.0-200.0).
Since many subjects were determined to have been exposed to more than one of the CAHs, sometimes even in the same job, Heineman et al. used logistic regression to examine, simultaneously, the effects of MC, carbon tetrachloride, tetrachloroethylene, and trichloroethylene, controlling for age, geographic area, and employment in electronics-related occupations/industries. MC was the only substance to show a statistically significant increase in risk as the probability of exposure increased (low: OR=0.9, CI=0.5-1.6); medium: OR=1.4, CI=0.6-3.1; high: OR=2.4, CI=0.9-6.4; chi-squared statistics for trend=2.08, p< 0.05). Risks associated with MC increased when adjustments for exposure to the other agents were made. In addition, subjects employed for 20 years or more in jobs with high average intensity MC exposure showed an eight-fold excess of brain cancer (OR=8.5, CI=1.3-55.5), taking all probabilities into consideration.
Among the six CAHs examined in this study Heineman et al. found the strongest association between brain cancer and MC-exposure, for which relative risks rose with probability, duration, and average intensity of exposure, though not with the cumulative exposure index.
According to Heineman et al., the major weakness of this study was not having direct information on exposure to solvents. Next-of-kin data, poor specificity of some work histories for specific solvents, and the interchangeability of solvents may have resulted in misclassification of individuals with respect to any of the exposure measurements used in this study. However, Heineman et al. pointed out that the potential sources of error probably did not significantly bias risk estimates away from the null or generate the observed trends.
Another limitation of this study, pointed out by Heineman et al., was that over one-third of the next-of-kin of eligible cases and controls were not interviewed. According to Heineman et al., this could have artificially created the associations seen in this study "only by underrepresenting cases who were unexposed, and/or controls who were exposed, to solvents in general, and MC in particular" [Ex. 113]. Heineman further remarked that differential misclassification was probably not a problem in this study because occupational histories came from next-of-kin of both cases and controls.
In light of the limitations of this study, however, Heineman et al. commented that the consistency of exposure-response trends for MC was surprising and suggestive. Moreover, Heineman et al. believed that the trends and consistency of the associations between brain cancer and MC could not be explained by chance alone.
Several commenters [Exs. 115-1, 115-31, 115-32, 115-36] indicated that Heineman et al. relied too heavily on next-of-kin information. Information provided by next-of-kin concerning jobs held, job descriptions, dates of employment, and hours worked per week may be flawed with recall bias. Next-of-kin may not be able to accurately recall job-related information, especially for jobs held early in life. If next-of-kin for cases or controls had better recall than the other group, differential misclassification could occur. HSIA [Ex. 115-36] stated that even small differences in error rates between cases and controls could produce false associations. Both HSIA and NIOSH [Ex.115-31] agreed that this indirect source of exposure information was likely to produce some degree of misclassification. However, NIOSH noted that misclassification "is a typical problem in population based case-control studies of this type [Ex. 115-31]" and that this misclassification could also explain the fact that no associations were found between brain cancer and the cumulative exposure score.
Organization Resources Counselors (ORC) [Ex. 115-2] and Abbott Laboratories [Ex. 115-30] were concerned that the lack of exposure verification made this NCI study unreliable for setting MC exposure limits. ORC stated that exposure values were assigned to all SIC and SOC codes, and not developed based on job history information, which would have given the study more validity. Kodak also expressed some concern regarding this study due to lack of accurate records of past exposures, reliance on expert judgement to a large degree, use of next-of-kin to determine potential exposure, and undocumented qualifications of those making judgements concerning the different occupations and industries involved. In addition, Kodak felt that the exposure data were "at best, unsubstantiated semi-qualitative judgements of likelihood and intensity of exposure [Ex. 115-1]." Organization Resources Counselors [Ex. 115-2] and Abbott Laboratories [Ex. 115-30] asserted that it was impossible to tell if those who died of cancer had been exposed to MC because there was no exposure verification. Vulcan Chemicals [Ex. 115-32] criticized the investigators for not going to work sites and determining the actual magnitude of exposure to the CAHs. HSIA [Ex. 115-36] argued that "concordance of proxy reports with actual work histories may range from 0-50% for decedents' first jobs and from 50-70% for last jobs." OSHA believes that exposure verification would have increased the validity of the findings of this study. However, lack of exposure verification does not nullify the results of the study. The Agency believes that the associations observed are suggestive of a human carcinogenic effect of MC.
Another issue that Kodak [Ex. 115-1] and Vulcan [Ex. 115-32] emphasized was the possible exposure to other chemicals or sources of potential human carcinogens, such as ionizing radiation, electromagnetic fields, smoking history, and place of residence. Vulcan [Ex. 115-32] noted that there may have been selection bias in this study because of the large ratio of astrocytic brain cancer tumors to the total number of brain tumors. Although they offered no explanation of how this selection bias would operate, Vulcan did suggest that this issue should be investigated further.
Vulcan was also concerned that the matching of controls and cases with respect to occupations and socioeconomic status may be inadequate. In particular, Vulcan criticized the Heineman study for not presenting the occupations of the control group and for not matching the socioeconomic status of the two groups. Similarly, Kodak [Ex. 115-1] stated that some adjustment should have been made in order to match across educational levels.
Kodak [Ex. 115-1] also believed that the estimates of trends observed in this study could have been affected, if workers in the longest duration or the higher probability of exposure categories had longer dates of employment, worked in more stable industries, and had better health benefits, better access to medical care, and more sophisticated diagnostic procedures. OSHA believes that there is no evidence that this is the case in this study.
HSIA [Ex. 115-36] criticized the methodology for assessing the number of industries with exposures to CAHs. HSIA argued that Gomez et al. did not fully explain how they determined that workplaces in the specific SICs would have CAH exposures. According to HSIA, Gomez et al. reported inaccurate information regarding industry use of MC. HSIA cited EPA's "Toxic Air Pollutant/Source Crosswalk, A Screening Tool for Locating Possible Sources Emitting Toxic Air Pollutants (EPA-450/4-87-023A, Dec. 1987)" which revealed a higher number of SIC codes using MC. In conclusion, HSIA asserted that Gomez et al.'s "exposure scenario" was incorrect.
Several commenters [Exs. 115-1, 115-31, 115-36] argued that the Heineman et al. study should only be considered a hypothesis-generating study and should not be used to adjust the PEL.
OSHA agrees with NIOSH that the Heineman et al. study was well-conducted because there was a systematic attempt to estimate exposure by work experience. Furthermore, there was a remarkably high correlation between exposure to MC and brain tumors. OSHA concludes that the results from this study strongly suggest a possible association between MC and brain cancer. However, in the absence of quantified exposure data for these workers, it remains relatively speculative to attempt to estimate a quantitative dose-response relationship. Therefore, OSHA concludes that the risk estimate based on the animal data is the best available and accordingly it retains that estimate for its significant risk analysis.
e. Summary of epidemiological studies. Considered as a whole, the available epidemiologic evidence did not demonstrate a strong, statistically significant cancer risk associated with occupational exposures to MC. However, the positive trend for biliary tract/liver cancer deaths, the association between occupational MC exposure and astrocytic brain cancer and the statistically significant excess prostate cancer results are suggestive of an association between MC exposure and cancer risk. In addition, the non-positive epidemiological studies summarized here are not of sufficient power to rule out the positive results from the animal studies. This issue is addressed further in the Quantitative Risk Assessment section of this document.
In summary, the epidemiological results are suggestive of an association between occupational exposure to MC and elevated cancer risk which offers supporting evidence to the positive animal bioassay results.
OSHA concludes from the mutagenicity, animal bioassay and human epidemiology data that MC causes cancer in test animals and that it is a potential occupational carcinogen. The Agency has determined that, because of the quality of the studies, the clear dose-response relationship and the appropriateness of the route of administration, the NTP rodent bioassay data are the best available for quantitative cancer risk assessment.
OSHA also concludes that the epidemiology data, in some cases, suggest a positive association between human MC exposure and cancer incidence, but the dose-response relationships are not clear. The Agency has determined that the remaining epidemiology data (the non-positive studies) are not of sufficient power to rule out the results obtained in the animal bioassay data and that the animal data provide the best available data for quantitative risk assessment.
E. Other Toxic Responses
1. Central Nervous System Toxicity
MC acts on the central nervous system (CNS) as a CNS depressant. CNS depression has been described in humans exposed to MC concentrations as low as 175 ppm (8-hour TWA). This depression in CNS activity was manifested as increased tiredness, decreased alertness and decreased vigilance. These effects could compromise worker safety by leading to an increased likelihood of accidents following MC exposure.
a. Animal studies. In the NPRM, OSHA reviewed two animal studies of MC CNS toxicity (briefly summarized below) and concluded that the CNS was potentially susceptible to reversible and irreversible effects due to MC exposure.
Savolainen et al. [Ex. 7-178] studied biochemical changes in the brains of rats exposed to MC. Rats were exposed to 500 ppm MC for 6 hr/d. On the fifth day, after 3 and 4 hours of exposure to MC, levels of acid proteinase in rat brains were significantly increased, but no change in brain RNA levels was reported. The authors suggested that the increase in acid proteinase may have been the result of increased levels of CO from metabolism of MC. OSHA believes that this study shows that MC can cause specific changes in the neurological system at a biochemical level. The Agency intends to monitor the scientific literature for additional developments on these effects, but has not used this information in setting the MC exposure limits because it is presently unclear how changes in acid proteinase are related to the observed CNS depressive effects of MC in humans.
Rosengren et al. [Ex. 7-56] looked at the effects of MC on glial cell marker proteins and DNA concentrations in gerbil brains after continuous exposure to 210, 350 or 700 ppm MC. Because of high mortality in the 2 higher doses, no data were collected at 700 ppm and exposure was terminated after 10 weeks at 350 ppm. Exposure to 210 ppm was continued for three months. Exposure to MC was followed by four months of no exposure before animals were examined for irreversible CNS effects. The authors found increased levels of glial cell marker proteins in the frontal cerebral cortex and sensory motor cortex after exposure to 350 ppm MC. These findings are consistent with glial cell hypertrophy or glial cell proliferation. Levels of DNA were decreased in the hippocampus of gerbils exposed to both 210 and 350 ppm and in the cerebellar hemispheres after 350 ppm MC. Decreased DNA concentrations indicate decreased cell density resulting from cell death or inhibition of DNA synthesis.
The neurotoxic mechanism of action of MC in gerbil brains is not understood. However, since the metabolism of MC to CO was determined to be saturated at both 210 and 350 ppm (COHb levels were equivalent at both exposure concentrations), the changes in glial cell proteins and DNA concentrations was attributed to either a direct effect of MC or an effect of a metabolite of the GST pathway. Although this study describes biochemical changes in the CNS subsequent to MC exposure, the high mortality of the experimental animals and the lack of MC toxicity data in the gerbil make it difficult to determine the significance of this study for extrapolation to other species. It is also unclear how these effects would relate to CNS depression observed in humans after MC exposure. In addition, continuous exposure to MC has been shown in other experimental situations [Exs. 7-14 and 7-130] to elicit more severe health effects than exposure to similar or higher concentrations when the animals are allowed a recovery period (for example, 6 hours' exposure per day). Exposure on a 6 or 8-hour per day schedule is also more like occupational exposure scenarios and therefore those experiments are generally easier to interpret when assessing risk to workers.
In summary, OSHA believes that the rat and gerbil data described above shows that MC can cause specific changes in the neurological system at a biochemical level. The Agency intends to monitor the scientific literature for additional developments on these effects to determine if these types of effects have implications for human CNS risks.
b. Human studies. The CNS depressant effects of MC have been well described in the literature [Exs. 7-4, 7-153, 7-154, 7-160, 7-175, 7-182, 7-183, 7-184]. MC causes CNS depression which is characterized by tiredness, difficulty in maintaining concentration, decreased task vigilance, dizziness, headaches, and, at high concentrations, loss of consciousness and death. Accidental human overexposures to MC [Exs. 7-18, 7-19] (for example, at concentrations greater than 10,000 ppm) have resulted in narcosis and death. CNS depression has been described after humans were exposed to experimental MC concentrations as low as 200 ppm [Ex. 7-175] and occupational concentrations as low as 175 ppm [Ex. 7-153].
i. Experimental studies. CNS depression was detected in human subjects exposed to MC at concentrations as low as 200 ppm for 4 hours or 300 ppm for 1.5 hours [Exs. 7-4, 7-160, 7-175, 7-182 and 7-184]. In these experiments, which measured subtle CNS depression (such as dual task performance and visual evoked response), it was not possible to determine a no observed effect level (NOEL), because the lowest experimental concentration used (200 ppm) elicited CNS effects. Since a NOEL was not determined for the CNS effects of MC, those effects may occur at lower exposures or after exposure for shorter durations.
The HSIA questioned whether bias was introduced into the results of these studies by inadequate procedures to establish a "double blind." This criticism raises a legitimate concern about the validity of the study. However, since Putz et al. did not describe the blinding procedures used in their experiments, the Agency concludes that there is not enough evidence publicly available to make the conclusion that the study is biased. OSHA believes that these studies were well conducted and is relying on the quality of the studies overall as evidence of the validity of the results. Absent evidence demonstrating the inadequacy of the blinding procedures, OSHA has determined that these studies show that MC can cause mild CNS depression in humans exposed at concentrations as low as 200 ppm.
NIOSH expressed concern regarding the potential for neurobehavioral impairment (expressed as CNS depression) at lower exposures and shorter durations, particularly in relation to the setting of a STEL for MC [Exs. 23-18 and 94]. In order to assess the potential impact of the CNS effects of MC, NIOSH looked at data gathered from several studies and compared breath concentrations of MC (as a surrogate for brain tissue MC concentrations) at different ambient exposure levels with the CNS depression described by Putz et al. [Ex. 7-175]. NIOSH concluded that:
At the proposed STEL of 125 ppm, increased uptake of MC in active workers may place them in the breath concentration range associated with mild neurobehavioral impairment. Although there are insufficient data to draw firm conclusions, extrapolation from existing studies suggests that the proposed STEL of 125 ppm may not fully protect physically active workers from CNS impairment. Therefore, a lower STEL should be considered, if feasible.
In response to concerns raised by NIOSH, the HSIA [Ex. 105] noted that NIOSH's analysis of breath MC concentration versus neurobehavioral impairment "seemed highly speculative." HSIA emphasized that the exposures which produced the reported neurobehavioral effects were observed only after 2 to 4 hours of exposure and that the effects were observed only when difficult tasks were measured.
To support their position, the HSIA asked Mr. Richard Reitz to use a PBPK model to estimate the concentration of MC in brain tissue. This analysis [Ex. 105] indicated that at exposures of 200 ppm for 15 minutes with persons exercising at 50 watts, the brain concentration of MC would be predicted to be similar to that observed in the Putz et al. study for subjects engaged in "light activity" for 2 hours at 200 ppm MC, which did not produce measurable CNS depression. (Putz et al. did not detect CNS depression in subjects exposed to 200 ppm for 2 hours). The model also predicted that 15-minute exposures to 125 ppm while the subject was exercising at 50 watts would produce brain MC concentrations substantially less than that predicted for the 4 hour exposure to 200 ppm MC.
OSHA considered the PBPK analysis presented by the HSIA, but was concerned that there has been no experimental validation of the predicted brain MC concentrations or any evidence as to what MC concentration would produce detectable CNS depression. OSHA believes the primary value of both the NIOSH and HSIA analyses is in demonstrating the relative effect that exercise and duration of exposure is likely to have on brain (or breath) concentrations of MC. The PBPK analysis clearly demonstrates that increasing exercise level increases brain concentration of MC, which is consistent with the detected CNS depression. Workers engaged in strenuous activity while exposed to MC should take special precautions, such as frequent breaks in fresh air, especially if dizziness or lightheadedness occurs.
Although OSHA found the PBPK model to be useful for demonstrating the interaction between exercise and brain concentration of MC, the Agency did not use the model quantitatively (for example, in determining the STEL). OSHA believes that the data suggest that there may be CNS effects at levels below those tested. There are no studies which directly address whether there are CNS effects after exposure to STEL concentrations of MC. To the extent that these effects occur, the STEL would not be protective. Mild and reversible CNS depression was detected at 200 ppm for 4 hours and 300 ppm for 1.5 hours. The Agency shares NIOSH's concern, based on extrapolation of breath MC concentrations, that the proposed STEL may not be adequately protective for physically-active workers.
OSHA concludes that there are clearly sufficient data to determine that a 125 ppm 15-minute STEL is needed to prevent a significant risk of material impairment to the CNS. Impairment of the CNS would also increase the risk from accidents. Measured data show risks at 200 ppm for four hours of exposure. A lower level at shorter duration is needed to avoid that risk. NIOSH's calculations show that for active workers a level lower than 125 ppm may be needed. However, because of feasibility concerns, which would be greater at lower levels and the suggestion that short duration of exposure (i.e., 15-minutes) may mitigate the effects, OSHA is retaining the proposed level, but will carefully monitor and follow up data to determine if this level eliminates significant risk.
ii. Occupational exposure studies. In the NPRM, OSHA summarized studies which it believed described a neuropathy associated with chronic occupational exposure to solvents. Weiss [Ex. 7-196] described the case of a 39-year old chemist who worked for 5 years with airborne concentrations of MC as high as 660 ppm to 3600 ppm in a room with poor ventilation. After 3 years of exposure, the worker developed neurological symptoms, characterized by restlessness, palpitations, forgetfulness, poor concentration, sleep disorders, and finally, acoustical delusions and optical hallucinations. No hepatic damage or cardiac toxicity was found. At the first appearance of symptoms, cessation of exposure produced an immediate cessation of symptoms. Later, longer and longer periods were required after termination of exposure in order to alleviate the symptoms. The increasing persistence of symptoms is consistent with a diagnosis of toxic encephalosis.
Hanke et al. [Ex. 7-195] examined 32 floor tile setters who were exposed primarily to MC at concentrations from 400 to 5300 ppm for an average tenure of 7.7 years. Clinical examination of 14 of the workers who had neurological symptoms (headache, vertigo, sleep disturbance, digestive complaints and lapses in concentration and memory) revealed changes in the EEG patterns of the exposed workers which persisted over a weekend pause in exposure. These EEG changes were characteristic of a toxic encephalosis produced by chronic intoxication with a halogenated solvent (MC). The persistence of the EEG changes over the weekend break indicated a prolonged effect of MC exposure on EEG patterns. (Additional changes in the EEG found during exposure could be attributed to an acute effect of MC). Although these studies represent a small number of cases with very high chronic exposures, the evidence is suggestive of a relationship between chronic MC exposure and toxic encephalosis.
In a case study report, Barrowcliff et al. [Ex. 7-123] attributed cerebral damage in a case study to CO poisoning caused by exposure to MC. Axelson [Ex. 7-150] has described an increased number of neuropsychiatric disorders among occupations with high solvent exposures.
In the NPRM, OSHA expressed the opinion that these studies, taken together, "provide suggestive evidence of a permanent toxicity [different from the observed reversible CNS depression] which may be the result of chronic exposure to MC." NIOSH stated that this assessment was too speculative and stated,
in the Hanke study, MC was apparently only one component of a solvent mixture and may not have been the only neurotoxic agent* * * In addition, the observation interval of 2.5 days was not long enough to provide convincing evidence of irreversible effect, regardless of the active agent.
Upon reexamination of these studies, OSHA agrees with NIOSH [Ex. 19-46] that although a prolonged effect (over a weekend break in exposure) of MC on EEG patterns has been demonstrated, these studies do not support a determination that MC exposure is associated with irreversible brain damage in humans.
OSHA reviewed several other studies of occupational exposure to MC for evidence of CNS effects of MC. The first study was provided as an English translation of a Czechoslovakian paper by Kuzelova et al. [Ex. 7-26]. These investigators examined workers in a film production plant who were exposed to MC concentrations from 29 to 4899 ppm. Several workers suffered frank MC intoxication and many workers showed signs of MC-induced CNS depression. Toxicity associated with chronic MC exposure was observed in workers exposed to MC for up to two years, but the authors recommended continuing studies of the long-term health effects.
OSHA believes that this study shows CNS depression in workers exposed to MC. The Agency agrees with the authors that this study was not sufficient to adequately characterize the long-term CNS health effects that may be induced by MC exposure.
Cherry et al. [Ex. 7-154] studied the effects of occupational exposure to MC at 28 to 175 ppm in two exposed populations. In a 1981 study, the authors found a marginal increase in self-reported neurological symptoms among exposed workers. This increase disappeared when an appropriate reference group was used for comparison. However, in a 1983 investigation, Cherry [Ex. 7-153] showed statistically significant increases in tiredness and deficits in reaction time and digit symbol substitution which correlated with MC in blood. Ambient MC exposures for this population ranged from 28 to 175 ppm for the full shift. This study demonstrated CNS effects due to occupational MC exposures below 200 ppm (the lowest dose which was administered in the experimental studies).
- The HSIA [Ex. 105, p. 34] commented as follows:
Decades of experience with worker populations exposed even at levels up to the current 500 ppm TWA have provided no evidence that such workers have higher rates of accidents or other signs of significant neurobehavioral impairment.
To the contrary, OSHA believes that the occupational studies discussed above demonstrate that MC has an effect on the CNS at occupational exposure levels as low as 175 ppm.
The Agency believes that the 1983 study by Cherry shows that occupational exposure to MC concentrations below the former 8-hour TWA PEL of 500 ppm can produce detectable CNS effects. Although the 1981 study, which relied on self-report of neurological symptoms, did not demonstrate a CNS effect, the 1983 study examined more objective measures of CNS depression and correlated the observed effects with a direct measure of MC exposure. OSHA believes that this study demonstrates that, although the CNS depression may be mild, it is demonstrable in occupational settings and at concentrations in the range of the STEL (although the exposures in this study were over an 8-hour work day). As described above, OSHA is sufficiently concerned about the potential for health effects at concentrations below the STEL of 125 ppm that it will continue to gather information and revisit this issue, if warranted.
2. Cardiac Toxicity
As described in the section on the metabolism of MC, MC is metabolized in vivo (in animals and humans) to CO and CO(2). Cardiovascular stress has been observed after exposure to CO, so it is reasonable to suspect that similar health effects would be observed after exposure to MC (and metabolism to CO) [Ex. 7-73, 4-33]. Carbon monoxide successfully competes with oxygen and blocks the oxygen binding site on hemoglobin, producing carboxyhemoglobin (COHb) and reducing delivery of oxygen to the tissues. This reduces the oxygen supply to the heart itself, which can result in myocardial infarction (heart attack) [Ex. 4-33].
Generally, humans have a baseline level of COHb of less than 1% COHb due to the endogenous production of CO from normal metabolic processes. The measured level of COHb in the general non-smoking population is from 1% to 3% because of direct exposure to CO from combustion sources such as automobiles, etc. In smokers, COHb generally ranges from 2% to 10% because of the additional CO exposure during smoking. CO generated from exposure to MC would be additive to the COHb burden already experienced by an individual from direct exposure to CO. The cardiac health effects anticipated from exposure to MC itself or CO as the result of metabolism of MC are described below.
a. Animal studies. There is no evidence from animal studies in the MC rulemaking record that MC has a direct toxic effect on cardiac tissue. After lethal doses of MC, death has been primarily attributed to CNS and respiratory depression [Exs. 7-27, 7-28]. Also, chronic studies (in which COHb levels have been maintained at 10% and higher) [Exs. 7-3, 7-8, 7-14, 7-130, 7-151] have not shown direct cardiotoxicity.
Chlorinated solvents have been shown to sensitize the cardiac tissue to epinephrine-induced fatal cardiac arrhythmias [Ex. 7-226]. However, MC is less effective in sensitizing cardiac tissue than other chlorinated analogues. MC caused sensitization of cardiac tissues only at doses well above doses which produce a narcotic effect. This finding indicates that compliance with an 8-hour TWA of 25 ppm MC would likely be sufficient to protect against such sensitization.
b. Human studies. The metabolism of MC to CO and measurement of COHb in human subjects exposed to MC were described in detail in the NPRM. In summary, it was found that exercising increased MC uptake and, subsequently, increased blood COHb levels compared to that of sedentary individuals [Ex. 7-222]. In addition, COHb levels due to smoking were found to be additive to the COHb produced by MC metabolism. Taken together, these results suggested that smokers or individuals engaged in physical exertion (as in a workplace) may be at increased risk from CO-induced toxicity from MC exposure. This risk may be especially elevated in individuals with silent or symptomatic cardiac disease who may be susceptible to very small increases in COHb because of an already impaired blood supply to the heart. Many American workers have silent or symptomatic heart disease. This increased OSHA's concern for the potential cardiac effects of MC and its metabolites.
Elevated COHb has been measured in humans experimentally and occupationally exposed to MC [Exs. 7-4, 7-5-R0327, 7-102, 7-115, 7-157, 7-159, 7-169, 7-174, 7-176]. The effects of elevated COHb are primarily increased risk of myocardial infarction, especially in susceptible individuals. Atkins and Baker [Ex. 7-198] described two cases of myocardial infarction in workers subsequent to CO exposure. COHb was measured at 30% and 24% in these individuals, which is much higher than normal general population levels of COHb. Humans exposed to MC would not be expected to experience COHb at those levels unless the exposure to MC was extremely high (greater than 500 ppm).
In a laboratory study of humans with coronary artery disease, subjects were exposed to CO and observed for cardiac health effects during exercise. In subjects with 3 to 10% COHb, decreased exercise tolerance and increased anginal pain were observed [Ex. 7-198]. In an epidemiological study submitted to OSHA by NIOSH during the MC public hearings, the investigators observed a statistically significant excess of ischemic heart disease mortality among tunnel workers when compared with rates for the New York City population [Ex. 23-18]. This increase in mortality is supported by clinical findings. Allred et al. [Ex. 23-18] observed that elevation of COHb from 0.6% to as low as 2% decreased time to myocardial ischemia and anginal pain during laboratory tests. OSHA believes that these studies, taken together, suggest that small increases in COHb can adversely affect persons with compromised cardiac health. The results observed in the tunnel workers are particularly relevant because they show an increased risk in a working population. NIOSH used these studies to support its recommendation that the COHb effects of MC be carefully considered in the MC rulemaking [Tr. 881-2, 9/21/92]. OSHA agreed with NIOSH that the effects observed at low levels of COHb are cause for concern about the risks of MC metabolism to CO.
In the NPRM, OSHA also reviewed case reports in which individuals exposed to MC experienced myocardial infarctions [Exs. 7-102, 7-73]. These case reports suggested that exposure to MC increased cardiac stress, although it was not determined whether this was a direct effect of MC or as the result of metabolism of MC to CO. OSHA believes that these case studies support the hypothesis that CO generated through metabolism of MC would have the same adverse health effects as direct CO exposure.
Two epidemiological studies (in film coating and fiber production workers) [Exs. 7-75, 7-76, 7-122, 7-163] examined cardiac mortality due to occupational exposure to MC. Ott [Ex. 7-76] compared mortality from a plant in South Carolina that used MC to a reference plant in Virginia. An increased risk ratio for ischemic heart disease (risk ratio = 3.1) was observed in the MC-exposed workers compared to the reference population.
This approach controls for the healthy worker effect by comparing two working populations, and excess risk was demonstrated. The authors believed that the apparent excess risk was due to geographical variability in the incidence of ischemic heart disease. The population from the reference plant was found to have an unusually low death rate due to ischemic heart disease in comparison to the general population rate.
In an update of the study [Ex. 7-75], the ischemic heart disease rate in the exposed population was compared to that in the surrounding York County, S.C. population instead of a reference plant. No difference in ischemic heart disease rates was detected between exposed workers and controls, although this approach would not control for the healthy worker effect. The SMR was 0.94 (32 observed, 34.2 expected).
NIOSH disagreed with the conclusion of the authors of this study, and indicated that the studies summarized above would be cause for concern regarding the cardiac effects of MC. NIOSH suggested that the raw data from the epidemiological studies of cellulose acetate film production workers and the studies of workers in cellulose acetate fiber manufacture be reviewed for cardiac mortality occurring during the period of occupational exposure for the workers. OSHA is concerned about the potential CO effects from metabolism of MC and will continue to monitor the scientific literature on this topic. However, the Agency is setting the exposure limits based on cancer and CNS effects and has not reached final conclusions on this issue.
3. Hepatic Toxicity
Chlorinated hydrocarbons as a class, such as carbon tetrachloride and chloroform, are toxic to the liver. In general, chlorinated hydrocarbons cause cytotoxicity (cell death) in rodent livers. Therefore, there was suspicion that the liver would also be a target organ for MC (a chlorinated hydrocarbon) toxicity. OSHA evaluated the available literature on the hepatic effects of MC in animal and human studies.
a. Animal studies. Studies of the effects of MC exposure on the rodent liver have not demonstrated significant acute liver toxicity, even at lethal or near-lethal doses. As summarized in the NPRM, Kutob et al. [Ex. 7-27] and Klaassen et al. [Ex. 7-28] conducted experiments on halogenated methanes and hepatotoxicity. MC was determined to be the least hepatotoxic of the halogenated methanes examined. The only injury described was a mild inflammatory response associated with lethal MC concentrations. These studies demonstrated that liver was not the primary target organ for the acute toxicity of MC.
Weinstein et al. [Ex. 7-181] examined the hepatic effects of MC on female mice who were continuously exposed for up to 7 days to MC concentrations of up to 5000 ppm. Mild, nonlethal injury to the livers was noted, characterized by balloon degeneration of the rough endoplasmic reticulum (RER), transient severe triglyceride accumulation (fatty liver), partial inhibition of protein synthesis and breakdown of polysomes into individual ribosomes. The injury is similar to a mild form of carbon tetrachloride toxicity (a structural analog of MC) and suggests that although the toxicity due to MC is not as severe as that produced by carbon tetrachloride, the mechanism of toxicity may be similar.
In subchronic experiments more severe effects were observed in the liver after continuous exposure. MacEwen et al. [Ex. 7-14] studied the effects of continuous exposure of mice, rats, dogs and rhesus monkeys to 1000 and 5000 ppm MC for up to 14 weeks. Fatty liver, icterus, elevated SGPT and ICDH were reported in dogs at both concentrations. These effects appeared at 6-7 weeks of exposure to 1000 ppm MC and at 3 weeks of exposure to 5000 ppm. Monkeys were less sensitive to hepatic injury, and showed no changes in liver enzymes and only mild to moderate liver changes at 5000 ppm MC. No liver alterations were detectable in monkeys exposed to 1000 ppm MC. Mice and rats developed liver toxicity at both exposure levels, characterized by increased hemosiderin pigment, cytoplasmic vacuolization, nuclear degeneration and changes in cellular organization.
Hepatic effects associated with chronic MC exposure were observed in lifetime cancer bioassays in three rodent species: rats, mice and hamsters. In studies conducted by the NTP and Dow Chemical Co., rats were exposed to inhalation concentrations of MC from 50 ppm to 4000 ppm 6 hours per day, 5 days per week [Exs. 7-8, 7-151, 7-173]. Hepatic effects were observed after exposure to MC concentrations as low as 500 ppm. These effects were characterized by increased fatty liver, cytoplasmic vacuolization and an increased number of multinucleated hepatocytes. At higher doses (greater than 1500 ppm), increased numbers of altered foci and hepatocellular necrosis became apparent.
Serota et al. [Ex. 7-180] administered 5 to 250 mg MC/kg body weight to rats in drinking water. Hepatic toxicity similar to that observed in the inhalation studies was reported at doses from 50 to 250 mg/kg.
In mice, the chronic hepatic effects of MC were investigated in two bioassays: NTP [Ex. 7-8] and Serota et al. [Ex. 7-179]. In the NTP study, mice were exposed by inhalation to 2000 or 4000 ppm MC. Cytologic degeneration was observed in both male and female mice and increased incidences of hepatocellular adenomas and carcinomas were found at both concentrations. The carcinogenic effects of MC are described in greater detail above, in the discussion of MC carcinogenicity.
In a drinking water study, Serota et al. found that mice exposed to 50 to 250 mg/kg/d MC had dose-related increases in the fat content of the liver (a sign of liver toxicity). Although some proliferative hepatocellular lesions were identified in this study, they were distributed across all exposure groups. Hepatocellular tumor incidences were not elevated above historical control incidences.
In the hamster, Burek et al. [Ex. 7-151] found minimal treatment-related changes in the livers of the MC-exposed animals after exposure to 500, 1500 or 3500 ppm MC. A dose-related increase in hemosiderin was found in male hamsters at 6 months and at 3500 ppm at 12 months. No other changes in liver physiology were reported.
OSHA believes that these studies demonstrate that the rodent liver is not sensitive to acute affects of MC, but that chronic exposure to MC caused toxic effects in rat and mouse liver and cancer in mouse liver. These studies appear to have been well conducted and the differences in toxicity observed across studies were likely due to differences in dose or route of exposure. The hamsters appeared to be insensitive to liver toxicity. OSHA believes that this is most likely due to inherent species differences in response to toxicants.
b. Human studies. OSHA evaluated epidemiological studies and case reports to determine the extent of hepatic effects detected after exposure of humans to MC. Liver toxicity was measured as alterations in the blood levels of any of several normal liver enzymes in these studies.
i. Epidemiological studies. In a cross-sectional analysis of the health of workers in an acetate fiber production plant in which workers were exposed to 140 to 475 ppm MC, Ott et al. [Ex. 4-33c] reported statistically significant increases in serum bilirubin and alanine aminotransferase (ALT) (also known as serum glutamic pyruvic transaminase (SGPT)) when compared with a reference group of industrial workers. The elevation in bilirubin levels showed a dose-response relationship, but the ALT levels were not associated with MC exposure. The authors felt that the increase in ALT in MC-exposed workers could not be attributed to MC because a dose-response relationship was not demonstrated and, therefore, the increase in ALT between the exposed and reference populations could be disregarded as a sign of liver toxicity. The authors concluded that although bilirubin elevation may be interpreted as a sign of liver toxicity, this interpretation was not supported by alterations in other liver parameters. OSHA feels that ALT cannot be disregarded as unrelated to MC exposure based on the lack of dose response within the exposure group. The high variability of this parameter and the low numbers of individuals within certain exposure subgroups (e.g., 10 men exposed at 280 ppm), make a dose-response relationship more difficult to demonstrate. Any mistake made in the characterization in an exposure group would result in obscuring the dose-response relationship. Although the evidence is not unequivocal, OSHA believes that the elevated bilirubin coupled with the elevated ALT values indicate suggestive evidence of a hepatotoxic response to MC exposure in this worker population.
In an update to the study described above, Cohen et al. [Ex. 7-75] found 4 cases of liver/biliary duct cancer in workers with more than 10 years of exposure to MC and after 20 years from first hire. Further description of this study can be found in the discussion of MC carcinogenicity, above.
In an English translation of a 1968 Czechoslovakian study, Kuzelova et al. [Ex. 7-26] found no liver enzyme abnormalities in workers exposed to MC concentrations from 29 ppm to 4899 ppm for up to two years. In contrast, in an English translation of a German study which focussed on neurological changes due to MC exposure, Hanke et al. [Ex. 7-195] observed pathological liver function tests and hepatomegaly (enlarged liver) in 4 of 14 floor tile setters examined. These workers were chronically exposed to MC at concentrations as high as 400 to 5300 ppm. The average tenure of employment of these workers was 7.7 years. The authors of the Hanke study noted that although MC with its impurities could be responsible for the liver damage, the evidence was not conclusive. OSHA has determined that there is insufficient evidence from the Kuzelova and Hanke studies to conclude that MC causes chronic human hepatotoxic effects.
ii. Case reports. In addition to the cross-sectional analyses of worker morbidity described above [Exs. 4-33c and 7-26], the relationship of MC exposure and hepatotoxicity has been studied by analysis of case reports. Welch [Ex. 7-73] collected 144 case reports of clinical disease reported subsequent to occupational MC exposure. Quantitative exposure estimates for individuals were unreliable, but the presence of MC in the work environment was ascertained for each employee. The most prevalent findings in these case reports were CNS symptoms, upper respiratory syndrome and alterations in liver enzymes. The patterns of alteration in liver enzymes were not consistent among individuals, but may be suggestive of a MC-associated hepatotoxic effect. One case of hepatitis of unknown etiology was identified. The case physician believed that the hepatitis was secondary to solvent exposure. The solvents to which this employee was exposed included xylene and methylethyl ketone as well as MC. OSHA believes that the confounding solvent exposures in the hepatitis case and the unknown exposure histories of the individuals with altered liver enzymes limit the interpretation of these studies. OSHA has determined that these case reports provide insufficient evidence to conclude that MC was the causative agent in these cases.
Analysis of cases of fatal and near-fatal human exposures [Exs. 7-18, 7-19] indicated no apparent acute alterations of liver function. Acute concentrations of MC which caused narcosis and even death were not associated with changes in liver enzymes.
OSHA concludes that limited evidence supports the hypothesis that MC causes human hepatotoxicity, based on the data in the Ott study. The remaining studies and case reports do not provide clear evidence of a causative role of MC in hepatotoxicity. The Agency has set the exposure limits based on cancer and CNS effects and has not reached final conclusions on this issue.
4. Reproductive Toxicity
There are only limited data available regarding the potential adverse teratogenic or reproductive effects due to MC exposure. Teratogenicity studies have been conducted in rats and mice and limited epidemiology and case reports have been described for humans.
a. Animal studies. A study [Ex. 4-5] using chicken embryos indicated that MC disrupts embryogenesis in a dose-related manner. Since the application of MC to the air space of chicken embryos is not comparable to MC administration to animals with a placenta, the exposure effect seen in the chick embryos can only be considered as suggestive evidence that an effect may also occur in mammalian systems.
The teratogenicity of inhaled MC has also been studied in rats and mice [Exs. 7-20, 7-21, 7-22]. In 1975, Schwetz et al. [Ex. 7-21] conducted a study on Swiss Webster mice. Mice were exposed to 1250 ppm MC for 7 hours/day, on days 6-15 of gestation. On day 18 of gestation, Caesarian sectioning of dams was performed. A statistically significant increase in mean maternal body weight (11-15%) was observed in dams exposed to 1250 ppm MC; however, food consumption was not measured. The only effect on fetal development associated with MC exposure was a statistically significant increase in the number of fetuses which contained a single extra center of ossification in the sternum. The incidence of gross anomalies observed in the MC-exposed fetuses was not significantly different from that in the control litters. Maternal COHb level during exposure reached 12.6%; however, 24 hours after the last exposure, COHb had returned to control levels.
In the same study by Schwetz et al. [Ex. 7-21], Sprague-Dawley rats were exposed to 1250 ppm MC via inhalation for 7 hours daily on days 6-15 of gestation. No MC-associated effects were observed in food consumption or maternal body weight. Among litters from MC-exposed dams, the incidence of lumbar ribs or spurs was significantly decreased when compared to controls, while the incidence of delayed ossification of sternebrae was significantly increased compared to controls. No increased incidence of gross anomalies were observed in the fetuses from exposed rats compared to fetuses from control litters. No MC-associated effects were observed on the average number of implantation sites per litter, litter size, the incidence of fetal resorptions, fetal sex ratios or fetal body measurements, in the 19 litters that were evaluated. As observed in the MC-exposed mice, there was significant elevation of the COHb level in the dams, but the level returned to control values within 24 hours of cessation of exposure.
In 1980, Hardin and Manson [Ex. 7-22] evaluated the effect of MC exposure in Long-Evans rats after inhalation of 4500 ppm for 6 hours/day, 7 days/week prior to and during gestation. Four exposure groups were described. The first group was exposed to MC for 12 to 14 days prior to gestation and during the first 17 days of pregnancy. The second group was exposed to MC only during the 12 to 14 days prior to gestation. The third group was exposed to MC only during the first 17 days of pregnancy. The fourth group (control group) was exposed only to filtered air. The purpose of this study was to test whether MC exposure prior to and/or during gestation was more detrimental to reproductive outcome in female rats than exposure during gestation alone.
In rats exposed to MC during gestation, there were signs of maternal toxicity, characterized by a statistically significant increase in maternal liver weights. The only fetal MC effects observed were statistically significant decreases in mean fetal body weights. No significantly increased incidence of skeletal or soft tissue anomalies was observed in the offspring.
In 1980, Bornschein et al. [Ex. 7-224] tested some of the offspring of the Long-Evans rats from Hardin and Manson's study described above. All four treatment groups were used to assess the postnatal toxicity of MC exposure at 4500 ppm. The general activity measurements of groups of 5-day old pups showed no exposure-related effects. At 10-days of age, however, significant MC-associated effects were observed in both sexes in the general activity test. These effects were still apparent in male rats at 150-days of age. This study showed that maternal exposure to MC prior to and/or during pregnancy altered the manner in which the offspring react and adapt to novel test environments at up to 150-days of age. These effects suggest that MC exposure prior to, or during pregnancy may influence the processes of orientation, reactivity, and/or behavioral habituation. No changes in growth rate, long-term food and water consumption, wheel running activity or avoidance learning were reported.
OSHA concluded from the animal studies that maternal exposure to high concentrations of MC during pregnancy may have some adverse effects on the offspring, in particular with regard to behavioral effects. The Agency has set the exposure limits based on cancer and CNS effects and has not reached final conclusions on this issue.
b. Human studies. Limited data have been collected on the reproductive effects of MC in male workers. In a study reported in the Occupational Safety and Health Reporter [Ex. 7-43], a greater risk of male sterility was found in male workers exposed to MC. In 1988, Kelly [Ex. 7-165] reported 4 cases of oligospermia in MC-exposed workers. This study was described in detail in the NPRM. Although the study provided some evidence of an effect of MC on male fertility, the observations were based on a small number of cases and OSHA believes that more research is necessary before causative conclusions can be drawn about the human male reproductive toxicity of MC.
The reproductive and developmental effects due to MC exposure in female workers have also been studied. According to information reported in an English translation of an abstract of a Russian article by Vozovaya et al. [Ex. 7-16], detectable levels of MC were found in the blood, milk, embryonal, fetal and placental tissues of nursing women exposed to MC in a rubber product plant. No other information was provided in the abstract. In a study by Taskinen et al. [Ex. 7-199], increased rates of spontaneous abortions were observed in female pharmaceutical workers exposed to MC. Exposure data were not reported in this study and it is unclear what confounding factors or other chemical exposures were present. OSHA believes that more research is necessary in order to evaluate the potential effect of MC on pregnancy outcomes, and so has not reached a conclusion on this issue.
Carbon monoxide has well known adverse reproductive effects in humans. Since MC is metabolized to CO, OSHA was concerned about the adverse reproductive effects of CO as a metabolite of MC. The EPA has reviewed the literature on the effects of maternal CO exposure on the development of the fetus in the Air Quality Criteria for Carbon Monoxide [Ex. 7-201]. Very high maternal CO exposures have resulted in fetal or infant death or severe neurological impairment of the offspring. CO reduces the amount of oxygen available to the tissues. The developing fetus is very sensitive to these effects. According to Fechter et al. [Ex. 7-200], low levels of CO exposure in animals have been shown to adversely affect the fetus, producing CNS damage or reduced fetal growth. These effects suggest that the fetus may be especially sensitive to the toxic effects of MC through its metabolism to CO.
As described above, OSHA is sufficiently concerned about the potential for reproductive health effects of carbon monoxide as a result of MC metabolism that it has decided to continue to gather information and revisit this issue, if warranted.
OSHA's determination that MC is a potential occupational carcinogen was based primarily on the positive findings of chronic inhalation bioassays in rodents. MC is carcinogenic to mice of both sexes, producing lung and liver neoplasms. In rats, MC produced dose-related increases in mammary tumors and increases in the number of tumors per tumor-bearing rat. The evidence in rodents is supported by epidemiologic findings from cellulose triacetate fiber production workers and a case-control study of individuals with astrocytic brain cancer. The study of fiber production workers suggests an association between liver and biliary cancer and long term (greater than 10 years) exposure to MC. The case-control study indicates an association between risk of astrocytic brain cancer and occupational exposure to MC. This evidence is further supported by the findings of genotoxic activity of MC in bacterial and mammalian cell systems. OSHA has set the 8-hour TWA PEL of 25 ppm primarily to protect employees from the risk of cancer due to MC exposure in the workplace.
CNS depression has been demonstrated in humans and animals at relatively low inhalation concentrations of MC. The CNS depression observed in those studies was relatively mild, although the effects occurred at concentrations in the range of the STEL of 125 ppm. OSHA believes that the STEL will be protective against CNS depression for most employees exposed to MC most of the time, but the Agency is sufficiently concerned about the potential for CNS health effects at concentrations below the STEL and have decided to continue to gather information and revisit this issue, if warranted.
- [62 FR 1494, January 10, 1997]
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