• Publication Date:
  • Publication Type:
    Proposed Rule
  • Fed Register #:
  • Standard Number:
  • Title:
    Occupational Exposure to Methylene Chloride


Occupational Safety and Health Administration

29 CFR Parts 1910, 1915 and 1926

[Docket No. H-071B]

Occupational Exposure to Methylene Chloride

AGENCY: Occupational Safety and Health Administration (OSHA), Department of Labor.

ACTION: Proposed rule; limited reopening of the rulemaking record.


SUMMARY: The Occupational Safety and Health Administration (OSHA) is reopening the record for the proposed revision of the methylene chloride (MC) standard (56 FR 57036, November 7, 1991) for comments on recently conducted research regarding MC metabolism. OSHA's proposed MC risk assessment for cancer was based primarily on extrapolations from mouse bioassay data. The proposal and the hearing notice (57 FR 24438, June 9, 1992) solicited input regarding the relevance of metabolic and physiological differences between mice and humans when assessing human cancer risk. As a result, the rulemaking record already contains considerable information, comment and testimony regarding this issue.

The new studies address the potential pathway(s) by which MC metabolites induce lung and liver cancer in mice and draw conclusions regarding the relevance of the mouse data to the assessment of human cancer risk. OSHA has determined that these studies are relevant to full consideration of concerns raised by the MC rulemaking. Therefore, OSHA is reopening the record to allow the public an opportunity to comment.

DATES: Written comments on the materials incorporated through the notice of reopening must be postmarked by November 24, 1995.

ADDRESSES: Comments are to be submitted in quadruplicate to the Docket Office, Docket No. H-071B, U.S. Department of Labor, Room N-2634, 200 Constitution Avenue, NW, Washington, DC 20210. telephone (202) 219-7894. Written comments limited to 10 pages or less in length may also be transmitted by facsimile to (202) 219-5046, provided that the original and 3 copies are sent to the Docket Office thereafter.

FOR FURTHER INFORMATION CONTACT: Anne C. Cyr, Office of Information and Consumer Affairs, Occupational Safety and Health Administration, U.S. Department of Labor, Room N-3647, 200 Constitution Avenue, NW, Washington, DC 20210. Telephone (202) 219-8148. Copies of the referenced studies are available for inspection and copying in the Docket Office and will be immediately mailed to persons who request copies by telephoning Christine Whittaker at (202)219-7174. For electronic copies, contact the Labor News Bulletin Board (202) 219-4784; or OSHA's WebPage on Internet at http://www.osha.gov/. For news releases, fact sheet, and other short documents, contact OSHA FAX at (900) 555-3400 at $1.50 per minute.


I. Background

On November 7, 1991, OSHA issued a notice of proposed rulemaking (56 FR 57036) to address the significant risks of MC-induced health effects. The proposed rule required employers to reduce occupational exposure to MC and to institute ancillary measures, such as employee training and medical surveillance, for further protection of MC-exposed workers.

OSHA convened public hearings (57 FR 24438, June 9, 1992) in Washington, DC on September 16-24, 1992 and in San Francisco, CA on October 14-16, 1992. The post-hearing period for the submission of additional briefs, arguments and summations ended on March 15, 1993. On March 11, 1994, OSHA reopened the rulemaking record for 45 days (59 FR 11567) to obtain public input on three documents incorporated into the rulemaking record, one of which examined the relationship between MC exposure and human carcinogenesis. The limited reopening, which ended on April 25, 1994, generated 37 comments.

OSHA relied primarily on the mouse bioassay performed by the National Toxicology Program (NTP) in assessing human cancer risks in the proposed rule. The Preliminary Quantitative Risk Assessment was based on a multistage model which used the applied dose from the NTP study in the dose-response analysis. The proposal and the hearing notice solicited input regarding the extent to which metabolic differences between mice and humans could be taken into account when assessing human cancer risk. The Agency generated a considerable amount of information, comments and testimony regarding this issue at the public hearings and in the post-hearing comment periods. Thus the rulemaking record upon which the final risk assessment will be based already includes substantial data for analysis using either administered-dose or pharmacokinetic models.

In September 1995, the Halogenated Solvents Industry Alliance (HSIA) submitted several recently-completed studies on this issue in which HSIA asserted that species differences in the enzymatic metabolism of MC make the mouse a poor surrogate for estimating human cancer risk. The utility of the mouse data in assessing human risk is one of the important issues in this rulemaking. Therefore, OSHA believes that it is appropriate, even at this late stage of the rulemaking process, to consider the HSIA-submitted studies in the drafting of the final rule. Accordingly, the Agency is reopening the rulemaking record to incorporate these studies and to provide the public with an opportunity to comment.

As discussed above, OSHA has been considering the impact of species differences on the MC risk assessment throughout this rulemaking, and has generated an extensive record over the nearly four years since the proposal was published. While the Agency agrees with HSIA that the new materials should be taken into account, the Agency still believes that every reasonable effort should be made to finish this rulemaking expeditiously. To that end, OSHA has concluded that it is appropriate to allow interested parties 30 days within which to submit any additional comments and information regarding this issue. OSHA will provide interested parties with copies of the newly incorporated materials, upon request, to facilitate full and timely public participation. Requests for copies should be addressed to Christine Whittaker, Room N-3718, Health Standards Programs, OSHA, U.S. Department of Labor, 200 Constitution Avenue, NW, Washington, DC 20210. Telephone: (202) 219-7174. Fax: (202) 219-7125.

In addition, OSHA notes that HSIA has submitted data on lung tissue obtained through the Zeneca Toxicology Laboratory in a preliminary communication (Ex. 124) but has not yet submitted a final report of this research. The Agency has determined, given the availability of the preliminary communication, that it would be inappropriate to delay the reopening until the final report was received. HSIA has indicated that the report will be submitted during the reopening period. As with the materials already docketed, OSHA will provide copies of that report, upon request, when it becomes available.

The materials added to the record consist of a transmittal letter from HSIA and seven technical submissions as discussed below.

Exhibit 117 Letter from Peter E. Voytek, Ph.D., of the Halogenated Solvents Industry Alliance to Joseph A. Dear, Assistant Secretary of Labor for Occupational Safety and Health, September 5, 1995

This letter introduces the HSIA studies covered by the notice of reopening and requests that OSHA "reopen the rulemaking record for the limited purpose of obtaining public comment (and additional scientific peer review, to the extent OSHA deems it appropriate) on this evidence." In particular, the HSIA letter states that the mice used in studies on which OSHA's risk assessment is based "are uniquely sensitive at high exposure levels to methylene chloride-induced lung and liver cancer, and that other species, including humans, are not at similar risk." The letter summarized the basis for this interpretation as follows:

As a result of this research program, it appears that there are no foreseeable conditions of human exposure in which the carcinogenic effects seen in mice could be expected to occur in man. Given the unique metabolism of methylene chloride by mice, the mouse cannot be considered an appropriate model for human risk assessment. The risk assessment that is the basis for the methylene chloride standard, which is in turn based on the increased liver and lung tumor incidence observed in the mouse bioassay, must be discarded in favor of scientific data that are relevant to human risk.

OSHA requests that commenters review the following technical studies to assess whether the conclusions summarized above are appropriate, in light of the evidence contained therein, considering factors such as: (1) The relevance, reliability, and sensitivity of the assays used (e.g., the DNA single-strand break assay reported in Exhibit 120 and the mRNA assay reported in Exhibit 124); (2) the existing evidence in the record indicating quantitative differences in MC metabolism between mice and humans; (3) the weight of evidence contributed by these in vitro studies evaluated in light of the other in vitro and in vivo information already in the record; and (4) other relevant factors. The Agency also requests that commenters address the extent to which these studies might also support alternative conclusions.

Exhibit 118 "Methylene Chloride Induced Mouse Liver and Lung Tumours", T. Green, Zeneca Central Toxicology Laboratory, July 31, 1995

This document summarizes the available information regarding the metabolism of methylene chloride in mice, rats, hamsters and humans. The researcher characterized this information as follows: "These results provide evidence that the mouse is unique in its response to methylene chloride and that it cannot therefore be considered an appropriate model for human health assessment."

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

This study reports MC metabolism by the cytochrome P-450 (CYP) and glutathione S-transferase (GST) pathways in mouse lung tissue, with particular reference to the Clara cell. The researchers reached the following conclusions:

1. Exposure levels of 1000 ppm MC and greater produced increased levels of DNA synthesis in Clara cells isolated from exposed mice in culture compared to controls, indicating that these cells are primed to respond when exposed to MC;

2. A minimum dose of between 1000-2000 ppm of MC is required to cause vacuolation [the development of cavities] in the Clara cell when given for 6 hours; and

3. The only biochemical change which correlated with exposure to MC was lung levels of non-protein sulfhydryl compounds;

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

The researchers measured DNA damage in lung and liver cells from mice, rats, hamsters and humans. They observed increased DNA single strand (ss) breaks in mouse liver cells, after exposure to 4000-8000 ppm MC for 6 hours and in mouse lung cells after exposure to 2000-6000 ppm MC. No increase in ss breaks was detected in rat livers after exposure to 4000 ppm for 6 hours or in rat lungs after exposure to 4000 ppm for 3 hours. Increased numbers of ss breaks were also not detected in hamster and human liver cells after exposure to MC in vitro at concentrations up to 90 and 120 mM. In experiments on isolated mouse Clara cells, the authors observed increased DNA ss breaks in cells exposed to concentrations of MC of 5 mM and above. According to the authors, the study suggests that humans, rats and hamsters are insensitive to MC-induced liver cancer, because those species lack the high level of GST metabolic activity found in the mouse Clara cell.

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

The researchers used a variety of chromatography methods to isolate two mouse glutathione S-transferases (MT-1 and MT-2) metabolizing MC, comparing the observed enzyme activity with that detected in rat GST (GST 5-5 and GST 12-12). The authors stated as follows:

The difference seen in total methylene chloride metabolizing activity between rat and mouse in vivo, or in cytosol fractions, is more than 10 fold which does not appear to be attributable to a higher specific activity of mouse MT-1 compared to rat GST 5-5. At present the labile nature of rat GST 12-12 and mouse MT-2 preclude an assessment of the relative activities of these enzymes in the two species. However, it seems probable that the higher activity in the mouse is attributable to greater expression of the one or both enzymes in that species.

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

This study investigated the mutagenicity of MC in mammalian cells by inducing mutations at the HPRT locus of CHO cells in mouse livers through exposure to MC GST metabolites, formaldehyde (a MC metabolite) and 1,2-dibromoethane (1,2-DBE) (the reference genotoxin).

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

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

The researchers compared the spectrum of DNA mutations induced by exposure to MC with the mutations induced by formaldehyde and 1,2-DBE. The results provided a spectrum analysis of MC and 1,2-DBE-induced mutagenesis in mammalian cells and extended the previous observation of formaldehyde mutagenesis in human lymphoblasts. The results suggested to the researchers that formaldehyde-induced DNA damage can contribute to MC mutagenicity, but that the majority of the mutations were derived from other types of DNA damage, probably via an interaction of S- chloromethylglutathione with DNA. The researchers noted that a glutathione conjugate also plays a role in the mutagenicity of 1,2-DBE. The increases above background mutation frequency detected through this study were 24.7-fold for 1,2-DBE, 4.7-fold for formaldehyde, and 8-fold for MC.

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

This preliminary communication summarizes the results of a study comparing the distribution of the glutathione S-transferase (GST) isozyme putatively responsible for metabolizing methylene chloride in the lungs and livers of mice, rats and humans. The distribution of enzyme was visualized using oligonucleotide anti-sense probes complementary to the nucleotide sequences for the transferases. The results indicated that the GST-specific mRNA could be found in lungs and livers of all three species. Mouse liver cells (particularly the nuclei) and mouse lung cells appeared to stain more heavily for the GST mRNA than the lung or liver cells from rats or humans. Although the amount of GST-specific mRNA was not quantified in this study, the authors interpreted the data to suggest that, "* * * mouse tissues are stained much more heavily than sections from either rat or human." Based on the distribution of the GST mRNA, the author concluded that,

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

II. Public Participation


Written comments regarding the materials incorporated into the MC record through this notice must be postmarked by November 24, 1995. Four copies of these comments must be submitted to the Docket Office, Docket No. H-071B, U.S. Department of Labor, Room N-2625, 200 Constitution Avenue, NW., Washington, DC 20210. (202) 219-7894. All materials submitted will be available for inspection and copying at the above address. Materials previously submitted to the Docket for this rulemaking need not be resubmitted.

III. Authority

This document was prepared under the direction of Joseph A. Dear, Assistant Secretary of Labor for Occupational Safety and Health, U.S. Department of Labor, 200 Constitution Avenue, NW., Washington, DC 20210.

It is issued under section 6(b) of the Occupational Safety and Health Act (29 U.S.C. 655), and 29 CFR Part 1911.

Joseph A. Dear,
Assistant Secretary of Labor.

[FR Doc. 95-26228 Filed 10-23-95; 8:45 am]