[Federal Register Volume 60, Number 205 (Tuesday, October 24, 1995)]
[Proposed Rules]
[Pages 54462-54465]
From the Federal Register Online via the Government Publishing Office [www.gpo.gov]
[FR Doc No: 95-26228]



=======================================================================
-----------------------------------------------------------------------

DEPARTMENT OF LABOR

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 

[[Page 54463]]
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 ae 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.

SUPPLEMENTARY INFORMATION:

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.

[[Page 54464]]

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 

[[Page 54465]]
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

Comments

    Written comments regarding the materials incorporated into the MC 
rulemaking 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]
BILLING CODE 4510-26-P