ions on the interpretation of the results. However, in NIOSH's judgement the collective data of these studies are sufficient to indicate the potential for carcinogenicity of these substances.
Also, methyl chloride induced degeneration and atrophy of the seminiferous tubules in treated male rats. It has also demonstrated teratogenicity in mice but not rats. No teratogenic effects were found in rats and rabbits exposed to methyl bromide.
There are several classifications for identifying a substance as a carcinogen. Such classifications have been developed by the National Toxicology Program,50 the International Agency for Research on Cancer,51 and OSHA in its "Identification, Classification, and Regulation of Potential Occupational Carcinogens" 29 CFR 1990.52 also known as "The OSHA Cancer Policy".53 NIOSH considers the OSHA classification the most appropriate for use in identifying potential occupational carcinogens* .54 All three of the monohalomethanes in this document have been shown to produce malignant neoplasms in experimental studies in either rats or mice. Methyl chloride has been tested in mice and found to be a teratogen. Based on this evidence, NIOSH that methyl chloride, methyl bromide, and methyl iodide be considered as potential occupational carcinogens and that methyl chloride be considered a potential occupational teratogen.
The excess risk of cancer to workers or the inductin of a teratogenic response in the children of workers exposed to specific airborne concentrations of these compounds has not yet been determined, but the probability of developing these adverse effects would be decreased by reducing exposure. As prudent public health policy employers should voluntarily assess the conditions under which workers may be exposed to these monohalomethanes and take all reasonable precautions to reduce exposures to the fullest extent feasible.
NIOSH is particularly concerned that the occupational exposure to methyl bromide may increase as a result of using this compound as a substitute fumigant for the recently restricted use of the fumigant ethylene dibromide (EDB).55 Chronic toxicology studies of methyl bromide are currently being conducted,37 and NIOSH recommends that the results of these studies be thoroughly evaluated before significantly expanding the use of methyl bromide.
Guidelines recommended in the Appendix for minimizing worker exposure to these monohalomethanes are general in nature and should be adapted to specific work situations as required.
*"'Potential occupational carcinogen' means any substance, or combination or mixture of substances, which causes an increased incidence of benign and/or malignant neoplasms, or a substantial decrease in the latency period between exposure and onset of neoplasms in humans or in one or more experimental mammalian species as the result of any oral, respiratory or dermal exposure, or any other exposure which results in the induction of tumors at a site other than the site of administration. This definition also includes any substance which is metabolized into one or more potential occupational carcinogens by mammals" (29 CRF 1990.103). [return to text]
- Tatken RL, Lewis RJ (ed): Registry of Toxic Effects of Chemical Substances. 1981-2 Edition. DHHS (NIOSH) Publication No. 83-107. U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, Cincinnati, Ohio, Volume II (June 1983).
- Davis LN, Strange JR, Hoecker JE, Howard PH, Santodonato J: Investigation of Selected Potential Environmental Contaminants--Monohalomethanes. U.S. Department of Commerce, National Technical Information Service, (NTIS PB 276 483) Springfield, Virginia (1977).
- Windholz M, Budavaris, Stroumtsos LY, Fertig MN: The Merck Index--An Encyclopedia of Chemicals and Drugs. Ninth edition. Merck and Co. , Inc., Rahway, New Jersey, pp. 788-89, 791 (1976).
- Faith WL, Keyes DB, Clark RL: Industrial Chemicals. John Wiley and Sons, Inc., New York, pp. 198-99, 418-22 (1950).
- Cohen JM, Dawson R, Koketsu M: