ndineae and papillary muscles). Exposure concentrations at 250 and 100 ppm were not teratogenic.40,41
Rats exposed to methyl chloride40 and rats and rabbits exposed to methyl bromide in another study42 showed no teratogenic effects. No teratology studies employing methyl iodide were found in the literature reviewed.
In addition to the previously described teratogenic effects, degeneration and atrophy of the seminiferous tubules were induced in male rats exposed by inhalation to methyl chloride at a concentration of 1,000 ppm.26 These effects were absent in rats exposed at 0, 50, and 225 ppm concentrations. Mice similarly exposed to methyl chloride exhibited no adverse reproductive effects.26
No studies on reproductive effects from exposure to methyl bromide or methyl iodide have been reported.
Human Health Effects
Most of the available information related to the toxic effects on humans exposed to methyl chloride, methyl bromide, methyl iodide is derived from accidental exposures to relatively high concentrations.
Symptoms of acute exposure to these compounds are relatively similar, consisting of headache, nausea, vomiting, drowsiness, dizziness, giddiness, diarrhea, confusion, ataxia, slurred speech, paralysis, convulsions, delirium, coma, and death.43-47 The lungs, liver, kidney, and brain appear to be the primary target organs in cases of severe poisoning.
No human studies to evaluate the possible mutagenic, carcinogenic, or teratogenic effects from exposure to these compounds are currently available.
The chemicals discussed in this bulletin, methyl chloride, methyl bromide, and methyl iodide, are alkylating agents that have been shown to induce cancer in rats and mice.
The studies which indicated the potential for these compounds to induce cancer in experimental animals are not without their shortcomings. The strains of animals used, the doses and routes selected for administration of the test compounds, and the fact that there was no coordinated study designed to test these compounds as a class impose limitations 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 th