OSHA's former limits for methyl chloride were 100 ppm as an 8-hour TWA, 200 ppm as a ceiling (not to be exceeded for more than five minutes in any three-hour period), and 300 ppm as a peak. The ACGIH has a 50-ppm 8-hour TLV-TWA limit and a 100-ppm 15-minute STEL for this substance, and NIOSH recommends the lowest feasible limit because it considers methyl chloride a potential occupational carcinogen. The proposed PELs were 50 ppm as an 8-hour TWA and 100 ppm as a 15-minute STEL; the final rule establishes these limits. Methyl chloride is a colorless, sweet-smelling gas.
There is considerable evidence in humans and some in animals demonstrating that exposure to methyl chloride by inhalation or dermal absorption produces narcosis and other central nervous system effects, including respiratory failure and death (ACGIH 1986/Ex. 1-3, p. 380). In animals, repeated exposures to 500 ppm or to higher concentrations can be life-threatening, but exposures to 300 ppm for 64 weeks caused no apparent effects (Smith and von Oettingen 1947/Ex. 1-527).
Reports in the earlier literature described by Fairhall (1969a/Ex. 1-848) indicate that moderate (not further specified) exposure causes ocular symptoms that may persist for weeks, while high (not further specified) exposure has severe effects on the central nervous system. Patty (1963a/Ex. 1-855) states that serious exposure causes central nervous system, liver and kidney, and bone marrow effects, with symptoms of ataxia, staggering gait, weakness, tremors, vertigo, speaking difficulty, and blurred vision. Symptoms may be of several weeks' duration or may even be permanent (Patty 1963a/Ex. 1-855).
The Dow Chemical Company (as cited in ACGIH 1986/Ex. 1-3, p. 380) studied the methyl chloride exposures of employees in 54 job classifications over a four-month period. Exposures ranged from 5 to 78 ppm methyl chloride (8-hour TWAs), averaged 30 ppm over the work shift, and occasionally included peaks as high as 440 ppm. Medical examination of these workers revealed no detectable effects of methyl chloride exposure. However, average eight-hour exposures in the range of 195 to 475 ppm caused symptoms of weakness, drowsiness, staggering gait, thickness of the tongue, and memory lapses in some of the exposed employees (Dow Chemical Company, as cited in ACGIH 1986/Ex. 1-3, p. 380).
In a study of six cases of industrial methyl chloride poisoning, workers chronically exposed to levels between 200 and 400 ppm developed neurotoxic symptoms after two or more weeks of exposure (Scharnweber, Spears, and Cowles 1974/Ex. 1-664). Symptoms included drowsiness, dizziness, mental confusion, clouded vision, staggering gait, and slurred speech, and symptoms sometimes recurred after apparent recovery and in the absence of renewed exposure.
Repko and co-workers (1976/Ex. 1-1165) found that workers exposed to concentrations of methyl chloride ranging from 7.4 to 70 ppm but averaging 33.6 ppm displayed a significant performance decrement, and that exposures below 100 ppm produced significant but transitory changes in functional capacity. OSHA will continue to monitor the toxicological evidence for methyl chloride and will re-evaluate the substance if this evidence suggests that this is appropriate.
OSHA received comments on methyl chloride from NIOSH and the Methyl Chloride Industry Association. NIOSH believes that methyl chloride is an appropriate substance for a Section 6(b) rulemaking because, in NIOSH's view, methyl chloride is a potential occupational carcinogen (Ex. 8-47; Tr. 3, pp. 97-98). The AFL-CIO (Ex. 194) agrees with NIOSH on this point. The Methyl Chloride Industry Association (MCIA) indicated its support of OSHA's proposed PELs for this substance and submitted material suggesting that methyl chloride may not be a potential occupational carcinogen (Ex. 148, pp. 2-4). MCIA submitted to the record a copy of the IARC monograph and recent supplement on methyl chloride, which conclude that the evidence for the carcinogenicity of methyl chloride is inadequate in both animals and humans. In the final rule, OSHA is establishing an 8-hour TWA of 50 ppm and a 15-minute STEL of 100 ppm for methyl chloride. The Agency concludes that these two limits together will substantially reduce the significant risk of neurotoxic effects, including functional impairment, performance decrements, headaches, dizziness, slurred speech, and staggering gait, which together constitute material impairments of health. These effects have been associated with exposure to this substance at the levels permitted by OSHA's former PEL. OSHA will continue to monitor the literature on the toxicity of methyl chloride to determine whether other action is appropriate.