Previously, OSHA had an 8-hour TWA limit of 350 ppm for methyl chloroform. The ACGIH has established the same TWA limit in addition to a TLV-STEL of 450 ppm; NIOSH recommends a 15-minute ceiling limit of 350 ppm. The Agency proposed to retain its 8-hour TWA limit and to add a STEL of 450 ppm; NIOSH concurs that these limits are appropriate but would express them as ceilings rather than as TWAs (Ex. 8-47, Table N7). The final rule retains an 8-hour TWA of 350 ppm and adds a STEL of 450 ppm for methyl chloroform, which is a clear, nonflammable liquid.
The primary health effects associated with exposure to methyl chloroform are anesthesia and cardiac sensitization. The oral toxicity of methyl chloroform is low, with LD(50) values ranging from 5.7 to 12.3 g/kg for rats, mice, rabbits, and guinea pigs. This substance does, however, defat the skin on contact, causing redness and scaling (Torkelson, Oyen, McCollister, and Rowe 1958/Ex. 1-768). Skin absorption is relatively insignificant: the acute percutaneous LD(50) in rabbits is greater than 16 g/kg, and slight, reversible irritation was observed from applications of 0.5 g/kg to rabbit skin for 90 days (Torkelson, Oyen, McCollister, and Rowe 1958/Ex. 1-768). Repeated exposures of animals to concentrations between 1000 and 10,000 ppm for three months produced anesthesia and lung and liver damage in some species, but exposure to 500 ppm of methyl chloroform vapor for seven hours daily, five days/week for six months caused no toxic changes in guinea pigs, rabbits, or monkeys (Torkelson, Oyen, McCollister, and Rowe 1958/Ex. 1-768). Other animal studies (Gehring 1968/Ex. 1-637; Plaa, Evans, and Hine 1958/Ex. 1-754; Rowe, Wujkowski, Wolf et al. 1963/Ex. 1-687) have reported that methyl chloroform has low hepatotoxicity, but cardiac sensitization has occurred at high doses (5000 to 10,000 ppm)(Rennick, Malton, Moe, and Seevers 1949/Ex. 1-864; Trochimowicz, Reinhardt, Mullin et al. 1976/Ex. 1-992). Tests in rats and mice for teratogenicity and carcinogenicity have demonstrated negative results (Schwetz, Leong, and Gehring 1975/Ex. 1-757; NIOSH 1976m, as cited in ACGIH 1986/Ex. 1-3, p. 382; Weisberger 1977/Ex. 1-694).
In humans, it has been reported that anesthetic effects may begin to occur at methyl chloroform concentrations approaching 500 ppm (Stewart, Gay, Schaffer et al. 1969/Ex. 1-529). Deaths from anesthesia and/or cardiac sensitization have been noted in employees working in confined areas (Patty 1963d/Ex. 1-856). Kramer and co-workers (1978/Ex. 1-515) conducted an epidemiological study of men and women exposed for periods ranging from several months to six years to methyl chloroform at levels that occasionally exceeded 200 ppm; when compared to matched-pair controls, no adverse exposure-related effects were found (Kramer, Ott, Fulkerson et al. 1978/Ex. 1-515).
Commenters supplied conflicting evidence to the record on the toxicity of methyl chloroform. The Workers Institute for Safety and Health (WISH) (Ex. 116, Tr. pp. 7-134, 135) noted that there is an extensive amount of recent information on this substance. In particular, WISH mentioned three recent studies (McLeod et al. 1987, Karlsson et al. 1987, and Mackay et al. 1987) that demonstrate that methyl chloroform causes chronic cardiac toxicity on long-term exposure, may have toxic effects on brain cells, and may cause behavioral changes after 3.5-hour exposures to 175 to 350 ppm. WISH believes that these studies and others warrant a further reduction in the PELs for methyl chloroform. However, the Halogenated Solvents Industry Alliance (Ex. 186) criticized these studies and believes that the PELs for methyl chloroform are appropriate.
In the final rule, OSHA is retaining its PEL of 350 ppm as an 8-hour TWA and adding a STEL of 450 ppm for methyl chloroform. The Agency concludes that this combined PEL-STEL limit will protect workers against the significant risk of narcotic and cardiac-sensitizing effects, which constitute material health impairments that are potentially associated with exposure to methyl chloroform at the elevated short-term levels permitted by an 8-hour TWA limit alone.