OSHA formerly had a limit of 5 ppm TWA, with a skin notation, for methyl iodide. The ACGIH has a TLV-TWA limit of 2 ppm, with a skin notation, for methyl iodide, and classifies it as a suspected human carcinogen (A2). NIOSH recommends reducing exposure to the lowest feasible limit, and also considers this chemical a carcinogen. The proposed PEL was an 8-hour TWA PEL of 2 ppm, with a skin notation; the final rule establishes these limits. Methyl iodide is a colorless, sweet-smelling liquid that turns yellow, red, or brown when exposed to light and moisture.
Methyl iodide has been reported to have an LD(50) in rats of 150 to 200 mg/kg; liver damage was evident after these lethal exposures (Kutob and Plaa 1962/Ex. 1-61). Fifteen-minute exposures to 3800 ppm were fatal in rats (Chambers et al. 1950, as cited in ACGIH 1986/Ex. 1-3, p. 399), and Bachem (1927/Ex. 1-1013) has reported that methyl iodide is six times as toxic in mice as methyl bromide. Inhalation studies have shown eye irritation and depressed body weight in rats as a result of 14-week exposures to 30 and 60 ppm (Blank, Nair, Roloff, and Ribelin 1984/Ex. 1-619). The same authors observed fatalities in rats within four weeks of exposure to 143 ppm; 10 ppm was reported to be a no-effect level.
In industry, fatalities have occurred from methyl iodide poisoning in chemical workers (Garland and Camps 1945/Ex. 1-1190; Appel, Galen, O'Brien, and Schoenfeldt 1975/Ex. 1-1076). However, the exposure levels associated with these fatal overexposures are not known (ACGIH 1986/Ex. 1-3, p. 399).
In tests of carcinogenicity, methyl iodide produced local sarcomas in rats injected subcutaneously and lung tumors in mice given intraperitoneal injections (Druckrey, Kruse, Preussman et al. 1970/Ex. 1-246; Poirier, Stoner, and Shimkin 1975/Ex. 1-686). These carcinogenic effects occurred at a dosage approximately equivalent to a daily 8-hour exposure to 20 or 25 ppm for an adult human (ACGIH 1986/Ex. 1-3, p. 399). OSHA received comments on methyl iodide's health effects from the American Industrial Hygiene Association (AIHA) (Ex. 8-16; Tr. 3-309) and from NIOSH (Ex. 8-47, Table N6A). The AIHA stated that "[a] number of potentially carcinogenic substances for which PEL revisions are proposed appear to have been misclassified concerning their toxic effect" (Ex. 8-16, p. 6). The AIHA includes methyl iodide in this group of substances. As discussed in the introduction to Section VI.C, OSHA did not intend the proposal's classifications to have regulatory implications; rather, both this classification and that of the final rule are intended only to reflect the health endpoint used by the ACGIH or NIOSH as the basis for selecting a particular PEL for a given substance, and to facilitate generic rulemaking. NIOSH (Ex. 8-47, Table N6A) agreed that the methyl iodide limit established by OSHA is appropriate, but pointed out that this substance could be classified as an occupational carcinogen.
In the final rule, OSHA establishes an 8-hour TWA limit of 2 ppm, with a skin notation, for methyl iodide. The Agency concludes that these limits will protect workers from the significant risk of irritation and liver and kidney damage, which are material impairments of health that are associated with exposure to methyl iodide in the workplace. The skin notation is needed to prevent dermal absorption of toxic amounts of methyl iodide.