OSHA formerly had no limit for methyl isoamyl ketone (MIAK). The ACGIH has established an 8-hour TLV-TWA of 50 ppm. NIOSH also recommends a 50-ppm TWA limit for MIAK. The proposed PEL was 50 ppm as an 8-hour TWA, with which NIOSH (Ex. 8-47, Table N1) concurs. The final rule establishes these limits. Methyl isoamyl ketone is a colorless, clear liquid with a pleasant odor.
The oral LD(50) value of methyl isoamyl ketone in rats is 1.67 g/kg (Smyth, Carpenter, Weil et al. 1962/Ex. 1-441). No data relating exposure levels to specific effects in humans have been reported. However, the ACGIH (1986/Ex. 1-3, p. 400) believes that MIAK is likely to be more irritating and a more potent narcotic than is the case for methyl isobutyl ketone.
The NIOSH criteria document on the ketones (1978f, as cited in ACGIH 1986/Ex. 1-3, p. 400) states that "because methyl isoamyl ketone contains one more carbon atom than does methyl isobutyl ketone, methyl [isoamyl] ketone might produce irritation and narcosis at concentrations at least as low as those at which methyl isobutyl ketone produces these effects," and NIOSH thus recommends a 50-ppm TWA for MIAK, corresponding to NIOSH's recommendation for methyl isobutyl ketone (NIOSH 1978f, as cited in ACGIH 1986/Ex. 1-3, p. 400). NIOSH submitted the only comments on MIAK.
In the final rule, OSHA is establishing an 8-hour TWA limit of 50 ppm for methyl isoamyl ketone. The Agency concludes that this limit will protect workers against the significant risk of narcotic and irritant effects, which constitute material health impairments that are associated with exposure to MIAK at levels above the new PEL.