The current OSHA limit for methyl n-amyl ketone is 100 ppm TWA. OSHA did not propose a revision to its current limit of 100 ppm, and this limit is being retained in the final rule. NIOSH (Ex. 150) agreed that the 100-ppm PEL was sufficiently protective.
Johnson et al. (1978/Ex. 1-335) found no neurologic impairment in rats and monkeys exposed to 131 ppm or 1025 ppm methyl n-amyl ketone for nine months. No gross or histopathologic changes were found (Johnson, Setzer, Lewis, and Hornung 1978/Ex. 1-335). Because of the absence of any human data indicating the concentration of methyl n-amyl ketone that produces sensory irritation, ACGIH (1986/Ex. 1-3, p. 374) believed it prudent to reduce the TLV-TWA from 100 ppm to 50 ppm. NIOSH (1978f, as cited in ACGIH 1986/Ex. 1-3, p. 374) concluded that there was no basis for revising the 100-ppm OSHA limit, since the evidence showed methyl n-amyl ketone's irritant effects to be equivalent to those of 2-pentanone, which had a recommended limit of 150 ppm.
No neurological or histopathological effects were observed at 131 ppm. The ACGIH's 50-ppm TLV applies an additional factor of safety to this no-observed-effect level, while the NIOSH REL is based on a judgment that such a reduction is unnecessary. In the NPRM, OSHA requested additional information on the health effects of methyl n-amyl ketone; however, no information was received into the record.
OSHA notes that the current 100-ppm PEL is well below the highest level (1025 ppm) reported to be associated with any adverse effects. Because histopathological examination was conducted on the organs of the rats and monkeys tested, OSHA is confident that the existing 100-ppm limit is not likely to be associated with adverse affects and that further reducing this limit would not result in a substantial reduction in risk. Therefore, OSHA is not revising its 100-ppm TWA limit for methyl n-amyl ketone at this time.