OSHA's former limit for o-methylcyclohexanone was 100 ppm as an 8-hour TWA, with a skin notation. The Agency proposed revising this limit to 50 ppm as a TWA and 75 ppm as a STEL, and to retain the skin notation; NIOSH (Ex. 8-47, Table N1) concurred with this proposal. These limits are established in the final rule and are consistent with the limits recommended by the ACGIH. ortho-Methylcyclohexanone is a somewhat viscous liquid with an acetone-like odor.
o-Methylcyclohexanone has both irritative and narcotic effects at relatively low concentrations. The commercial product contains a mixture of isomers; however, toxicity data describe the effects of the ortho isomer only. Gross (as cited in Lehman and Flury 1943a/Ex. 1-962) reported that 450 ppm had irritative effects on the eyes and respiratory systems of rabbits, and 2500 ppm produced narcotic effects (Gross, as cited in Lehman and Flury 1943a/Ex. 1-962). Treon et al. (1943a/Ex. 1-393) reported the oral LD(50) to be between 1 and 1.25 g/kg for rabbits. Eye problems were observed at about 500 ppm, but exposure to 182 ppm produced no adverse effects (Treon, Crutchfield, and Kitzmiller 1943a/Ex. 1-393).
Rowe and Wolf (1963, as cited in ACGIH 1986/Ex. 1-3, p. 386) reported that concentrations of 100 ppm had no narcotic effects in humans but could cause irritation. No comments, other than NIOSH's, were received on OSHA's proposal to revise the limit for this substance.
Because a level of 100 ppm may represent an effect level for irritation in humans (Rowe and Wolf 1963, as cited in ACGIH 1986/Ex. 1-3, p. 386), OSHA finds that a reduction in its 100-ppm PEL is warranted. The Agency considers the irritation caused by exposure to this substance to be a material impairment of health. Therefore, OSHA is revising its limit for o-methylcyclohexanone to 50 ppm as an 8-hour TWA and 75 ppm as a 15-minute STEL. OSHA is also retaining its skin notation for this substance.