OSHA previously had a ceiling limit of 100 ppm for alpha-methyl styrene. The Agency proposed revising this limit to 50 ppm TWA with a STEL of 100 ppm, and NIOSH (Ex. 8-47, Table N1) concurred with OSHA's proposed limits for this substance, which are established in the final rule. alphaMethyl styrene is a polymerizable, colorless liquid.
OSHA's former ceiling limit of 100 ppm is based on data developed in 1955 by the Dow Chemical Company (as cited in ACGIH 1986/Ex. 1-3, p. 410) and by Wolf, Rowe, McCollister et al. (1956/Ex. 1-404). These data demonstrated that seven-hour-per-day, five-day-per-week exposures to 200 ppm alpha-methyl styrene for six months produced no ill effects in rats, guinea pigs, rabbits, or monkeys.
In humans, however, these authors reported that a two-minute exposure to 200 ppm caused eye irritation and complaints about this substance's unpleasant odor. OSHA received no comments, other than NIOSH's, on its proposal to revise the limit for alpha-methyl styrene.
Therefore, to ensure that workers are protected against the acute irritant effects of this substance, OSHA is establishing a 50-ppm 8-hour TWA limit and a 100-ppm 15-minute STEL in the final rule. The Agency concludes that these combined limits will substantially reduce the exposure-related risk of irrita-tion and odor effects, which together constitute material impairments of health.