MESITYL OXIDE

OSHA comments from the January 19, 1989 Final Rule on Air Contaminants Project extracted from 54FR2332 et. seq. This rule was remanded by the U.S. Circuit Court of Appeals and the limits are not currently in force.

CAS: 141-79-7; Chemical Formula: (CH₃)₂C = CHCOCH₃

OSHA’s previous limit for mesityl oxide was 25 ppm as an 8-hour TWA. The Agency proposed revising this limit to 15 ppm as an 8-hour TWA and 25 ppm as a 15-minute STEL, based on the ACGIH (1986/Ex. 1-3) recommendation. NIOSH has a 10-ppm REL for mesityl oxide. The final rule establishes a 15-ppm 8-hour TWA and a 25-ppm 15-minute STEL for mesityl oxide, which is an oily, colorless liquid with a peppermint odor.

Silverman, Schulte, and First (1946/Ex. 1-142) found that a majority of test subjects experienced eye irritation on exposure to 25 ppm mesityl oxide and nasal irritation at 50 ppm. A toxicity data sheet published by the Shell Chemical Corporation (1957, as cited in ACGIH 1986/Ex. 1-3, p. 361) confirms 25 ppm as the maximum comfort level. Smyth, Seaton, and Fischer (1942/Ex. 1-378) reported liver and kidney damage among rats and guinea pigs exposed to 100 ppm mesityl oxide for six weeks; no adverse effects were reported for animals exposed to 50 ppm. After reviewing these data, the ACGIH (1986/Ex. 1-3, p. 361) concluded that the former TLV of 25 ppm should be reduced to 15 ppm TWA and 25 ppm as a 15-minute STEL because of the greater systemic toxicity of mesityl oxide compared with that of other saturated ketones. NIOSH (1978f, as cited in ACGIH 1986/Ex. 1-3, p. 361), relying on the same data, recommended a limit of 10 ppm as a 10-hour TWA.

Studies indicate that eye irritation occurs following brief exposure to 25 ppm of mesityl oxide, and nasal irritation is experienced at the 50-ppm level. Animal studies show liver and kidney damage in experimental animals exposed to 100 ppm. NIOSH’s comment (Ex. 8-47, Table N2; Tr. p. 3-86) was the only one received by the Agency on its proposal to revise the limits for mesityl oxide. NIOSH based its lower recommended limit on a belief that the eye irritation caused by exposure to mesityl oxide might be more severe than the irritation caused by exposure to the other ketones because mesityl oxide has a higher molecular weight than the lower ketones. OSHA is not persuaded by this argument because the evidence that brief exposure to 25 ppm mesityl oxide causes eye irritation is based on actual human exposures to mesityl oxide at that level; that is, NIOSH’s argument would be reasonable if the 25 ppm short-term limit were being established by analogy to the effects of another (lower-molecular-weight) ketone.

After reviewing the health evidence for this substance, OSHA finds that the proposed 15-ppm TWA and 25-ppm STEL limits are protective against both the acute and chronic effects demonstrated to be caused by exposure to this substance. In the final rule, OSHA concludes that a TWA PEL of 15 ppm and a STEL of 25 ppm are necessary to protect employees both from the possible liver and kidney damage associated with chronic exposures and the eye irritation resulting from elevated short-term exposures to mesityl oxide. The Agency considers both the systemic and the irritant effects of exposure to mesityl oxide material impairments of health and functional capacity. To reduce these risks, OSHA is establishing limits for mesityl oxide of 15 ppm as an 8-hour TWA and 25 ppm as a 15-minute STEL.