Previously, OSHA had an 8-hour TWA limit of 100 ppm for propylene oxide. The ACGIH has a limit of 20 ppm TLV-TWA. The proposed PEL was an 8-hour TWA of 20 ppm, and the final rule establishes this revised limit. Propylene oxide is a colorless, highly flammable, volatile, and ethereal liquid.
The health hazards associated with exposure to this substance are primary skin, eye, and respiratory irritation, as well as central nervous system depression. The oral LD(50) values reported for rats and guinea pigs are 930 mg/kg and 690 mg/kg, respectively. In mice, the inhalation LC(50) has been reported to be at 1740 ppm for 4 hours. Dogs and guinea pigs exposed for 4 hours at 2000 and 4000 ppm, respectively, died (NIOSH 1977i/Ex. 1-1182). Although only some species tolerate daily exposures to 200 ppm, all species tested tolerated 100 ppm without ill effects (Rowe, Hollingsworth, Oyen et al. 1956/Ex. 1-609). Jacobson and associates (1956/Ex. 1-702) considered the toxic effects of propylene oxide to be one-half to one-third as intense as those of ethylene oxide (Jacobson, Hackley, and Feinsilver 1956/Ex. 1-702).
Corneal burns and skin necrosis, as well as respiratory and pulmonary irritation, have been reported in humans as a result of direct contact with the liquid or vapor (Patty 1963h/Ex. 1-857); central nervous system effects include ataxia, incoordination, and general depression.
OSHA received several comments on propylene oxide. Lawrence Birkner, Manager of Safety and Industrial Hygiene for ARCO Petroleum and Chemical Company (Tr. 3-229/3-245), reported that his company has an internal limit for propylene oxide of 20 ppm and that about "98 or 99 percent of...[ARCO's] exposures are [presently] below the current ACGIH TLVs" (Tr. 3-243).
Richard E. Sanderson, Director of the Office of Federal Activities for the EPA, commented that the discussion of propylene oxide's health effects in the proposal neglected to mention this substance's carcinogenicity or its ability to cause adverse reproductive effects (Ex. 3-746). NIOSH (Ex. 8-47, Table N6B) agrees with EPA that propylene oxide is a potential occupational carcinogen that warrants a full Section 6(b) rulemaking. NIOSH bases its inclusion on an NTP bioassay in rats and mice that demonstrates "some evidence" of carcinogenicity in rats and "clear evidence" of carcinogenicity in mice (Ex. 8-47). In response to these commenters, OSHA states that the Agency is aware of propylene oxide's serious health effects and is monitoring the literature on this substance closely.
OSHA establishes an 8-hour TWA limit of 20 ppm for propylene oxide in the final rule to protect workers against the significant risk of primary irritation and CNS depression, which constitute material health impairments that are associated with exposure to propylene oxide at levels above the revised PEL. The Agency concludes that this limit will substantially reduce these significant risks.