OSHA formerly had an 8-hour TWA limit of 25 ppm for n-propyl nitrate. The ACGIH has a 25-ppm TWA and a 15-minute STEL of 40 ppm; these limits were proposed by OSHA. NIOSH (Ex. 8-47) concurred with these proposed limits, and these limits are established in the final rule. n-Propyl nitrate is a pale yellow liquid with a sickly sweet odor.
Rats inhaling propyl nitrate vapor for four hours at a concentration of 10,000 ppm exhibited cyanosis and methemoglobinemia before they died (Hood 1953, as cited in ACGIH 1986/Ex. 1-3, p. 505). The intravenous LD(50) in unanesthetized rabbits has been reported to be between 200 and 250 mg/kg; in anesthetized dogs and cats, intravenous doses of between 100 and 200 mg/kg were usually fatal (Murtha, Stabile, and Wills 1956/Ex. 1-649). Murtha and associates (1956/Ex. 1-649), who conducted these studies, concluded that n-propyl nitrate exerts a direct action on the vascular smooth muscle and that the ensuing cardiac effects and respiratory depression contribute to the compound's hypotensive action (Murtha, Stabile, and Wills 1956/Ex. 1-649). Inhalation trials in mice, rats, hamsters, guinea pigs, and dogs have established 4-hour LC(50) values ranging from 9000 to 10,000 ppm for rats, 6000 to 7000 ppm for mice, and 2000 to 2500 ppm for dogs. Dogs survived repeated exposures (six hours/day, five days/week) at 260 ppm for six months, although slight clinical signs were observed during the first two weeks of exposure (Rinehart, Garbers, Greene, and Stoufer 1958/Ex. 1-524). The percutaneous toxicity of n-propyl nitrate is low but may cause inflammation and thickening of the skin after repeated exposures; these effects are sometimes transient (ACGIH 1986/Ex. 1-3, p. 505). To protect against cardiovascular and respiratory depressant effects requires both TWA and STEL limits. NIOSH (Ex. 8-47) was the only commenter to the rulemaking record for this substance.
In the final rule, OSHA is retaining the PEL of 25 ppm TWA and adding a STEL of 40 ppm for n-propyl nitrate. The Agency concludes that this combined PEL-STEL limit will protect workers against the significant risk of cyanosis, methemoglobinemia, and hypotension, all material health impairments are potentially associated with exposure to n-propyl nitrate at levels above the 8-hour TWA PEL.