OSHA had an 8-hour TWA limit of 2 ppm for nitric acid. The ACGIH has the same TWA limit and a 15-minute STEL of 4 ppm, and NIOSH recommends a TWA limit of 2 ppm. The proposal retained the 8-hour TWA PEL of 2 ppm for nitric acid and added a STEL of 4 ppm; NIOSH (Ex. 8-47, Table N1) concurred with these limits, which are established in the final rule. Nitric acid is a fuming colorless or yellowish liquid.
Rats receiving a single exposure to nitric acid mist at a concentration of 63 mg/m3 exhibited no apparent adverse effects (Diggle and Gage 1954/Ex. 1-729). Chronic exposure to airborne nitric acid vapor or mist at unspecified levels was reported to cause chronic bronchitis, pneumonitis (Fairhall 1957i, as cited in ACGIH 1986/Ex. 1-3, p. 428), and tooth erosion (Lynch and Bell 1947/Ex. 1-793). Nitric acid's irritant potential is considered similar to that of other strong acids; it typically exists in conjunction with nitrogen dioxide, which is regarded as being more hazardous (ACGIH 1986/Ex. 1-3, p. 428). No comments, other than NIOSH's, were submitted on this substance.
OSHA is retaining the 8-hour TWA PEL of 2 ppm and adding a STEL of 4 ppm for nitric acid in the final rule. The Agency concludes that this combined limit will protect workers against the significant risk of irritation, chronic pulmonary disease, and dental corrosion, which together constitute a material impairment of health.