ing pulmonary edema, can occur several hours after an acute exposure and are probably related to inflammation resulting from cellular necrosis in lung tissues.

Several studies have shown that exposure to HNO3 at concentrations up to 0.2 ppm for 4 hr produces no observable effects in healthy humans. Concentrations as low as 0.05 ppm might cause some mild respiratory effects in asthmatic individuals. The subcommittee estimates that a no-effect level for HNO3 is 1.0 ppm (ceiling) for periods up to 1 hr for healthy individuals and 0.05 ppm (ceiling) for periods up to 1 hr for asthmatic individuals and others with compromised respiratory function. However, as described in the following subsections, an exposure-response function (ERF) cannot be established for HNO3 in healthy individuals or in asthmatic individuals, because too few data have been published on the effect levels of HNO3 for animals or humans.

HEALTHY ADULTS

The no-observed-effect level (NOEL) for humans exposed to HNO3 is 0.2 ppm for 4 hr (Aris et al. 1993) (see Chapter 3 and Appendix F). In that study, 10 persons were exposed and examination by bronchoscopy 18 hr later, revealing no adverse effects. If Haber's rule applies, an acceptable exposure concentration is 0.8 ppm (i.e., 0.2 ppm × 4) for 1 hr and 1.6 ppm for 30 min. Those concentrations are based on observing no effects in 10 individuals. When a small number of subjects are used (10 in this case), little confidence can be placed in the observation of no effects, and a confidence limit should be calculated for the observation. The true probability of an observable effect at a concentration of 0.2 ppm for 4 hr could be as high as 0.067, with a 50% chance of observing no effects in 10 individuals. That is, the probability of not observing an effect in 10 individuals is (1-0.067)10 = 0.50. That calculation provides a means of obtaining a conservative risk estimate even though no responses occurred in a small sample. If the probability of an observable effect is 0.067 at 0.2 ppm for 4 hr, and if Haber's rule applies, an exposure at 0.8 ppm for 1 hr implies the same risk of 0.067, or an incidence of 67 per 1,000 individuals. Thus, an incidence of 0.067 at a contamination of 0.8 ppm for 1 hr is one data point that might be used in establishing an ERF.

Abraham et al. (1982) exposed seven normal sheep to HNO3 vapor



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