from 1.0 to 1.5 ppm, but the Russians reported that they had little confidence in the values produced by the “GANK” measurement system they were using. Measurements taken in recent months with a new measurement system (Draeger CMS) have indicated that ammonia concentrations were below the detection limit of 2 ppm for the new system.
The SMACs for exposure durations of 1 h to 180 d were set by King-Lit Wong in 1994 (Table 3-1) and were based on mucosal irritation, which is the most sensitive toxic end point for exposures at low to moderate concentrations of ammonia. To establish the SMACs, Wong used data from subjects who were not inured to ammonia because, in adapted workers, mild inflammation (conjunctival erythema) was reported even in those who did not complain of discomfort at exposures to 20 ppm (Vigliani and Zurlo 1955; Wong 1994). The 1- and 24-h SMACs were set at 30 and 20 ppm, respectively, to permit no more than slight mucosal irritation during emergency situations, whereas the SMACs for 7, 30, and 180 d were set at 10 ppm, the estimated maximum nonirritating concentration.
The 1-h SMAC was based on a report (MacEwen et al. 1970) that exposure of volunteers to 30 ppm of ammonia for 10 min was not perceptible in three of five noninured subjects and was barely perceptible in the other two. Thus, a 1-h exposure to 30 ppm of ammonia is expected to produce no more than mild irritation, which is acceptable for emergency situations.
For the 24-h SMAC, a lower concentration was desired so as to reduce the degree of discomfort that astronauts would have to endure during a longer emergency situation. A concentration of 20 ppm was selected, because it produced only eye and respiratory discomfort in workers (Vigliani and Zurlo 1955; Furguson et al. 1977).
No data were available on the maximum concentration that would be non-irritating for longer-term exposures to ammonia. Therefore, the 7-, 30-, and 180-d SMACS were based on a comparison of dose-response data from occupational and laboratory studies in humans (Vigliani and Zurlo 1955; Verberk 1977; MacEwen et al. 1970). A lowest-observed-adverse-effect level (LOAEL) of 20 ppm was used to set the 24-h SMAC, but, rather than apply the traditional safety factor of 10 for extrapolation from the LOAEL to a no-observed-adverse-effect level (NOAEL), dose-response data from several studies in the literature were used to decrease the safety factor from 10 to 2. Verberk (1977) reported that, for 1-h exposures, a reduction in the ammonia concentration from 120 to 80 ppm decreased the reported degree of eye irritation from “nuisance” to between “just perceptible” and “distinctly perceptible,” whereas a reduction from 80 to 50 ppm decreased the irritation ratings from “just perceptible” and “distinctively noticeable” to “just noticeable.” MacEwen et al. (1970) reported that, for 10-min ex-