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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR GB (SARIN) 29 min/m3 in monkeys exposed to hot temperatures (80â95Â°F) and moderate relative humidity (35% to 55%) (Oberst et al. 1952). On the basis of the monkey and human studies, a lethal Ct (concentration Ã time) for humans was estimated to be slightly higher than 12,800 mg-min/m 3 (McGrath et al. 1951). Silver (1953) estimated the LCt50 to be about 15,000 mg-min/m3; the estimate was based on studies of McGrath et al. (1951) who investigated the inhibition of cholinesterase (ChE) in monkeys and men exposed to GB. The subcommittee believes that, on the basis of available data, 10,000 mg-min/m3 is a conservative estimate of the human LCt 50. The preliminary studies involved exposure to the right forearm, and data from these studies were used to determine the whole-body exposures. Therefore, the subcommittee concludes that CDEPAT's LCt50 estimate of 10,000 mg-min/m3 for GB is scientifically valid. ECt50 for Threshold Effects CDEPAT's proposed ECt50 estimate for threshold (minimal) effects following percutaneous exposure to GB vapor is 1,200 mg-min/m3, assuming light clothing and exposure durations of 30 to 50 min. There is no existing ECt50 estimate for threshold effects (CDEPAT 1994). The Army's proposed estimate of 1,200 mg-min/m3 is based on human data using the most appropriate study (McGrath et al. 1951). Exposures of 190 to 1,010 mg-min/m3 (concentrations of 21 to 92 mg/m3; durations of 9 to 11 min) resulted in ChE levels of 95% to 108% of baseline; all subjects were asymptomatic, supporting a no-effect level of < 1,000 mg-min/m3 (McGrath et al. 1951). Humans exposed at 1,255 to 1,850 mg-min/m3 (concentrations of 81 to 109 mg/m3; exposure durations of 11.5 to 20 min) had ChE activities ranging from 31% to 90%. Two of nine individuals were asymptomatic, and the other seven experienced sweating that persisted for a minimum of 24 hr and a maximum of 30 days (McGrath et al. 1951). Therefore, the subcommittee concludes that CDEPAT's proposed ECt 50 estimate for threshold effects is scientifically valid. INHALATION VAPOR EXPOSURE Lethal Effects (LCt50) CDEPAT's proposed LCt50 estimate following inhalation exposure to GB vapor is 35 mg-min/m3, assuming minute volumes of 15 liters and exposure