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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR GA (TABUN) 21 and are exposed for 30 to 50 min. The existing LCt 50 is 20,000 mg-min/m 3 (Wood 1949). The original human vapor exposure estimate corresponding to an LCt 50 was 20,000 mg-min/m3 but appears to have been established without supporting data or scientific rationale. LCt50 data from animal studies varies with species (CDEPAT 1994). For example, exposure of dogs and guinea pigs to GA vapor for 10 min resulted in a higher LCt50 value (approximately > 6,100 mg-min/m3) than exposure of mice for the same time (2,500 mg-min/m3). The LCt50 for the monkey was estimated to be 5,000 to 9,000 mg-min/m3 for exposure durations of 132 to 305 min (Krackow and Fuhr 1949). The LCt50 in rabbits for exposure durations of 120 to 282 min was estimated to be > 20,000 mg-min/m3 (Marquand and Kethley 1946). The rabbit LCt50 was > 20,000 mg-min/m3 (Marquand and Kethley 1946). However, the mouse, guinea pig, and dog studies involving exposure durations of 10 min and the monkey and rabbit studies involving exposure durations of > 120 min (120 to 305 min) are not applicable in deriving LCt50 values for humans. Krackow and Fuhr (1949) exposed 16 men at Cts (concentration Ã time) of 520 to 2,000 mg-min/m3 for 10 to 40 min. The men used gas masks and wore only shorts, socks, and shoes. The exposure caused a slight decrease in ChE activity. The authors concluded that such exposure was safe at Cts as high as 2,000 mg-min/m3. Those human data provide support for CDEPAT's proposed LCt50 estimate of 15,000 mg-min/m3. The subcommittee concludes that the proposed estimate is scientifically valid. ECt50 for Threshold Effects ECt50 for threshold (minimal) effects is the vapor exposure that would result in a significant ChE inhibition (< 15%) but without any identifiable adverse biological consequences. CDEPAT's proposed ECt50 estimate for threshold effects of percutaneous exposure to GA is 2,000 mg-min/m3 for exposure durations of 30 to 50 min in moderate temperatures. CDEPAT's confidence in this estimate is relatively high and is based on significant ChE inhibition (CDEPAT 1994). There is no existing ECt50 estimate for threshold effects (CDEPAT 1994). Changes in ChE activity have been reported in men and monkeys after percutaneous vapor exposure. The human data indicate that a slight but