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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR GA (TABUN) 23 CDEPAT based its proposed LCt50 estimate for humans primarily on the assumption that GA is probably about 0.5 times as potent as GB (HEC 1960). The subcommittee accepts CDEPAT's approach of assuming that GA is 0.5 times as toxic as GB in deriving its LCt50 estimate for this route. The subcommittee recommends that the LCt50 estimate for GA be lowered, as was done for GB by the subcommittee. The subcommittee also recommends that further research be conducted to establish the LCt50 estimate with a greater degree of confidence. ECt50 for Severe Effects CDEPAT's proposed ECt50 estimate for severe effects following inhalation exposure to GA is 50 mg-min/m3, assuming exposure durations of 2 to 10 min and minute volumes of 15 liters. CDEPAT's degree of confidence in this estimate is moderate. There is no existing toxicity estimate for ECt50 for severe effects following inhalation of GA vapors (CDEPAT 1994). CDEPAT's proposed ECt50 estimate of 50 mg-min/m3 was derived on the basis of the study by Cresthull et al. (1957), which indicated that the ratio of the incapacitation vapor dose (ICt50) to the LCt50 is about 0.75. The reported ICt50s for 2-min and 10-min exposures were 102 and < 180 mg-min/m3, respectively. The subcommittee believes that CDEPAT's approach of estimating the ECt50 for severe effects is reasonable in its assumption that the ratio of ICt50 to LCt50 is 0.75. The subcommittee recommends that CDEPAT's estimate of 50 mg-min/m3 be lowered to correspond to the lowered estimate for LCt50 until further research is conducted to establish the ECt50 for severe effects with a greater degree of confidence. ECt50 for Mild Effects CDEPAT's proposed ECt50 estimate for mild effects (miosis or rhinorrhea) following exposure to GA vapor is 0.5 mg-min/m3, assuming a 2-min to 10-min exposure period. The Army's existing ECt 50 estimate for mild effects is 0.9 mg- min/m3; the duration of exposure was 5 min (Mumford 1950). Human data are available that indicate that the proposed ECt50 esti
