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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR GD (SOMAN) 38 tent to GB for severe effects via the inhalation route (CDEPAT 1994). The basis of this assumption is a study by Cresthull et al. (1957) in which the ratio of ICt50 to LCt 50 for GB was 0.7. Because GB and GD have similar structures and similar modes of action, CDEPAT assumed that the ratio of 0.7 for GB would also hold true for GD. However, the subcommittee concludes that the confidence in the ECt50 value is low because of sparse data on both compounds. The subcommittee recommends that CDEPAT's proposed estimate of 25 mg-min/m3 be lowered to correspond to the subcommittee's recommendation for lowering the ECt50 for GB until further research is done to establish the estimate with a greater degree of confidence. ECt50 for Mild Effects CDEPAT's proposed estimate for ECt 50 for mild (ocular and/or nasal) effects for GD is 0.2 mg-min/m3, assuming exposure durations of 2 to 10 min and moderate temperatures. This local effect is not affected by minute volume. There is no existing ECt50 estimate for GD (CDEPAT 1994). Sufficient human data are not available to calculate an ECt50 for mild effects following ocular exposure to GD. One study in which rabbits were exposed under identical conditions to GB and GD showed that GD is a 2.5-times more potent miotic agent than GB via inhalation exposure (Callaway and Dirnhuber 1971). Thus, in deriving the ECt 50 estimate for GD, CDEPAT assumed that GD is 2.5 times more potent than GB for ocular effects (Callaway and Dirnhuber 1971). The subcommittee agrees with CDEPAT's approach of assuming that the ocular toxicity of GD is 2.5 times greater than that of GB. The subcommittee also agrees with the conclusion of CDEPAT that the confidence in the ECt50 estimate is low because of sparse data on both compounds. In addition, because the subcommittee recommends raising the ECt50 for GB, it concludes that the estimate for GD should be raised correspondingly for ocular effects until further research is done to establish the estimate with a greater degree of confidence. PERCUTANEOUS LIQUID EXPOSURE Lethal Effects (LD50) CDEPAT's proposed estimate for the LD50 value is 350 mg for a 70-kg
