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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR GD (SOMAN) 36 min. There is no existing LCt50 estimate for exposures to percutaneous vapor (CDEPAT 1994). No human data on GD are available for this effect. LCt50 values derived from studies in monkeys range from 1,750 to 28,000 mg-min/m3 (Cresthull 1957). Because data on the effects of percutaneous exposure to GD vapor are extremely limited and are insufficient for developing human-toxicity estimates, CDEPAT recommended that estimates for GD be based on toxicity information on GB, assuming that GD is 4 times more potent than GB. This assumption is supported by empirical evidence in dogs and rabbits, showing that the percutaneous potency of GD vapor is at least 2 times and maybe 5 times that of GB (Van de Wal and Zeffert 1970). The subcommittee concludes that the approach used by CDEPAT to estimate the LCt50 of GD is scientifically valid and defensible, because (1) GD and GB belong to the same class of chemicals (organophosphates) and are structurally similar, and (2) animal data support the assumption that GD is 4 times more potent than GB. On the basis of the available data, the subcommittee concludes that the CDEPAT's LCt50 estimate of 2,500 mg-min/m3 is scientifically valid. ECt50 for Threshold Effects CDEPAT's proposed estimate for the ECt50 for threshold (minimal) effects after percutaneous exposure to GD vapor is 300 mg-min/m3, assuming light clothing, exposure durations of 30 to 50 min, and moderate temperatures. There is no existing toxicity estimate for threshold effects from exposure to percutaneous vapor (CDEPAT 1994). No human data are available to estimate this effect level. In deriving the ECt50 for threshold effects, CDEPAT assumed that GD vapor is 4 times more potent than GB vapor (CDEPAT 1994). This assumption was based on a report (Cullumbine et al. 1954) showing that the LCt 50 for GD was 4 times lower than GB. However, CDEPAT has a low degree of confidence in this estimate because of insufficient data. The subcommittee agrees with CDEPAT's approach of basing the ECt50 estimate for GD on toxicity data from GB and assuming that GD is 4 times more potent than GB. The subcommittee also agrees with CDEPAT that the confidence in the ECt50 estimate is low to moderate because of sparse data on both compounds. The subcommittee recommends that CDEPAT's proposed estimate of 300 mg-min/m3 serve as an interim value