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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR HD 60 The LCt50 value for humans is difficult to estimate from the available animal data, because the data indicate that animal species vary in their sensitivity to HD. The LCt50 ranged from > 6,300 to 20,000 mg-min/m3 in monkeys (NDRC 1943a, 1944), from 2,948 to 4,885 mg-min/m 3 in mice (NDRC 1943b,c), and from 4,750 to 6,430 mg-min/m3 in rabbits (NDRC 1943a, 1944). The current LCt50 estimate for HD exposure of humans is 10,000 mg-min/m3. That estimate is based on data from monkeysâone of the least-sensitive species tested for lethality. The authors of the CDEPAT report recommend lowering the estimate to 5,000 mg-min/m 3, an estimate that is more consistent with results from studies in other animal species. In the subcommittee's opinion, CDEPAT's approach is reasonable, but the estimate is not overly conservative, because data indicate that humans might be one of the most-sensitive species to HD exposure. Rats and mice are the most-sensitive animal species. Although the proposed CDEPAT estimate is more appropriate than the existing estimate, the subcommittee believes that the new estimate might still be too high. Therefore, the subcommittee recommends that the proposed estimate be lowered. 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 estimates for severe effects from exposure to HD are 500 mg-min/m3 for moderate temperatures and < 200 mg-min/m 3 for hot temperatures, assuming exposure durations of 30 to 50 min. The existing estimates for moderate and hot temperatures are 2,000 and 1,000 mg-min/m3, respectively (CDEPAT 1994). The estimates are derived from data reported in human studies conducted 50 years ago (CDEPAT 1994). The proposed estimate of < 200 mg-min/m 3 for hot- temperature exposures is supported by a study of 10 men (PCS 1946), who exercised and sweat profusely in perspiration-drenched clothing and were exposed to HD at 220 mg-min/m3 at 90Â°F and 85% relative humidity for 57 min. All the men had severe scrotal burns. Thus, CDEPAT's recommended estimate is based on actual human data with a reasonable number of subjects. CDEPAT's recommended estimate for exposures at moderate temperatures (70Â°F, 48% humidity) is 500 mg-min/m3 and is supported by a study