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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR GA (TABUN) 22 significant reduction in ChE activity occurs after percutaneous vapor exposures above 1,000 mg-min/m3 (Krackow and Fuhr 1949). Data are also available that indicate that men wearing only shorts, socks, and shoes and using gas masks could be safely exposed (that is, without degradation in performance) to vapor doses as high as 2,000 mg-min/m 3 (Krackow and Fuhr 1949). Masks were used to avoid exposures via inhalation and to protect against ocular effects. On the basis of the human data, the subcommittee concludes that CDEPAT's proposed ECt50 estimate of 2,000 mg-min/m3 is scientifically valid. INHALATION VAPOR EXPOSURE Lethal Effects (LCt50) CDEPAT's proposed LCt50 estimate for inhalation exposure to GA vapor is 70 mg-min/m3, assuming exposure durations of 2 to 10 min and minute volumes of 15 liters. The existing LCt50 value is 135 mg-min/m3 (CDEPAT 1994). The LCt50s estimates for GA vary with time. In one study, they were 4 mg- min/m3, 8 mg-min/m3, and 16 mg-min/m3 for exposure durations of 48, 40, and 19 min, respectively (Wills and DeArmon 1954). Questions about the accuracy of those data and the longer exposure time make those data inappropriate for calculating human-toxicity estimates for a 2- to 10-min exposure. Other estimates of the LCt50 for humans ranged from 400 to 500 mg-min/m3 (Welchman 1946). However, the rationale and justification for making such estimates are obscure. A number of animal studies using a variety of species have been conducted to establish LCt50 values following inhalation exposure to GA vapor. Unfortunately, most of those studies were conducted over 50 years ago, and few details concerning the exposure and monitoring aspects of the study were recorded. In general, the LCt50 values for GA ranged from 135 to 500 mg-min/m3 for exposures of 10 min or less. CDEPAT's estimates were largely based on studies using rhesus monkeys in which the LCt50s were 135 and 187 mg-min/m3 for 2-min and 10-min exposures, respectively (Cresthull et al. 1957). The poor quality of the animal data used in estimating the LCt50 provides little confidence in the ability to predict the human LCt 50. Existing human data are also inadequate for estimating LCt50 values. Thus,