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Suggested Citation:"ECt50 for Severe Effects." National Research Council. 1997. Review of Acute Human-Toxicity Estimates for Selected Chemical-Warfare Agents. Washington, DC: The National Academies Press. doi: 10.17226/5825.
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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR GB (SARIN) 30 durations of 2 to 10 min. The existing LCt50 estimate is 70 mg-min/m 3 (CDEPAT 1994). The LCt50 data for inhalation exposure for several animal species (mouse, rat, primate, dog, rabbit, cat, and pig) provide an LCt50 estimate for humans of 60 mg-min/m3 for 10-min exposures. The average ratio (of LCt50 for GA, GB, and GF) for 10-min and 2-min exposures was calculated to be 0.6 (CDEPAT 1994) and that ratio was also supported by a classified study. Using a factor of 0.6 to estimate the 2-min LCt50 from the 10-min LCt 50, CDEPAT obtained a value of 35 mg-min/m 3 (60 × 0.5 35). Human data from the Adamek Report (as cited in Wills and DeArmon 1954) showed deaths in four of four subjects exposed at 4 mg/m3 for 10 min (a Ct (concentration × time) of 40 mg-min/m3). Data were available for 48 other subjects, all of whom received some type of post-exposure therapy. Using data from exposed and unexposed individuals, the authors calculated an LCt50 of 24 mg-min/m3 (Wills and DeArmon 1954). However, on the basis of the 100% lethality observed in humans exposed at 40 mg-min/m3, the subcommittee recommends that the CDEPAT's proposed LCt50 estimate of 35 mg-min/m3 be lowered. The subcommittee also recommends that further research be conducted to establish the LCt 50 estimate for inhalation with a greater degree of confidence. ECt50 for Severe Effects CDEPAT's proposed ECt50 estimate for severe effects following inhalation exposure to GB vapor is 25 mg-min/m3, assuming minute volumes of 15 liters and exposure durations of 2 to 10 min. The existing ECt 50 estimate is 35 mg- min/m3 (CDEPAT 1994). In the absence of adequate data on GB for this effect, CDEPAT's proposed estimate is based on the assumption that the ratio of ICt50 (incapacitation dose for 50% of a given population) and LCt50 is about 0.7. The ratio is supported by a study conducted in monkeys (Cresthull et al. 1957). The proposed ECt50 estimate of 25 mg-min/m3 was calculated by multiplying the LCt50 of 35 mg-min/m3 by 0.7, which equals 25 mg-min/m3. The subcommittee believes this approach is reasonable. However, the subcommittee recommended that the LCt 50 for inhalation exposure be lowered; therefore, the ECt50 should be lowered correspondingly. The subcommittee recommends that further research be conducted to establish the ECt50 for severe effects with a greater degree of confidence.

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No reliable acute-exposure1 standards have been established for the particular purpose of protecting soldiers from toxic exposures to chemical warfare (CW) agents. Some human-toxicity estimates are available for the most common CW agents—organophosphorus nerve agents and vesicants; however, most of those estimates were developed for offensive purposes (that is, to kill or incapacitate the enemy) and were intended to be interim values only. Because of the possibility of a chemical attack by a foreign power, the Army's Office of the Surgeon General asked the Army's Chemical Defense Equipment Process Action Team (CDEPAT) to review the toxicity data for the nerve agents GA (tabun), GB(sarin), GD (soman), GF, and VX, and the vesicant agent sulfur mustard (HD) and to establish a set of exposure limits that would be useful in protecting soldiers from toxic exposures to those agents. This report is an independent review of the CDEPAT report to determine the scientific validity of the proposed estimates.


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