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Suggested Citation:"ECt50 for Threshold 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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Page 44

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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR GF 44 moderate temperatures. The existing estimate is 15,000 mg-min/m3 (CDEPAT 1994). In a study conducted with rhesus monkeys, 10 animals each weighing 2.7 to 5.9 kg were exposed to GF vapor. An LCt50 value of 10,000 mg-min/m3 was reported (McGrath et al. 1953). It is important to note that the animals were clipped, allowing for maximum skin exposure. Exposure durations varied from 31 to 345 min with an average atmospheric concentration of 105 mg/m3. Animals were exposed until they died. That type of exposure is a serious limitation of the study, because exposure until death necessarily results in higher LCt50 values. Had animals been exposed to lower vapor Cts (concentration X exposure time) and followed for 24 to 48 hr, the LCt50 might be lower. CDEPAT proposed lowering the existing estimate of 15,000 mg-min/m3 to 2,500 mg-min/m3. The subcommittee agrees with CDEPAT's proposal to lower the estimate, but it believes that insufficient evidence was provided for lowering the estimate to 2,500 mg-min/m3. Therefore, the subcommittee recommends that CDEPAT's proposed estimate of 2,500 mg-min/m3 serve as an interim value until further research is done. The subcommittee also recommends that a study be conducted at different exposure levels and observations made 24 to 48 hr after exposure to obtain data to more accurately determine the LCt50. ECt50 for Threshold Effects CDEPAT's proposed ECt50 estimate for threshold (minimal) effects from exposure to GF is 300 mg-min/m3, assuming exposure durations of 30 to 50 min and moderate temperatures. There is no existing ECt 50 estimate for threshold effects (CDEPAT 1994). No human or animal data are available for estimating the ECt 50 for threshold effects (CDEPAT 1994). The proposed estimate is based on the assumption that GD and GF are equipotent (Cullumbine et al. 1954). The subcommittee believes that, in the absence of adequate data on GF exposure of humans and animals, CDEPAT's approach of assuming equipotencies for GF and GD and thus proposing the same ECt50 estimates is reasonable. The subcommittee recommends that CDEPAT's proposed estimate of 300 mg-min/m3 serve as an interim value until further research on GF is conducted to establish the ECt 50 estimate 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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