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Suggested Citation:"ECt50 for Mild 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 62

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REVIEW OF ACUTE HUMAN-TOXICITY ESTIMATES FOR HD 62 LCt50. CDEPAT had no confidence in any study. Therefore, CDEPAT had no basis for determining which animal species best reflected the human response. CDEPAT performed some modeling studies, but they did not provide useful information. In the absence of better data, CDEPAT averaged the toxicity data from several studies to estimate the human LCt50. The subcommittee believes that this approach is reasonable. The subcommittee agrees with the proposed estimate but would prefer to see a range of proposed values to indicate the confidence bounds. ECt50 for Severe Effects CDEPAT's proposed ECt50 estimate for severe (ocular) effects from inhalation exposure to HD vapor, assuming exposure durations of 2 to 10 min, was lowered from the existing value of 200 mg-min/m3 to the value of 100 mg- min/m3 (CDEPAT 1994). The eye is one of the organs that is most sensitive to the effects of HD vapors. Available data indicate that temporary blindness might be produced by HD vapor exposures of 200 mg-min/m3, but other eye effects will be experienced at lower exposures. Therefore, the authors of the CDEPAT report reduced their estimates for severe nonlethal effects by 50%. From a battlefield perspective, the soldiers will first experience eye effects which will lead to the removal of soldiers from the battlefield. The subcommittee agrees with CDEPAT's proposed estimate. ECt50 for Mild Effects CDEPAT's proposed estimate for the ECt50 for mild (ocular) effects from exposure to HD is 25 mg-min/m3, assuming exposure durations of 2 to 10 min. The existing estimate is > 50 mg-min/m3 (CDEPAT 1994). As with its ECt50 estimate for severe effects, CDEPAT based its ECt 50 estimate for mild effects on the eye. Pre-1940 data indicate effects on the eye at vapor doses of 5 to 10 mg-min/m3 (Reed 1920). Later studies, which are considered more reliable because of improved techniques, indicate that the eye can withstand a higher exposure (that is, 70 mg-min/m3) (Guild et al. 1941). The subcommittee believes that CDEPAT's proposed estimate of 25 mg-min/m3 for mild effects from inhalation exposure to HD vapor is supported by human data. Therefore, the subcommittee concludes that the proposed ECt50 estimate for mild effects is scientifically valid.

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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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