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Classified as a blister agent (vesicant), Lewisite is named after the American military scientist, W. Lee Lewis, who produced this organoarsenic compound as a prototype chemical warfare (CW) agent in 1918. Because it was developed so late in World War I, Lewisite was never used in that conflict. The only military use of Lewisite known probably occurred in China during the Sino-Japanese conflict (ca. 1937-1942). Unlike mustard, which has delayed onset of clinical symptoms, the extreme irritation to eyes and skin begin almost immediately, with redness and blisters forming hours later. Significant exposure to Lewisite can cause blindness. Because of the rapid onset of pain, however, most exposures to Lewisite will result in less damage to the eyes as victims will attempt to avoid further contact by closing eyelids and avoiding the area. Lewisite, in its pure form, is an oily, colorless liquid, with no detectable odor. However, some have described impure batches of Lewisite as being of amber or dark brown color and having an odor of geraniums.
Depending on environmental conditions, Lewisite is a semi-persistent agent, and can penetrate a variety of rubber products, including those used in protective garments. Liquid at low temperatures (well below freezing), Lewisite was sometimes employed to mix with mustard to keep both CW agents solvent for use in winter conditions. Concentrations of Lewisite that can cause injury and death closely resemble those of mustard agent (also a vesicant). The median lethal concentration (LCt50) for Lewisite is about 1.5 grams-min/m3 for inhalation, and the median lethal dose (LD50) on the skin is estimated at 30mg/kg, or about 2.5 grams for an adult male of 180 pounds. The chemical formulation of Lewisite is relatively simple, and many countries, including those in the developing world, are capable of producing it in militarily significant quantities.

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