Phosgene is a highly toxic gas with widespread use in the civilian chemical industry. First introduced as a weapon in World War I, phosgene causes severe irritation of the lungs and can cause pulmonary edema (the filling of lung spaces with fluid) in low concentrations. Effects of phosgene are delayed between exposure and onset of symptoms, usually hours after inhalation. Phosgene was responsible for about 80 percent of all deaths due to chemicals in World War I. In the mid-1990s, the Japanese cult Aum Shinrikyo used phosgene in at least one attack on a journalist, but without causing significant injury. Due to its gaseous state and low toxicity relative to other chemicals such as nerve agents, phosgene has not figured prominently in chemical threat assessments in the modern era. There has been some speculation that phosgene may have played a role in the 1984 Bhopal, India chemical disaster. However, it would appear that phosgene was not a significant factor during the release of methyl isocyanate that killed some 2,500 people. Victims of phosgene exposure, once having reached the stage during which breathing assistance is required, nearly always have poor outcomes. Although some can detect phosgene by its odor, likened to freshly cut hay or grass, lethal concentrations of phosgene can be present without any olfactory warning. With its potential for military use, phosgene is classified as a Schedule 3 chemical in the Chemical Weapons Convention, which also recognizes its multitudinous roles in the worldwide chemical industry.
Phosgene owes its toxicity primarily to its reactive character and interaction with vital molecules and enzymes in the pulmonary system. After coming into contact with water, phosgene also liberates chlorine and forms hydrochloric acid, exacerbating subsequent injury to the lung tissue. The median lethal concentration of phosgene in humans is approximately 3.2 grams-min/m3, and the and the median incapacitating dose (ICt50) is estimated at half this amount (1.6 grams-min/m3).
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