phosgene , colorless poison gas , first used during World War I by the Germans (1915). When dispersed in air, the gas has the odor of new-mowed hay. The gas is highly toxic; when inhaled it reacts with water in the lungs to form hydrochloric acid and carbon monoxide. Because the upper respiratory tract is little affected, warning signs of exposure are slight, and symptoms may fail to appear for from 2 to 24 hours after exposure. However, the release of hydrochloric acid in the lungs causes pulmonary edema and may also cause bronchial pneumonia and lung abscesses; in severe cases death may result within 36 hours. Phosgene is now used in chemical synthesis. It may be prepared by the reaction of carbon monoxide with chlorine in the presence of a catalyst or by the oxidation of chloroform or carbon tetrachloride. Phosgene has the formula COCl 2 .