mustard gas, chemical compound used as a poison gas in World War I. The burning sensation it causes on contact with the skin is similar to that caused by oil from black mustard seeds. The compound is not a gas but a colorless, oily liquid with a somewhat sweet, agreeable odor; it boils at 217°C. A powerful vesicant, mustard gas causes severe blistering even in small quantities. Highly irritating to the eyes, it quickly causes conjunctivitis and blindness. If inhaled, it attacks the respiratory tract and lungs, causing pulmonary edema. Some effects of exposure to mustard gas are delayed up to 12 hr; death may result several days after exposure. Mustard gas was introduced by the Germans in warfare against the British at Ypres, Belgium, in July, 1917, and took a heavy toll of casualties. It is dispersed as an aerosol by a bursting shell. Chemically, mustard gas is a thioether, 2,2′-dichlorodiethyl sulfide, (ClCH2CH2)2S. It can be prepared by reacting ethylene with sulfur monochloride, S2Cl2, or by other methods. Its vesicant property is readily destroyed either by oxidation or by chlorination (e.g., with bleaching powder).