Mustard gas. Computer graphic of a molecule of the chemical warfare agent mustard gas (dichloroethyl sulphide, C4.H8.S.Cl2). The atoms (spheres) of the molecule are colour-coded: carbon (orange), hydrogen (white), chlorine (green) and sulphur (yellow). First used in World War I, the gas kills by stripping off the mucous membranes of the lung airways. It is also an anti-cancer agent, and a closely-related compound, nitrogen mustard, is used in anti-cancer research and chemotherapy. The nitrogen mustard compounds have the sulphur atom of mustard gas compounds replaced by a nitrogen atom.