Phosgene is the chemical compound with the formula Cl₂CO. This colourless gas gained infamy as a chemical weapon during World War I, but it is also a valued industrial reagent and building block in organic synthesis. In low concentrations, its odor resembles freshly cut hay or grass. In addition to its industrial production, small amounts occur naturally from the breakdown of chlorinated compounds and the combustion of chlorine-containing organic compounds. Approximately 5000 tonnes were produced in 1989.