Phosgene is the chemical compound with the formula COCl2. It is a colorless gas which gained importance as a chemical weapon during World War I.Phosgene was synthesized by the British chemist John Davy in 1812 by exposing a mixture of carbon monoxide and chlorine to sunlight.

In low concentrations, its odor resembles freshly cut hay or grass.

It is also an industrial reagent in synthesis of pharmaceuticals and other organic compounds. In addition to its industrial production, small amounts occur naturally from the breakdown and the combustion of organochlorine compounds, such as those used in refrigeration systems.Phosgene is produced by passing purified carbon monoxide and chlorine gas through a bed of porous activated carbon, which serves as a catalyst.

Upon ultraviolet (UV) radiation in the presence of oxygen, chloroform slowly converts into phosgene. To suppress this photodegradation, chloroform is often stored in brown-tinted glass containers.

The great majority of phosgene is used in the production of isocyanates, The isocyanates are precursors to polyurethanes.

Significant amounts are also used in the production of polycarbonates. Polycarbonates are an important class of engineering thermoplastic found, for example, in lenses in eye glasses.

Phosgene is an insidious poison as the odor may not be noticed and symptoms may be slow to appear.  Its high toxicity arises from the action of the phosgene on the proteins in the pulmonary alveoli, which are the site of gas exchange. The damage to the alveoli disrupts the blood-air barrier, causing suffocation.Sodium bicarbonate may be used to neutralise liquid spills of phosgene. Gaseous spills may be neutraised with ammonia.
Following the extensive use of phosgene gas in combat during World War I, it was stockpiled by various countries as part of their secret chemical weapons programs.Phosgene was frequently used by the Imperial Japanese Army against the Chinese during the Second Sino-Japanese War.
Chemical Weapons