large quantities of these compounds were routinely used by industry. Phosgene levels in the troposphere were around 20-30 pptv at the time (peak 60 pptv), however these levels had not decreased significantly nearly 30 years later, despite organochloride production becoming restricted under the Montreal Protocol. Phosgene in the troposphere can last up to about 70 days and is removed primarily by hydrolysis with ambient humidity or cloudwater. Less than 1% makes it to the stratosphere, however is expected to have a lifetime of several years here since this layer is much drier and phosgene decomposes slowly through UV photolysis. Consequently, it does play a minor part in ozone depletion. Combustion Carbon tetrachloride (CCl4) can turn into phosgene when exposed to heat in air. This was a problem as carbon tetrachloride is an effective fire suppressant and was formerly in widespread use in fire extinguishers. There are reports of fatalities caused by its use to fight fires in confined spaces. Carbon tetrachloride's generation of phosgene and its own toxicity mean it is no longer used for this purpose. Biologically Phosgene is also formed as a metabolite of chloroform, likely via the action of cytochrome P-450. History Phosgene was synthesized by the Cornish chemist John Davy (1790–1868) in 1812 by exposing a mixture of carbon monoxide and chlorine to sunlight. He named it "phosgene" from Greek φῶς (phos, light) and γεννάω (gennaō, to give birth) in reference of the use of light to promote the reaction. It gradually became important in the chemical industry as the 19th century progressed, particularly in dye manufacturing. Reactions and uses The reaction of an organic substrate with phosgene is called phosgenation. Synthesis of carbonates Diols react with phosgene to give either linear or cyclic carbonates (R = H, alkyl, aryl): HOCR2−X−CR2OH + COCl2 → [OCR2−X−CR2OC(O)−]n + 2 HCl An example is the reaction of phosgene with bisphenol A to form polycarbonates. Synthesis of isocyanates The synthesis of isocyanates from amines illustrates the electrophilic character of this reagent and its use in introducing the equivalent synthon "CO2+": RNH2 + COCl2 → RN=C=O + 2 HCl (R = alkyl, aryl) Such reactions are conducted on laboratory scale in the presence of a base such as pyridine that neutralizes the hydrogen chloride side-product. On an industrial scale, phosgene is used in excess to increase yield and avoid side reactions. The phosgene excess is separated during the work-up of resulting