Phosgene

• 1. Introduction

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  Phosgene is a colorless gas with the formula COCl2, and in low concentrations it smells like freshly cut hay or grass. That pastoral scent belongs to one of the most dangerous substances industrial chemistry has ever handled. The same molecule was named after sunlight, used on the battlefields of a world war, and is still made in huge quantities today. Why would a gas with such a mild smell become a weapon that armies stockpiled? How can a person inhale a lethal dose and feel nothing for hours? And why do modern factories, which depend on the gas, take such care never to hold much of it at once? The answers begin with a chemist in Cornwall.

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• 2. Child of Sunlight

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  In 1812, the Cornish chemist John Davy, who lived from 1790 to 1868, exposed a mixture of carbon monoxide and chlorine to sunlight. The light drove a reaction, and the product was a new gas. Davy gave it a name built from two Greek words: phos, meaning light, and gennao, meaning to give birth. The name records its origin, a compound born of light.

  As the 19th century went on, phosgene gradually became important to the chemical industry. It found a particular home in the manufacture of dyes. Chemists had learned that this simple gas could be coaxed into reacting with many other substances.

  Structurally, the molecule is flat. Its carbon-oxygen distance is 1.18 angstroms, its carbon-chlorine distance is 1.74 angstroms, and the angle between the two chlorine atoms is 111.8 degrees. Chemists sometimes picture it as formaldehyde with both hydrogen atoms swapped for chlorine.

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• 3. Weapon of Trenches and Caves

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  In 1915, French forces became the first to use phosgene as a chemical weapon in World War I. The gas is heavy, so it sank into enemy trenches and filled them quickly. It is a powerful irritant of the lungs. Armies often released it mixed with an equal volume of chlorine, which helped carry the denser phosgene across the ground. Phosgene proved more potent than chlorine, though some symptoms took 24 hours or more to appear. Across the war, it was responsible for 85,000 deaths.

  After 1918, various countries kept stockpiles. The Imperial Japanese Army used phosgene only infrequently against Chinese forces during the Second Sino-Japanese War. Gas weapons of this kind, phosgene among them, were produced by the army's Unit 731.

  During World War II, American planners weighed phosgene against Japanese troops sheltering in bunkers and caves. Such fortifications had served the Japanese defenders very well at Luzon and Okinawa. Tests by the Chemical Warfare Service at Dugway Proving Ground found that the gas would "pour into a cave almost like water". It spread rapidly and reached concentrations high enough to kill even people wearing gas masks. Operation Olympic, the planned invasion of Kyūshū, included provisions for its use.

  Phosgene is less toxic than nerve agents such as sarin. Yet it is still regarded as a viable weapon, because it is far simpler to make than tabun, a first-generation nerve agent. Phosgene sits on Schedule 3 of the Chemical Weapons Convention, and any site producing more than 30 tonnes a year must be declared to the OPCW.

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• 4. A Delayed Injury

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  The smell of phosgene can be detected at 0.4 ppm, which is four times the threshold limit value for a time-weighted average. A worker may therefore notice the odor only after safe exposure has already been passed. Some people describe the scent as sweet, or even like rotten banana peels. Others notice nothing at all, which is why the gas is called an insidious poison.

  Inside the lungs, phosgene attacks the OH, NH2 and SH groups of proteins in the alveoli, where gas exchange happens. It forms ester, amide and thioester links, and the barrier between blood and air breaks down. Fluid then builds up in the lungs as pulmonary edema.

  The damage depends mainly on dose, not on concentration alone. Dose can be approximated as concentration multiplied by the duration of exposure. For that reason, workers in places where a leak could happen wear indicator badges near the nose and mouth. The badges show roughly how much has been inhaled, so treatment can start at once if the figure climbs too high.

  Above about 150 ppm times minutes, edema often develops. It shows up on X-ray and in falling blood oxygen. Death can follow within hours, or as late as two to three days after exposure. Lower doses call for monitoring and precautionary treatment, and the patient is usually released after several hours.

  The real danger is the silence before the edema. A person may feel fine for hours, and by the time symptoms arrive, medical help may be too late. Nearly all deaths in accidental industrial releases happened this way. Edemas treated promptly usually heal over weeks without major consequences.

  Chronic low-level exposure is a separate problem. Many synthetic chemists who worked with phosgene, among them Leonidas Zervas, reportedly suffered long-term respiratory trouble and eventual respiratory failure.

  If a leak does occur in a plant or laboratory, ammonia gas can mitigate it. For liquid spills of phosgene solutions or diphosgene, an absorbent and sodium carbonate are applied.

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• 5. Made on Demand

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  World production of phosgene was estimated at 2.74 million tonnes in 1989. The recipe is short. Purified carbon monoxide and chlorine gas are passed through a bed of porous activated carbon, which acts as the catalyst. The reaction gives off heat and is typically run between 50 and 150 degrees Celsius. Above 200 degrees, the product breaks back down into carbon monoxide and chlorine.

  Because the gas is so hazardous, it is generally considered too dangerous to ship in bulk. Plants instead make it and use it on site, in an on-demand process. Production and consumption are kept in step, so only a small amount exists in the system at any moment. A leak therefore has less to release. Some batch production continues, but operators work to keep stored quantities low.

  Where phosgene must be stored, it goes into metal cylinders. In the United States, the valve outlet on such a cylinder is a tapered thread called CGA 160, which is used for phosgene and nothing else.

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• 6. Unintended Phosgene

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  In the 1970s and 1980s, phosgene in the lower atmosphere measured around 20-30 parts per trillion by volume, peaking at 60. Part of the source was industry itself. Simple organochlorides slowly turn into phosgene under ultraviolet light in the presence of oxygen. Before the ozone hole was discovered in the late 1970s, industry routinely used organochlorides in large quantities, and they inevitably reached the air. Nearly 30 years later, levels had not fallen significantly, even though the Montreal Protocol restricted organochloride production.

  In the troposphere, phosgene lasts up to about 70 days. Humidity and cloudwater remove it mainly by hydrolysis. Less than 1% reaches the stratosphere, where the dry air and slow breakdown by UV light give it a lifetime of several years. It plays only a minor part in ozone depletion.

  Carbon tetrachloride offers a harsher lesson. Heated in air, it can turn into phosgene. It was once widely used in fire extinguishers because it suppresses flames so well. There are reports of deaths when it was used against fires in confined spaces. Between that hazard and its own toxicity, it is no longer used this way.

  The body can make the gas too. Phosgene forms as a metabolite of chloroform, likely through the action of cytochrome P-450.

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• 7. Plastics and Foams

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  In 2013, about 75-80% of the world's phosgene went into isocyanates, 18% into polycarbonates and about 5% into other fine chemicals. The common thread is phosgenation, the reaction of an organic substance with phosgene. Reacting it with bisphenol A, for example, produces polycarbonates. Reacting it with diamines produces diisocyanates such as TDI, MDI, HDI and IPDI, which are precursors to polyurethanes.

  Manufacturers use phosgene in excess to raise yield and limit side reactions. The surplus is separated out and recycled, and whatever remains is decomposed in water over activated carbon. The biggest production units are in Texas and Louisiana, Germany, Shanghai, Japan and South Korea. Dow Chemical, Covestro and BASF are the most important producers.

  Phosgene can also turn carboxylic acids into acyl chlorides, though thionyl chloride is commonly used instead. In the research laboratory, safety concerns now limit its use. It can make isocyanates from amines, with a base such as pyridine to neutralize the hydrogen chloride. It can make chloroformates such as benzyl chloroformate, and it reacts with amino acids to give N-carboxyanhydrides. Substitutes include diphosgene, a liquid, and triphosgene, a crystalline solid. Carbonyldiimidazole, or CDI, is safer but expensive, and it is itself made by reacting phosgene with imidazole.

  Phosgene also yields monoisocyanates, which serve as pesticide precursors. One of them is methyl isocyanate, abbreviated MIC, and its name returns in the record of industrial accidents.

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• 8. Accidents

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  In May 1928, eleven tons of phosgene escaped from a war surplus store in central Hamburg. Three hundred people were poisoned, and ten of them died. It is regarded as the first major phosgene incident.

  In the second half of the 20th century, several fatal incidents involving phosgene occurred in Europe, Asia and the United States. Authorities investigated most of them and made the outcomes public. The Bhopal disaster is a notable case of mistaken blame. Phosgene was initially suspected, but investigations showed that MIC caused the many poisonings and deaths.

  Two recent accidents involved single workers. In January 2010, a release of phosgene at a DuPont facility in West Virginia killed an employee, and the US Chemical Safety Board released a video detailing the accident. In May 2016, a leak at a BASF plant in South Korea left a contractor with a lethal dose.

  In 2023, a freight train carrying vinyl chloride derailed and burned in East Palestine, Ohio. The fire released phosgene and hydrogen chloride into the air, and the Ohio River was contaminated. No one had set out to make phosgene there. Burning vinyl chloride was enough.

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