The French produced phosgene which proved to be more powerful than the chlorine.
From the en.wikipedia.org
In the research laboratory phosgene still finds limited use in organic synthesis.
From the en.wikipedia.org
Chlorination of CO is the industrial route to the important compound phosgene.
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Phosgene can be detected at 0.4 ppm, which is four times the Threshold Limit Value.
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Burning it releases gases that are ten times as toxic as the chemical weapon phosgene.
From the sciencedaily.com
They are short-lived and they decay rapidly, but they decay into phosgene.
From the sciencedaily.com
In the process, phosgene gas would be created due to the thermal reaction.
From the en.wikipedia.org
During prolonged storage in the presence of oxygen chloroform converts slowly to phosgene.
From the en.wikipedia.org
Phosgene may also be produced during testing for leaks of older-style refrigerant gasses.
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More examples
A colorless poisonous gas that smells like new-mown hay; used in chemical warfare
Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a chemical weapon during World War I. It is also a valued industrial reagent and building block in synthesis of pharmaceuticals and other organic compounds. ...
Chemical formula COCl2, phosgene is a very toxic gas with a TLV of 0.1ppm. Phosgene has a moldy, hay like odor and is irritating. Phosgene is a reaction product that can form during chlorinated furnace cleans if sufficient oxygen isn't present.