Chemical weapons (in the past often called “poison gas”) have been used through history, beginning with primitive materials such as noxious fumes from burning sulfur, and moving to extremely toxic synthetic chemicals that were developed during the first half of the twentieth century.

Large scale use of toxic chemicals as weapons occurred during the First World War (1914 -1918). Over one hundred thousand tonnes of chemicals were used on the battlefields. About ten per cent of the total tonnage of chemical warfare agents used during the First World War (WWI) were chemicals intended to harass rather than kill, namely tear gases, lung irritants and vomiting agents. Use of more lethal chemicals followed. These caused some 1.3 million casualties during the war, of whom some 90,000 are estimated to have died.

In the Second World War, between 1939-45 substantial stocks of chemical weapons were accumulated. Following the end of the Second World War, systematic surveys continued to find ideal chemical warfare agents.

Despite the screening of thousands of chemicals, only some 60 or so had the appropriate physical, chemical and toxicological properties to be useful as chemical warfare agents. Analysis of the casualties caused by particular chemicals in WW1 reduced the number of effective chemical warfare agents down to about twelve. More agents have been added to this list since then, and some of the early chemical warfare agents were replaced by newer types.