The only exceptions are research establishments such as Britain's Chemical Defence Establishment at Porton Down in Wiltshire, producing small quantities of chemicals designed to permit experiments relating to defensive measures. Under the CWC, each state may keep one ton of the most lethal agents for such purposes.
The nations which still cause concern - none of which have signed the convention - are Iraq, Libya and Iran. Iraq is subject to a special United Nations regime and international observers are therefore less concerned about it than about Libya, where there have been allegations that chemical weapons are being manufactured at Tarhuna, south-east of Tripoli. There has also been concern about a plant at Rabta. Libya denied the latter, saying the $20m (pounds 13m) plant produced pharmaceuticals
Iran has allegedly produced mustard gas, chlorine, phosgene and hydrogen cyanide and is believed to be able to produce nerve gases such as sarin.
Chemical weapons are relatively easy to make. There are four main categories. The first, initially used in 1915, are simple "choking" agents - chlorine and phosgene. Next came "blister" agents - dichlorethyl sulphide, or mustard gas, first used in 1916. Besides being able to penetrate clothing, these agents can also be very persistent, remaining in the ground for decades after their release. Then came "nerve" agents, invented by the Germans in the Thirties but quickly adopted by the Soviet Union and the US, which give rise to involuntary nerve impulses, causing convulsions and death.
The fourth type are "blood" agents such as hydrogen cyanide, which are lethal but disperse very quickly. There are also toxins, which are of biological origin but act as poisons.
Iraq's use of chemical weapons has been the most blatant in recent years. In 1988, during the Iran-Iraq war, the Iraqis used it against the Kurds at Halabja, killing an estimated 5,000 people. After the Gulf war, the Allies denied that the Iraqis had made any use of their chemical stocks, but has emerged since that there were several large releases.