Gas victims in the Iraqi town of Halabja

Main agents: VX, sarin, tabun, soman.

Effects: Nerve agents are extremely deadly - much more so than strong poisons such as cyanide.

Amounts small enough to fit on a pin head can prove fatal in under two minutes. Death usually occurs within quarter of an hour of exposure.

Nerve agents can kill after being inhaled in vapour form or if the skin is exposed to them in liquid form. In extreme cases convulsions and respiratory failure are followed by death.

Basic nerve agents are known as G-types, and include sarin and tabun. V-series agents, such as VX, are more potent.

Recognition: V-series agents are odourless, colourless liquids. G-series ones are often odourless, but can have a slightly fruity smell or a brown appearance.

Use as a weapon: The strength of VX gas means it is very dangerous to use in a small space, especially if the wind were to blow it back on those releasing the gas. Releasing it in a spray form is effective.

V-series agents can be treated with adhesive to enable them to stick to targets.

History: Sarin was invented in Germany in the 1930s but was not used in combat during World War II. After the war most major powers developed nerve gas, with the British inventing VX.

Tabun and sarin were among the agents used by the Iraqi government when it killed 5,000 Iraqi Kurds in Halabja in 1988. Perforated bags of liquid sarin were left in the Tokyo underground in the 1995 attack by the Japanese Aum Shinrikyo cult. Twelve people died.

It is uncertain whether some coalition forces were exposed to small traces of nerve agent during the 1991 Gulf War.

Production: The materials needed to produce nerve agents are relatively cheap and easy to come by, but manufacture would require sophisticated laboratories and technical expertise. The Japanese Aum Shinrikyo cult only managed to create its own gas by recruiting scientists. Both the US and Russia admit to having made nerve agents.