BBC News Online looks at the use and effects of weapons such as mustard gas, which burns the eyes, skin, and lungs.
Mustard gas was part of Iraq's chemical weapons stockpile
Main agents: Mustard gas, lewisite.
Effects: Agents burn skin, eyes and lungs as they are absorbed, causing large blisters on skin and inside lungs and windpipe. Effects are delayed for up to 12 hours after exposure, which can allow the agent to cause severe damage before it is detected.
Blister agents cause large numbers of casualties, but fatalities are relatively rare unless the agent is undetected. Long term disabilities and cancer are common in victims.
Recognition: Blister agents do not smell strongly, but have hints of garlic, geraniums or fish. At room temperature, mustard is a liquid which gives off a vapour, but it can also be made into an aerosol or dust.
Use as a weapon: Blister agents can be used to fill shells and bombs.
History: Mustard gas was first used by Germany in World War I against Russia. It has been used by Italy in Ethiopia in 1935-36, Egypt in Yemen in the 1960s and Iraq against Iran in the 1980-1988 war.
Production: Cheap and simple, but special equipment is needed.
Protection: Blister agents attack the skin and lungs in both vapour and liquid form, so full body protective suits are required as well as gas masks. They can linger for some time.