The deadly nerve agent tailor-made for mayhem

If a terrorist group wants to use - or threatens to use - chemical weapons, sarin is an obvious choice. In its purest form it is 500 times more lethal than cyanide, and kills within two to 15 minutes. It disperses rapidly, except in very cold weather, and evaporates about as fast as petrol. It would be particularly effective in a hot underground train.

Any university laboratory can prepare the ingredients and it can be mixed from two relatively innocuous components on the spot - a "binary" chemical weapon, making it relatively safe to store and transport.

President Saddam Hussein used it against the Kurds at Halabja, in northern Iraq, in 1988, as part of a chemical attack which killed an estimated 5,000 people.

Experts were surprised at the low level of casualties in yesterday's attack. Although several thousand people reported symptoms, only six died, suggesting that the agent was impure.

"I'd have expected a much higher proportion of people killed," said Julian Robinson, an authority on chemical weapons at Sussex University.

Less than a milligram of the liquid - about the size of a pin-head - will kill a person through the skin and the vapour is lethal to anyone breathing it in at a concentration of 100 milligrams per cubic metre. The small quantities required to produce yesterday's mayhem could have been manufactured in a laboratory.

The first nerve agent, tabun, was discovered in 1936 by Dr Gerhard Schrader, a German scientist working at IG Leverkusen, while he was conducting research into pesticides. A year later he discovered a related compound, almost 10 times as lethal, which was christened sarin. The name is an acronym of the initials of people working in Schrader's group.

Germany manufactured 300,000 tons of it for use during the Second World War but did not use it, even when close to defeat, because the Germans feared retaliation from the Allies who had far better means of delivery.

Pure sarin is a colourless liquid. Its chemical name is iso-propyl-methylphosphoro-fluoridate. In a binary chemical shell it is produced by mixing iso-propanol with methyl-phosphonyl-difluoride. The two precursors combine rapidly as the shell spins. Any impure mixture is likely to produce an acrid smell, which may account for reports yesterday of a smell like "paint stripper".

Nerve agents work by inhibiting the action of cholinesterase, the chemical which switches off the action of nerve cells in a person. The first symptom is dimmed vision, caused by contraction of the pupils in the eye. Low doses produce tightness of the chest and breathing difficulties. Higher doses produce headaches, muscle weakness, tremors, vomiting and incontinence. Lethal doses cause the heart and lungs to stop and are often accompanied by convulsions.

The main antidote is atropine, which acts as a partial substitute for the cholinesterase. Oxime tablets can help to break the bond between the nerve agent and the cholinesterase.

These tablets formed the "nerve agent pre-treatment sets" taken by troops in the Gulf War, which may have been responsible for some cases of Gulf War syndrome.