It is considered 500 times more powerful than cyanide, used to kill millions in the gas chambers of Nazi death camps in the closing stages of World War Two.

Like other gases originally developed for war, Sarin was never used on the battlefield.

The fact that both Germany and the Allies had their own stockpiles of the deadly weapon proved a sufficient deterrent to its use.

First developed as an organophosphate pesticide, Sarin works by being inhaled or absorbed through the skin and kills by crippling the nervous system.

Symptoms include nausea and violent headaches, blurred or tunnel vision, drooling, muscular convulsions, respiratory arrest, loss of consciousness and then death, according to the US Centres for Disease Control and Prevention.

In high doses, Sarin paralyses the muscles around the lungs and prevents chemicals from "switching off" the body's secretions, so victims suffocate or drown as their lungs fill with mucus and saliva.

Even a tiny dose of sarin - which, like other nerve gases such as Soman, Tabun, and VX, is odourless, colourless, and tasteless - can be deadly if it enters the respiratory system, or if a drop comes into contact with the skin.

Permanent damage

Key facts

  • Developed but not used by Nazis during World War Two
  • Extremely toxic; affects the nervous system
  • Suffocates victims by paralysing muscles around the lungs
  • Single drop can kill a person in a few minutes
  • Death preceded by convulsions, excessive salivation and blurred vision
Even if it does not kill, Sarin's effects can be permanent, inflicting lasting damage to the victim's lungs, eyes and central nervous system.

Being heavier than air, Sarin can linger in an area for up to six hours, depending on weather conditions.

It is made from widely available chemicals including organic phosphorous, sodium fluoride, alcohol, but specialist knowledge and apparatus are needed to make pure and long-lasting sarin.

Experts say it is surprising that more people did not die in the 1995 and 1994 attacks.

The Aum members who carried out the subway attack left punctured packages of impure liquid Sarin in subway carriages and stations, which gave officials time to seal off affected areas after the first signs of trouble.

If purer Sarin had been released, particularly as an aerosol, the toll from the attack might have been far worse.

In the Matsumoto attack, the gas killed every living being within 100 metres: dogs in a garden, fish in a pond, while birds dropped out of the sky.

Treatment

Sarin victims can be treated with atropine, a general antidote for nerve agents.

Another family of drugs, known as oximes, are also effective.

Carbamates, a category of chemicals used in medicine and insecticides, can increase human resistance to sarin and other nerve agents if taken before exposure.

Some countries desperate to acquire weapons of mass destruction have been drawn to Sarin and other poison gases, seeing in them a cheaper alternative to nuclear weapons.

Only Russia and the United States have officially declared that they have Sarin stockpiles, but other countries - including Egypt, Iran, Libya, and North Korea - may also possess supplies, experts say.

In May 2002, the United States accused Syria of having Sarin.