Sarin, the deadly nerve gas which the US says was unleashed last month by the Syrian regime in a Damascus suburb, was developed by Nazi scientists in 1938.
Originally conceived as a pesticide, sarin was used by Iraqi dictator Saddam Hussein's regime to gas thousands of Kurds in the northern town of Halabja in 1988.
A cult also used the odorless, paralysing agent in two attacks in Japan in the 1990s.
US Secretary of State John Kerry on Sunday said tests on hair and blood samples taken from the emergency workers who rushed to the scene of the Damascus attack on August 21 had shown indications of sarin.
He said the samples had been given to the US independently, outside of an outgoing UN probe.
Washington has squarely blamed the Syrian regime of President Bashar al-Assad for the attack, which it says killed more than 1400 people including hundreds of children.
Inhaled or absorbed through the skin, the gas kills by crippling the respiratory centre of the central nervous system and paralyses the muscles around the lungs.
The combination results in death by suffocation, and sarin can contaminate food or water supplies, according to the US Centers for Disease Control and Prevention (CDC), which notes that antidotes exist.
"Sarin is 26 times more deadly than cyanide gas. Just a pinprick-sized droplet will kill a human," according to the World Health Organisation.
Exposure symptoms include nausea and violent headaches, blurred vision, drooling, muscle convulsions, respiratory arrest and loss of consciousness, the CDC says.
Nerve agents are generally quick-acting and require only simple chemical techniques and inexpensive, readily available ingredients to manufacture.
Inhalation of a high dose - say 200 milligrams of sarin - may cause death "within a couple of minutes", with no time even for symptoms to develop, according to the Organisation for the Prohibition of Chemical Weapons.
Exposure through the skin takes longer to kill and the first symptoms may not occur for half an hour, followed by a quick progression.
Even when it does not kill, sarin's effects can cause permanent harm -- damaging a victim's lungs, eyes and central nervous system.
Heavier than air, the gas can linger in an area for up to six hours, depending on weather conditions.
UN inspectors, who have been in Syria investigating allegations of the regime's use of chemical weapons, left the country on Saturday. The analysis of their samples could take up to three weeks, UN experts have said.
