More than a month after Israeli and American officials first alleged the use of chemical weapons in Syria, United Nations investigators say they have "reasonable grounds" to believe they’ve been used in attacks this spring.

Specifying the chemical weapon as sarin gas, France went further, with its foreign minister saying there was no doubt the Syrian government had attacked rebels with sarin at least once.

The allegations are putting a spotlight on a little-known and seldom-used lethal chemical weapon that can be difficult to detect. Here’s what’s known about this toxin.

What is sarin?

Sarin is a man-made liquid toxin that is clear, colourless, and tasteless. Once it’s released, it evaporates into a vapour that can spread through the air and enter the body through the eyes, nose and mouth.

The chemical also mixes easily with water and can be used to poison water supplies or food. Ingesting sarin can be fatal, but even touching sarin-contaminated water can lead to poisoning.

Sarin was developed as a pesticide in Germany in 1938, but during the Second World War Nazi scientists realized they could synthesize it into a chemical weapon.

The agent was reportedly used several times during the Iran-Iraq war in the 1980s. More recently, members of a doomsday cult in Japan used sarin gas in two attacks in 1994 and 1995.

In the first attack, they attempted to kill three judges be releasing it out of a refrigerator truck. Wind changes caused the assailants to miss their target, but eight bystanders died.

The same group used the gas during attacks on five crowded subway cars in Tokyo the following year. To release the gas, the assailants punctured newspaper-wrapped plastic bags with sharpened umbrella tips, sending out a gas that sickened thousands of passengers and killed 13.

How does it work?

Sarin can be deadly because it interferes with a key neurotransmitter that acts as an “off switch” for glands and muscles. When the neurotransmitter malfunctions, the muscles and glands become overstimulated.

Exposure to sarin gas triggers symptoms within a few seconds. Low doses can cause a runny nose and blurred vision, drooling or frothing at the mouth, nausea and vomiting. Larger doses can cause convulsions, paralysis, respiratory failure and eventually death.

Exposure to liquid sarin is slower to take effect, from a few minutes to as much as 18 hours depending on the dose.

People exposed to low or moderate doses in gas or liquid form usually recover completely, though some neurological problems can linger for one to two weeks.

Exposure to high doses is usually fatal within minutes. There are antidotes available to counteract its effects, but the treatment must be administered soon after exposure.

How is it identified after an attack?

As a gas, sarin disperses and evaporates quickly, though it does leave a residue in the area where it was used and can be detected if the investigation is fast enough. In the Tokyo attacks, it took police just three hours to confirm the use of sarin gas.

The gas also leaves traces in the body and can be detected in blood, urine or hair samples.

But detection becomes difficult within just a few weeks, when the level of the chemical may no longer be measurable at the scene of a suspected attack, or in a victim's body.

In the suspected attacks in Syria, witnesses said they saw regime forces fire projectiles that caused rebel fighters to throw up, struggle to walk, and drop to the ground.

Some of the videos posted by activists have shown rows of civilians in what appear to be makeshift hospitals, sometimes twitching as they struggle to breathe with the aid of oxygen masks.

On Tuesday, both the French and British governments said they had confirmed that sarin was used "multiple times and in a localized way" in Syria, including at least once by the regime.

The British government did not say when or where the samples that it tested were obtained. But the French government said it was able to test samples of blood, urine and hair from sickened rebel fighters. It said it received the samples after reporters from the French newspaper Le Monde persuaded Syrian doctors to hand the samples over with the promise they would analyze them for the toxin.

The test results were passed to the head of a United Nations chemical weapons investigation team on Tuesday in Paris.