Experts say it is too early to say whether sarin or a mix of substances was used in the attack on rebel-held Khan Sheikhun

What happened in Khan Sheikhun ?

Syrian government planes carried out a dawn raid on the rebel-held town of Khan Sheikhun on Tuesday morning. Following the airstrikes, residents reported whole families found dead in their beds, with victims and injured survivors showing symptoms that match poisoning by nerve agents.

These symptoms included pupils shrunk to the size of pin pricks, foaming at the mouth, breathlessness and convulsions.

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The most likely poison is thought to be sarin, which killed hundreds in an attack on the Damascus suburb of Ghouta in 2013. After those deaths the Syrian president, Bashar al-Assad, agreed to give up his chemical weapons supply for destruction.

But a large supply of sarin was allegedly unaccounted for when the stockpile was removed, according to Hamish de Bretton-Gordon, a former commanding officer of the UK Chemical, Biological, Radiological and Nuclear (CBRN) Regiment and director of Doctors Under Fire.

Even experts who say it is too early to confirm sarin use say the deaths have all the hallmarks of a chemical weapons attack, delivered through airstrikes.

“It is possible it’s sarin but also possible it could be something else, or a mix of things. We mustn’t fall into the trap of thinking that [only] one substance was used, when it could have been more than one,” said Richard Guthrie, a British chemical weapons expert.

“The key thing I’m confident of here is that a material has been deliberately dispersed in order to cause harm. People flying the aircraft wanted to kill other people with poison.”

Médecins Sans Frontières, whose doctors treated some of the victims, said both a nerve agent and chlorine appeared to have been used.

“Victims smelled of bleach, suggesting they had been exposed to chlorine,” the group said in a statement, after detailing symptoms of neurotoxins. “These reports strongly suggest that victims of the attack on Khan Sheikhun were exposed to at least two different chemical agents.”

What is sarin and how does it work?

Sarin is a colourless, odourless liquid at room temperature. It can kill in very small doses through contact with the skin or inhalation when it is in vapour form.

When used in weapons, it is usually fired in a rocket or shell that on impact disperses the liquid as an aerosol – a cloud of tiny droplets fine enough to be inhaled or mist the skin and eyes. Some sarin will also evaporate into gas.

Once inside the body, Sarin interferes with an enzyme called acetylcholinesterase, which helps control the nervous system. A few drops can be lethal, with children particularly vulnerable, and death can come within minutes.

It was first manufactured in 1938 by scientists in Nazi Germany, who were working on pesticides and stumbled across the lethal poison. They passed the recipe to the army and it was loaded into shells although never used against allied forces.

The main treatment is injection with a chemical antidote called atropine, which blocks the effect of sarin on the nervous system and can revive victims who appear almost dead. It is in short supply in rebel-held Syria.

Rinsing less badly-affected survivors with water can help with decontamination, and oxygen can ease breathing difficulties, but neither can halt the action of sarin or reverse any damage it has caused to nerves.

Russia has denied that Syria launched a chemical weapons attack. Does their argument have any credibility?

Syria’s military has “categorically denied” responsibility for the attack. Russia, which is heavily backing the Assad government, said a Syrian government airstrike had hit a “terrorist warehouse” holding “toxic substances”.

That claim does not fit with facts on the ground, for several reasons. An airstrike on a weapons depot with high explosives would have destroyed much of the sarin immediately, and distributed any that survived over a much smaller area.

“The pattern of casualties isn’t right for the distribution of materials that you would get if you had a location with toxic materials breached by an airstrike. It’s more consistent with canisters that have distributed [chemical weapons] over a wider population,” Guthrie said.

While it is impossible to assess the exact amount of chemical agent used immediately, the extent and distribution of the casualties are consistent with the use of hundreds of kilos.

Sarin is too complicated and expensive for rebels to have manufactured themselves, and while they might potentially have obtained some supplies of stolen nerve agents or other gas, it is very unlikely to be more than a few kilos.

“If they have [sarin], it would be in minute quantities, maybe a kilo or so,” said De Bretton Gordon. The high numbers of woman and children among the casualties was not consistent with a military depot, he added.

Finally, the Syrian manufacturing process for sarin involves creating and storing two key components, both far more stable than the nerve agent itself. They are mixed to create sarin hours – or at most days – before it is used, said Dan Kaszeta, a chemical weapons expert and former officer in the US Army’s chemical corps.

So an airstrike on a storage facility would be unlikely to release sarin itself. And because one of the two components is highly flammable isopropyl alcohol, or rubbing alcohol, you would expect a fireball, which has not been observed.

Can anything be done to stop future attacks?

Doctors and experts have called for greater supplies of gas masks and the antidote to sarin to be sent to rebel-held areas of Syria to help limit casualties in the case of another attack.

That response is in part recognition of the futility of international efforts to limit use of chemical weapons in Syria, after the initial outrage that followed the 2013 attack in Ghouta.

Although Assad destroyed much of his stockpile then, there are allegations he kept some supplies. The military also has ready access to chlorine, which can be used as a weapon but is also an industrial chemical needed for peaceful uses, including water purification.

After Barack Obama publicly abandoned his “red line” on the use of chemical weapons in Syria, there has been little overt military threat to Assad for deploying powerful and once-taboo weapons against rebel-held areas.

There has been no response, other than toothless censure, to UN reports confirming use of chemical weapons by government troops.

“Chemical weapons are incredibly successful, that’s why Assad is using them,” said de Bretton Gordon. “What we need is safe zones, particularly places like Idlib, to protect civilians.”

But the Syrian airforce and anti-air defences have always made the prospect of trying to control its skies risky.

Russia’s decision in 2015 to double down on military support for Assad would make any attempt to enforce a no-fly safe zone even more dangerous and carry a serious risk of escalation.

What does this attack mean for the future of chemical weapons?

There is growing concern that increasing deployment of once taboo chemical weapons in the Syrian civil war, with apparent impunity, is eroding decades of work to control their manufacture and use.

“In 2016 we saw the highest frequency of use of chemical weapons since 1916,” said De Bretton Gordon.

Chemical weapons have helped Assad hold on to power during a long and brutal civil war, at the cost of immense civilian suffering. The fact that he has faced no real consequences for their use risks setting a dangerous precedent, with implications that carry far beyond Syria, weapons expert Guthrie said.

“You would have expected the use of chemical weapons, as deployed by Assad in 2013, would result in him being deposed by his own people or by the international community.

“That hasn’t happened, so what is the disincentive for a dictator to think, ‘This is a weapons system that might help me survive.’”