Inspectors from the international chemical weapons watchdog are to start an investigation into an attack on a town outside Damascus that killed more than 40 people, injured up to 500, and led to the threat of military strikes against Syrian government targets.

The team from the Organisation for the Prohibition of Chemical Weapons will begin work on Saturday to determine whether chemical weapons were used in Douma on 7 April.



The town had been the last opposition stronghold in the Syrian capital. It was recaptured in the days following the attack, which has drawn broad condemnation by the US, France and UK, but strident denials from Russia that it actually took place.



The World Health Organisation said on Wednesday that close to 500 people sought treatment at medical facilities it supports. Many had symptoms consistent with exposure to toxic chemicals, notably chlorine gas and a nerve agent. Russian military officials who visited the site said no patients had complained of such symptoms.

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With the war of narratives raging in the aftermath of the attack, attention is focusing on the science surrounding what took place. The investigation will focus on gathering samples from the people affected and the immediate surroundings. The inspectors will also interview doctors, patients and other witnesses.

Establishing whether chlorine was used will be harder than identifying any nerve agent. Blood and urine samples rarely retain any useful signatures from a chlorine attack, and it will be extremely challenging to find telltale signs in the environment. The gas will have gone, leaving chloride behind, but there is so much chloride in the environment already that confirming any was produced in a gas attack is not easy.



Facebook Twitter Pinterest UN inspectors in Syria in August 2013. Photograph: Shaam News Network/EPA

Instead, inspectors will look for a compound in the environment that only tends to form in the presence of chlorine gas. Details of the chemical signature are secret, a chemical weapons specialist said, because the forensic procedure has not yet been published.



Greater attention will be devoted to whether a nerve agent, such as sarin, was deployed. There have been two previous confirmed examples of large-scale sarin use in the Syrian conflict – in both cases the OPCW found that the sarin used against opposition targets came from regime-held stockpiles. Horrific images showed large numbers of dead and dying people in Ghouta in 2013, and Khan Sheikhun in April last year.

If a nerve agent was used, evidence should not be hard to find, experts say. One starting point could be the basement of the house where many of those killed or injured were sheltering.



“That would be the best place to start,” one expert told the Guardian. “An enclosed space traps the gas and some will be absorbed on to different types of material.”

Russian military teams inspected the house and declared nothing had happened there, raising concerns among the families of the dead that the scene had been tampered with. However, the experts said specimens gathered from materials including window seals may retain traces of any nerve agent used, even after a clean-up attempt.



“It’s very hard to clean a place so well that you cannot find it. You always find something,” one expert said.

If no sarin is found, a reliable smoking gun for the agent is the compound isopropyl methylphosphonic acid (IMPA), but it can be hard to find after about a week. It leaves behind another compound, methylphosphonic acid (MPA), but this is also a residue from other agents, including VSX and soman.



Facebook Twitter Pinterest A Syrian man collects samples from the site of a suspected toxic gas attack in Khan Sheikhun in April 2017. Photograph: Omar Haj Kadour/AFP/Getty Images

The inspectors will also gather blood and urine from those apparently affected by the attack. When sarin enters the blood, it binds with proteins to form “protein adducts” which can be detected more than a month after exposure. The US government said on Thursday it had received urine specimens from Douma which contained the presence of a nerve agent.

With their work facing intense scrutiny, the team must confirm that people who claim to be victims were at the site at the time of the alleged attack, and oversee directly the collection of samples. Along with others from the environment, these samples will be flown with a courier to the OPCW lab in the Hague. From there, the samples will be split and sent to OPCW-affiliated labs around the world for analysis.



Last year, chemical weapons inspectors released a report that used signature impurities to trace the trail of sarin used in an April attack in Khan Sheikhun back to stockpiles held by the Syrian regime. In Douma, the inspectors have no mandate to trace the origins of any chemical weapons to their source.