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A man, affected by what activists say is nerve gas, breathes through an oxygen mask in Damascus suburbs August 21, 2013 in this picture provided by Shaam News Network. Syrian activists accused President Bashar al-Assad's forces of launching a gas attack that killed more than 200 people on Wednesday, in what would, if confirmed, be by far the worst reported use of chemical arms in the two-year-old civil war. Syrian state television denied government forces had used poison gas and said the accusations were intended to distract a team of United Nations chemical weapons experts which arrived three days ago.

Three authorities on chemical weapons tell The Globe and Mail's Daniel Bitonti whether they believe the attack in Syria involved poison gas - and why proving it definitively is so difficult

Was this a chemical-weapons attack?

All three experts say they believe a chemical weapon was used, based on the kinds of symptoms people were displaying in picture, video and anecdotal evidence.

Stephen Johnson, an expert in weapons and chemical explosives at the U.K.'s Cranfield Forensic Institute, said the symptoms victims have displayed in the widely circulated videos and photographs point squarely to a nerve agent.

"The salivation, the pinpointing of the pupils, with the shaking of the limbs, and unconsciousness. And while there are other things that can cause each of those symptoms, the scale and the number of these videos really does build quite a convincing case for investigation," he said.

Hamish de Bretton-Gordon, a former commander of British biological and chemical counter-terrorist forces, said he's been studying the apparent attack, "and talking to doctors and others in Damascus. I think first of all, ... likely over 2,000 people have died here and they haven't died from a conventional weapon," he said.

"And the suggestion that it has been faked has pretty much been disproved."

Mr. de Bretton-Gordon said that staging pictures of dead babies and children is nearly impossible to do.

What kind of chemical agent was likely used?

Two of the three experts interviewed by The Globe believe that the nerve agent used in the apparent attack was sarin, which causes severe respiratory distress, limpness and death. Victims of sarin show no visible wounds. The experts base their assessments on the symptoms they observed, as well as the fact that the Syrian government is known to have large stockpiles of the chemical.

"We know that sarin has been used in Syria before. British and American leaders have publicly stated that they have analyzed samples …that have proven positive for sarin," Mr. de Bretton-Gordon said. "We also know that [Syrian President Bashar] al-Assad has large stockpiles of sarin."

Mr. Johnson says sarin was likely used based on the regime's history. In April, British Prime Minister David Cameron said the U.K. had gathered credible evidence proving that sarin was used by Syrian government forces in a March 19 attack on Utaybah, as well as on an April 13 attack on Sheikh Maqsood. U.S. Defence Secretary Chuck Hagel also said in April that sarin had been used on a small scale by Syrian government forces.

"Many of the toxins act on the nervous system … but evidence is being produced previously suggesting a use of sarin. It seems incredibly likely that sarin is part of the Syrian weapons arsenal," Mr. Johnson said.

Dan Kaszeta isn't convinced.

"Certainly a chemical seems to be used, I just don't think it is one of the normal, traditional chemical-warfare agents," said Mr. Kaszeta, an independent chemical and biological defence consultant who formerly served in the U.S. Army's Chemical Corps as well as the Secret Service. "The symptoms don't add up consistently, and the symptoms that are most prevalent are ones that are generic: People having difficulty breathing, people with irritation in the eyes."

He said reports of Syrians having smelled an odd odour in the area after the apparent attack also suggest that sarin wasn't used because it is odourless.

While Mr. Kaszeta didn't say specifically what he thought was used, he believes it was a toxic industrial chemical. "Probably not something corrosive because nobody seems to have any burns; probably not something terribly flammable because you would have a lot more fires," he said.

Mr. de Bretton-Gordon said, however, that reports of odd odours don't rule out sarin.

"When it comes to smells, in the fog of war there are other conventional artillery used at the time, so smells could be coming from virtually anything," he said.

What needs to be done to prove (or disprove) a chemical attack?

All three experts agree that getting inspectors immediate blood and hair samples, as well as access to the area, will be key in proving with certainty that chemical weapons were used in the attack. But the experts also say this will be difficult. UN inspectors are already in Syria investigating earlier alleged uses of chemical weapons, but they have not been able to access the area of Wednesday's attack.

"I think we're going to be reduced to maybe hopefully medical evidence. And that gets difficult because Islamic cultures don't typically like graves being exhumed and postmortems being done," Mr. Kaszeta said. "Blood samples are not good for very long, and it's not the type of thing a normal hospital is going to be able to look for. This is very, very sophisticated toxicology forensics."

Rebels have said they have smuggled out blood and hair samples. According to Mr. Johnson, scientists will be looking for a byproduct of sarin found in the blood called IMPA.

"Certainly if you find IMPA in the bloodstream, a cursory survey of the academic literature doesn't really give any other reason than exposure to sarin," he said. "But obviously to find those markers that are unambiguous, you really want to be looking at blood samples in the first week."

Scientists can also test pools of water and soil for sarin, but that, too, can prove a challenge.

"To actually get the sampling, you have to set up a forensic crime scene, white tape around it and get on your hands and knees to collect evidence," said Mr. de Bretton-Gordon. "From my experience in war zones, that's just not going to happen."

How will this affect the international community's ability to declare that a "red line" has been crossed?

France has already said that if it's proven that chemical weapons have been used by the Syrian regime, military force will be the consequence. But all of the experts say justifying international intervention will be quite hard to do.

"Proving when the red line gets crossed, particularly in active war zones where people are actively killing each other, and evidence is getting blown up and witnesses are saying contradictory things…it's a lot harder to do than people think," Mr. Kaszeta said. "You have to remember Saddam Hussein gassed the Kurds… did it brazenly and openly. And there are still some people who deny it happened or blame the Iranians."

Mr. de Bretton-Gordon described it as an "impossible situation" for world leaders. "From a legal point of view, they need that hard evidence. It strikes me that the UN is not going to go in there because the regime is not going to let them in," he said.

He added that the best the international community can do at this point is help protect the opposition. "I have been urging the British government for some time to give protective equipment to the opposition. These people are unprotected and had no chance of survival," he said. "Some protective equipment and some information to know how to survive these things means that if it's used again, we won't see mass casualties."