By Debora MacKenzie THE US has had a change of heart: its government is now reasonably confident that the Syrian military has used chemical weapons, specifically sarin. But things still don’t add up. On 24 April, US defence secretary Chuck Hagel dismissed chemical weapons claims as “suspicions”. However, on 25 April, in a letter to US senators, a White House spokesman said that US intelligence has decided “with varying degrees of confidence, that the Syrian regime has used chemical weapons on a small scale”. Barack Obama has declared the use of chemical weapons a “red line” that would elicit a response by the US, potentially embroiling the country in Syria’s civil conflict. However, the letter also called for “a comprehensive United Nations investigation that can credibly evaluate the evidence”. Even if UN inspectors receive permission to enter Syria, it is not clear how much evidence of sarin use they will uncover. Obama’s red line might be safe for now. The change of heart was based, said the letter, on “physiological samples” from victims. These were reportedly from an attack by Syrian president Bashar al-Assad’s forces on the city of Homs, and an attack on the village of Khan al-Aisal outside the northern city of Aleppo. Assad says the rebels killed 25 of his troops using chlorine gas at Khan al-Aisal, but rebels insist Assad’s troops used chemical weapons against them. Assad asked UN secretary-general Ban Ki-moon to send an inspection team to investigate his claims. Ban declared that all the sites should be investigated. Possibly as a result, the inspection team that Assad invited is still waiting in Cyprus for permission to enter Syria. There are clearly some chemicals being used, but footage of victims raises more questions than it answers. After the attack on Homs, doctors described symptoms that included pin-point pupils and convulsions – signs of sarin – but also fluid in the lungs that lead to death, which is not. US officials later declared that the incident involved CS gas, a tear gas used legally as a riot-control agent. Jean-Pascal Zanders of the European Union Institute for Security Studies in Paris says images of the alleged attacks do not suggest a nerve gas like sarin. “There are no convulsions or dead bodies,” he says, “only single patients being treated in crowded emergency rooms. I’d expect clusters of casualties, and deaths.” Moreover, no one seems to have been exposed to nerve agents when handling victims. There are no convulsions or dead bodies. I’d expect clusters of casualties if sarin had been used Only one of four such videos “shows any real poisoning symptoms”, says Richard Guthrie, an independent chemical weapons expert in London, but those could be caused by exposure to industrial chemicals released when factories are bombed. Zanders says it would be nearly impossible to get chlorine to levels that would kill as many fighters as Assad claims. But no evidence of the chemical would be left for inspectors to find by now. Traces of sarin should still be detectable in blood, urine or soil samples. But finding it wouldn’t tell us where it came from. Charles Blair of the Federation of American Scientists says it would be in the interests of rebel forces to involve the US in their fight against Assad – and that the origins of the samples coming out of Syria cannot be guaranteed. Some say a sarin-tainted sample would be hard to fake. For one thing, the US believes that Assad controls all of Syria’s chemical munitions. But that may not be the only source. When the US had chemical weapons, the army, until 1969, gave out small vials of agents, including sarin, to teach soldiers to recognise their smell. Similar vials used to train Syrian soldiers might have tempted the rebels. This article appeared in print under the headline “What chemical weapons have been used in Syria – if any?