IRAN AND TOXIC GAS
Chemical weapons expert Dan Kaszeta, an associate fellow at the Royal United Services Institute (Rusi), said that "finding the alleged causative substance is often the only useful evidence, but can be extremely difficult". As substances can dissipate or degrade, collecting a sample "pretty much requires you to be there, with the right equipment, at the time of exposure," he tweeted. Many witness accounts from Iran have focused on smells - describing a tangerine or rotten fish odour - but this can be misleading, he said. "The various odours described in the Iranian incidents are difficult to tie to particular chemical hazards," he said. In some videos, girls can be heard complaining about tear gas, which has been widely used during recent anti-government protests. Mr Kaszeta said that was "plausible in some way", as poorly-made tear gas can release off "a lot of junk" with a range of odours.