The Novichok nerve agent has everyone worried, but just how dangerous is it? One of its developers is Russian scientist Vil Mirzayanov, now living in Princeton, and he warns it is appalling.
"It is the same as nerve gas but ten times, at least ten times more powerful. There's no cure. It's very nasty stuff. Antidotes exist, but what does antidote mean? You saving a person who is exposed to this gas, temporarily. They won't immediately die, but they'll be invalid for the rest of their lives".
"I am guilty, I participated in this criminal enterprise," continues Mirzayanov. "Because of that, I am probably the most firey enemy of these chemical weapons. It's a weapon of mass killing, mass murder!"
The Soviets worked from the 1970s on a range of chemical, biological and nerve agents, and Mirzayanov blew the whistle on the entire programme in 1992. He was initially arrested and imprisoned in Russia, before being allowed to leave for the USA.