There was an article the other day, I think in the Telegraph, that the agent used has a long shelf life and when deployed, takes a long, long, lonnnnnng, time to "evaporate", which was the word the chem weapon expert used that they cited.
Nasty stuff.
oddly enough, as I was watching the news conference, I remember thinking “And this is what it looks like when we the people are being massively lied to. It almost feels like post 9-11, when we were urged to shop in defiance of the bombers.”