I thought this was interesting, I learned a lot and felt marginally reassured - hopefully ATS will to. Worth the click on the link to read the whole
thing:
www.chiefengineer.org...

So a chemical attack will have its best effect an hour or so either side of sunrise or sunset. Also, being vapors and airborne particles, the
agents are heavier than air, so they will seek low places like ditches, basements and underground garages. This stuff won’t work when it’s
freezing, it doesn’t last when it’s hot, and wind spreads it too thin too fast.
Attackers have to get this stuff on you, or get you to inhale it for it to work. They also have to get the concentration of chemicals high enough to
kill or injure you: too little and it’s nothing, too much and it’s wasted. What I hope you’ve gathered by this point is that a chemical weapons
attack that kills a lot of people is incredibly hard to achieve with military grade agents and equipment. So you can imagine how hard it would be for
terrorists. The more you know about this stuff, the more you realize how hard it is to use.