Here is the first part of an article on the "Real Deal" with NBC weapons.
sourced at the Washington Compost
Words of Wisdom About Gas, Germs, and Nukes By SFC Red Thomas, Armor Master Gunner
U. S. Army (Ret) 10.19.01
Since the media have decided to scare everyone with predictions of chemical, biological, or nuclear warfare on our turf I decided to write a paper and keep things in their proper perspective. I am a retired military weapons, munitions, and training expert.
Lesson number one: In the mid 1990s there was a series of nerve gas attacks on crowded Japanese subway stations. Given perfect conditions for an attack, less than 10% of the people there were injured (the injured were better in a few hours) and only one percent of the injured died. CBS-Television's 60 Minutes once had a fellow telling us that one drop of nerve gas could kill a thousand people. He didn't tell you the thousand dead people per drop was theoretical. Drill Sergeants exaggerate how terrible this stuff is to keep the recruits awake in class (I know this because I was a Drill Sergeant too).
Forget everything you've ever seen on TV, in the movies, or read in a novel about this stuff, it was all a lie (Read this sentence again out loud!). These weapons are about terror, if you remain calm, you will probably not die.
This is far less scary than the media and their "experts" make it sound. Chemical weapons are categorized as Nerve, Blood, Blister, and Incapacitating agents. Contrary to the hype of reporters and politicians, they are not weapons of mass destruction. They are means of "Area Denial," effective to keep an enemy out of a particular zone for a limited period of time: terror weapons that don't destroy anything. When you leave the area you almost always leave the risk.
That's the difference; you can leave the area and the risk. Soldiers may have to stay put and sit through it and that's why they need all that spiffy gear.
These are not gasses; they are vapors and/or airborne particles. Any such agent must be delivered in sufficient quantity to kill or injure, and that defines when and how it's used.
Every day we have a morning and evening atmospheric inversion where "stuff," suspended in the air gets pushed down. This inversion is why allergies (pollen) and air pollution are worst at these times of the day.
So, a chemical attack will have its best effect an hour of 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.