The short answer to Rommie's question: It depends on the agent.
There are chemical agents that are designed to hang around and keep causing problems. Think mustard gas from WWI.
There are agents that are designed to be short acting.
Some agents are intended to incapacitate and others are intended to be lethal (think nerve agents).
The standard chemical weapons being used in the middle east are probably a variant on mustard gas. Mustard gas is especially nasty because it's a blistering agent that causes severe chemical burns. If the victim inhales the gas into the lungs the lungs become blistered and stop functioning. If the victim gets in on the skin it can cause the skin to peel off due to the chemical burns.
The usual method of delivery is by an artillery shell. One shell by itself probably only covers a small area. The standard approach is to lay down a barrage of artillery shells that blanket an area. A few hundred shells could blanket several square blocks.
What makes a weapon like this so damaging is that it is indiscriminate. If you lob it into a city block packed with several hundred people virtually every one will suffer some damage.
Believe it or not most military organizations hate the stuff. It's hard to control because the best way to deliver it is as a gas (for best dispersion) which is very vulnerable to shifting winds and atmospheric conditions. And it's relatively easy to defend against with the right equipment. Plus, it does way too much collateral damage to civilian populations.
for those who just can't resist knowing more >>>>
http://en.wikipedia.org/wiki/Chemical_weapon
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