Before the national anxiety over bioterrorism drives you to buy a gas mask at the nearest army surplus store, take a deep breath and brace yourself for what experts say about them -- most of it is not good. One could never know where the exposure is; you'd have to wear the mask at all times. Even if you could stand to do that, the risks to your health would probably be higher than the likelihood that wearing the mask would prevent contamination.
And wearing a mask could be particularly dangerous if you have a respiratory disease or asthma, which already hampers your breathing. There were 20 deaths during the Gulf War associated with wearing the gas masks improperly. And those were people who were trained to use them. But if you insist, buy the best.
The device used by firefighters or emergency medics who get to a disaster first is a self-contained breathing apparatus, which has its own oxygen canister, good for about a half-hour of breathing time. First responders also wear impermeable clothing designed to protect from agents that penetrate the skin.
The second-most protective masks are those that fit either over the full face or over half the face and contain filters. These masks can protect against breathing in most gases and biological agents as long as the person is wearing the correct filter, which only can be used once.