+1Don't assume a gas mask, no matter what make or model it is, will protect you from the chemical being used. In Bosnia we were required to carry our gas masks in response to threats of using Chlorine Gas against us. The issued masks offer zero protection against chlorine.
Different chemicals will rise or fall according to their weight compared to the atmosphere. Chlorine for example will sink into low points. Climbing out of low-lying areas will help in this case but that won't necessarily be the case with every chemical.
When possible use plants, animals and those weaker than you as chemical detectors. If you start seeing pigeons doing the funky chicken out in the park it may be time to imshee out of the area.
yeah it is and one of the biggest stock piles in the US is in mine and snakebites (0n this forum) back door.Plus many chemicals are absorbed through the skin anyway so a mask will do nothing; you would need a chem suit if you were really concerned about it. Pretty scarey crap really.