While I was in the Navy our unit received 15 hours of instruction concerning chemical warfare. We were taught to:
1. Recognize different kinds of gases.
2. Anticipate symptoms from each of the gases.
3. Properly use a gas mask.
Then we had a graduation exercise. We stood at attention with our gas masks at our feet. The instructor told us what good students we had been and how much he enjoyed being with us. Meanwhile, someone was behind us dispersing a cloud of tear gas. By the time someone called "Gas!" all of us had our eyes and gas masks filled with the gas. I never saw another gas mask while I was in the Navy.
What is the lesson? If you buy a gas mask, you will not get any benefit from it unless you wear it all the time. By the time you discover that you need it, it will be too late.