The gas mask is working for me. I used the third-floor conference area of a Germantown-based company for "health and personal security" to strap one on. Then I cut up an onion raw with paring knives. I didn't cry even though the bulb was causing a chemical reaction in my eyes.
I didn't even detect any onion's distinct tang. A gas mask can protect you from terrorists who attack with diced onions. A complete gas mask can be purchased. Onion isn't what people worry about these days. VX is. And GB, HD, and other terrifying combinations of the alphabets. Gas masks or, more generally, respirators are feared by many consumers.
The federal government does not support the idea of civilians wearing a gas masks. The use of gas masks and public hoods during chemical threats is not recommended because of legitimate security issues. Safety worries are not trivial. Use a gas mask incorrectly and you can be killed by your own fumblings before the attacker has the opportunity to get you.
This is not to say that the Department of Homeland Security is totally anti-mask. The Web site does mention that "filter masks" can help keep out germs from a biological attack or debris from an explosion, and it says that "something over your nose and mouth in an emergency is better than nothing.