In the wake of 9/11, thousands of people went out and bought gas masks. As it turns out, the kind of gas masks they were buying would not serve the function for which they were purchased.

We've all seen gas masks before. But how exactly do they work? Today we'll learn about a gas mask that works on an air purifying system. The part that goes over the mouth has a simple particle filter, basically a cloth with tiny holes to filter out even something as small as a spore of anthrax bacteria. If you need to breath in chemically polluted air, a particle filter is no good. That's where the activated charcoal filter comes in.

The charcoal is "activated" by being treated with oxygen to create millions of tiny spaces that absorb different kinds of chemicals like a sponge. Since the charcoal filter can't absorb all chemicals, the last filter uses chemical reactions to clean the air. Say I'm breathing in chlorine-saturated air, like soldiers during World War I. The chemical filter reacts with the chlorine to neutralize it and make it harmless.