Instructions
Locate a new gas mask. They are generally found in ammo and weapon caches on the surface, or on the bodies of dead men and women. Any gas mask you can equip will glow white.
Press and hold "A" on your controller. Your character will automatically exchange his gas mask for the one you have found.
Press "Down" on the 360's controller to swap out filters. Each filter for your gas mask gives you five minutes of breathing time. When you are running out of time, your character will breathe more heavily and the gas mask becomes increasingly foggy.
Avoid damage at all costs. When damaged, the mask will show visible shows of degradation, starting with small cracks in the corners. Eventually the mask will show large cracks, at which point the next time you are injured it will break, immediately killing you.