Gas Masks and Filters in Metro 2033

If you're playing Metro 2033, there's an important concept that you must come to terms with..
Have plenty of filters and don't let your Gas Mask take damage.
The sooner you get familiar with these things, the easier it would be to play the game.

There's another small problem... You don't get a lot of ammo. What you have is not going to be enough to blast your way out of attacks by blindly spamming bullets. Remember, it's a shooter... but it's not Modern Warfare 2 ;)