We have discussed previously, the many things that a gas mask are not good for; however, that doesn't change the fact that there are certain situations which a gas mask will not help you in, but which you dearly need help to survive. A fire for instance, produces an environment where a gas mask won't help you at all, regardless of the fact that you really need help.
The problem comes whenever you run into a situation that leaves you with an oxygen depleted environment. Underwater, for instance, is actually a very oxygen rich environment. The problem is that the oxygen is mixed with hydrogen, and is therefore useless to us for respiration. The same is true for a smoky room. That oxygen is mixed with carbon and can't be breathed. The answer for a situation such as this is to have an apparatus that offers pure oxygen to the lungs, much like SCUBA gear does for divers.
Firemen use what's known as a SCBA. This acronym stands for Self Contained Breathing Apparatus. (SCUBA means Self Contained Underwater Breathing Apparatus) And that is what you will need if the oxygen in an area is depleted to the point that the chemical irritants can't be separated from it. The gas mask is best used outside, against chemical irritant that doesn't mix with oxygen molecules well. There are civilian models of SCBAs that are affordable and which work well if you would like to keep some in your house for your family; however, bear in mind that they require some maintenance as the oxygen goes stale after a while.