People routinely live above 4000 m, where oxygen partial pressure is under 130 mbar. True, we often suffer mountain sickness there, and take days adapting to the height as we walk up. But airplanes are only pressurized to 760 mbar total, 160 mbar oxygen. And that drop, from 210 mbar to 160 mbar, happens in a few minutes as the plane climbs, with the few problems being caused by pressure changes, not lack of oxygen. Now, note that while oxygen undergoes exothermic reactions to burn, very few substances (like Mg and Li) are capable to fix nitrogen. Nitrogen takes up heat. Therefore in an environment where oxygen concentration is lower than 21 %, even though oxidation reactions like breathing still happen and release heat, the heat is spread to larger amount of nitrogen molecules, the peak temperature reached is lower, and that temperature most reactions are slower - and this hampers thermal runaways like fire. Many common flammable substances cease to burn at surprisingly high oxygen concentration - like wood already at 17 %. So... Would it be safe to fill a room with gas of 12...13% oxygen, the rest nitrogen? To prevent accidental suffocation on pure nitrogen of 0 % oxygen, it would make sense if the nitrogen is already mixed with 12...13% oxygen while stored. True, 12...13% oxygen entered into rapidly (under a minute) might cause a mild impairment. But breathing fire and smoke also causes rapid impairment of a man. Also, if air is stored in metal pressurized bottles, at room temperature and around 200 bar pressure, and suddenly released in case of need, it cools by Joule-Thompson effect. IIRC by about 30 degrees. So, cold air flow at about -10 Celsius can also be mildly impairing to man, but not seriously so. Would a blast of cold air at -10 Celsius and 12 % nitrogen be effective in blowing away fires and cooling overheated objects? Now, 12...13 % oxygen is mildly impairing to man. Would 15...17 % oxygen be a comfortable and safe environment to work in permanently, to avoid fire breaking out in the first place?