From Unofficial Stationeers Wiki

Air is defined in game as a nitrox mixture of 75% nitrogen to 25% oxygen. It is primarily used to create breathable, pressurized rooms that eliminate the need for EVA suits for humans. It is, however, ill-fitted for use in an EVA suit's oxygen supply, as the nitrogen will not be filtered out of the helmet, which, over time, lowers oxygen concentration in the helmet to unbreathable concentrations. When filling tanks in a nitrox environment, it is advised to use nitrogen filters alongside carbon dioxide filters in the EVA suit to keep the oxygen concentration high in the helmet.