Typical atmospheric air is mostly nitrogen (78%) and oxygen (21%) with only a small amount of carbon dioxide (0.03%). Other gases are present in trace amounts: Hydrogen, Argon, Neon, Helium and more.
When you breathe your lungs absorb oxygen from the air, but they also absorb all the other gases as well. The oxygen is used by your body for all its functions, but what about nitrogen? Air has more than three times more nitrogen than oxygen, so your body is dealing with quite a lot of it. The answer is that our body is saturated with inert nitrogen, it offgasses through your skin and quite a lot of it filtered by your organs and passed as nitrates in your urine.
When your tanks are filled for scuba diving they are usually filled with dry filtered air. Pumps and filters remove most of the particulates (dust, pollen, airborne pollutants). Water vapor is also present in air; the amount varying greatly depending on temperature, pressure and weather. Water condensation can rust the inside of a scuba tank, so water vapor is filtered as well. The result is a tank containing a normal mixture of dry atmospheric gases in normal proportions.
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