The amount of Oxygen in the air we breath is only about 20%. The rest is about 78% Nitrogen and a combination of other gases such as Carbon Dioxide.
What might also be surprising is we only use about 5% of this Oxygen that we inhale and the 15% is exhale back into the air. Keep in mind this is for a body at rest. Of course, if a person is exercising, they would require more oxygen to function.
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