How much oxygen does a human need per average breath of air?

In an average gasp of air, only 15-20% is oxygen. When you exhale you release around 10%. In short, 10% would be the ideal minimum.
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Do crayfish need air to breathe?

  no craw fish do not they have gills like fish and get there oxygen from the water but you do need a air pump so oxygen will get into the water

Thanks for the feedback!