Why do hospitals use 99% O2, when only 21% of the regular air we breath is oxygen?

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During an oxygen shortage (like with Covid), could hospitals use a lower concentration, maybe 70-80%? From what I understand, the 99% gets diluted by different apparatuses, such as nasal cannulas (more dilution) and face masks (less dilution). I know quality control and having a reliable baseline is important too, but why is 95-99% the needed concentration?

In: Biology

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