O2 Concentrator instead of O2 tank

Tony writes:

Would you care to explain how the Apollo experience, with very fit men
doing well on 4.8 psi PP O2 is in any way related to my assertion that
you, or the general population, would have difficulty surviving in a
pure oxygen atmosphere of 3 psia?

Fitness has no effect on susceptibility to altitude sickness and
hypoxia. Adaptations are slow to occur and readily lost and rarely
practical, even for astronauts. However, a pure oxygen atmosphere at
4.8 psi apparently works just fine, for anyone in normal health.

The Apollo spacecraft were regulated to between 4.8 and 5.0 psi,
roughly, as I recall. Spacesuits were set to even lower pressures,
around 3.8 psi.

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