[stonemetal12]

It does affect them, during a treatment they will take "air breaks" where they return to normal oxygen levels so they don't get oxygen poisoning.

On the other side Astronauts run pure oxygen at low pressure (8psi) in their EVA suits, it can be tolerated indefinitely.

[belorn]

Oxygen exposure is best seen in terms of absolute molecules for oxygen poisoning. If you are 90m water depth, 2.1% oxygen is the same as 21% at the surface. At ~0.2 atmospheric pressure, 100% oxygen would be about the same as at surface.

8 psi is 0.54 atmospheric pressure, so breathing in 100% oxygen under that pressure would be similar to a 54% nitrox mix at surface. Not as bad as 100% at surface, but might still be something that a doctor would monitor for potential signs. It is important to remember that people tolerance for oxygen can vary between people, and that Astronauts is a heavily selected group in term of fitness and health.