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.
"Pulmonary and ocular toxicity result from longer exposure to increased oxygen levels at normal pressure."