Here’s a News Flash for you: Oxygen in high concentrations kills.
The Wikipedia page for the International Space Station says that it has a fairly Earth-like, sea-level atmosphere: 21% oxygen
"The Wikipedia page for the International Space Station says that it has a fairly Earth-like, sea-level atmosphere: 21% oxygen, balance nitrogen at 101.3 kPa. Supposedly it's because a pure-oxygen environment is dangerous as in the Apollo 1 disaster, but in that case "pure-oxygen" meant 1.15 atm of O2. It seems like a pure, 0.21 atm O2 atmosphere (or even lower) with no inert balance gas should be fine for people and would be ~ 80% less structurally demanding."
https://space.stackexchange.com/questions/5690/why-is-the-breathing-atmosphere-of-the-iss-a-standard-atmosphere-at-1-atm-conta
Who knows about the O2 levels in confined spaces.