I've been re-watching Enterprise and I have the same question I had the first time.
In episode 12 of season 4, Babel One, Tucker and Reed are running low on air while in environmental suits, and Tucker uses 100% pure oxygen to refill the suits. Is there something I've missed about the suits that makes this non lethal?
I'm not interested in the biology as that is certain death, but more about the suits from any series or movie, there must be something to prevent the oxygen being lethal - I'm looking for an explanation on that.
Do they somehow manufacture nitrogen or another inert gas and if so why don't they just manufacture the oxygen, why didn't they die?
All the questions are linked so answering any of them answers them all.