I would have guessed that oxygen would be similarly prone to causing the bends (it is in fact a fair bit more soluble in water than nitrogen). It appears this isn't the case.
Actually, I think it is. But you can adapt a little more quickly because you can use up the dissolved O2
, unlike dissolved N2
that you have to breathe back out.
As you probably know, one of the changes made after the Apollo 1 fire was the use of a 60% O2
+ 40% N2
mix in the cabin during prelaunch and ascent. The astronauts still breathed 100% O2
; presumably the risk of a fire occurring within a suit was considered acceptable. Over the first few days of the mission the cabin was slowly vented to allow the remaining N2
to escape and be replaced by pure O2
from the life support system.
Apparently this mix, though substantially enriched in O2
over sea level air, reduced the fire hazard enough to be worthwhile.
The 60-40 mix has the property that at 5 psi (nominal cabin pressure in flight) the ppO2
is the same as in sea level air, meaning the astronauts could breathe it immediately after reaching orbit. But it occurred to me that it would have been possible to launch with ordinary air in the cabin if, instead of bleeding the cabin down to 5 psi during ascent and holding it there, it had been bled down to vacuum and then refilled with 100% O2
. The crew would be in their suits and breathing 100% O2
But I can see two problems with my idea, and they're probably why it wasn't used. First, bleeding the cabin to vacuum would mean pressurizing the suits and making it more difficult to move and operate controls at a critical time. Second, such a sudden drop from 15+ psi to 3.75 psi (the nominal suit pressure) might have caused the bends even when breathing pure O2
. Only dropping to 5+ psi was probably safer.