This thread asking about air locks and saving air on a Martian base got me wondering again about this and I didn’t want to clutter up that thread.
What are they breathing on the I.S.S.? NASA, since Mercury, preferred a 100% O[sub]2[/sub] at 5.5 psi, though since the Apollo 1 fire, at least for Apollo, they used an oxygen/nitrogen mix at 16 to over-pressurize and test the seals on the pad and then vented it during the climb until it was 5.5 psi O[sub]2[/sub] by the time it reached orbit and the crew could unhelmet and deglove. The Russians preferred an N[sub]2[/sub] O[sub]2[/sub] mix and for the Apollo/Soyez mission there was an adapter which was just really an airlock and appropriate docks.
I don’t know what they breathed on the Shuttle either.
The US Shuttle Orbiter maintained an atmosphere of 22% oxygen and 78% nitrogen at approximately sea level pressure. The ISS maintains roughly the same environment. This became an issue when the American “Freedom Station” became the hybrid ISS because the Russians wanted to operate at a higher oxygen concentration and slightly lower pressure (~12 psi, I think), which wouldn’t have been compatible with the then designed American modules and the shuttle. The problems with significantly higher oxygen concentrations are well known, but according to Soviet research a high ox/lower pressure environment provides some physiological and health advantages in free-fall.
Related question - does it feel different breathing oxygen at such a low pressure? 5.5psi is equivalent to about 27,000ft above sea level, if my calculations are correct, and only a little over a third of sea-level pressure. Obviously lack of oxygen won’t be a problem like it is with ordinary air at that altitude, but do you feel the difference?
I’ve spent some time in unpressurized aircraft above 25,000’ - and in a chamber at the equivalent pressure - breathing oxygen through a mask. I can’t say I’ve noticed anything different attributable to the lower pressure and gas density. But the mask is probably enough of a change to distract you from other small differences.
The air feels a little thinner, like being up on a high mountain but without the oxygen starvation. The most pronounced evvect, however, is the change in convective heating, and to some extent, evaporation. You don’t feel as warm at a given ambient temperature, and sweat or liquids evaporate quicker and more completely.
The “bends” (decompression sickness) only occurs because of excess nitrogen accumulation in the bloodstream under high ambient pressure and the body returns to lower pressure more quickly than the nitrogen can be removed. When going to a lower pressure, oxygen rich environment it is important to pre-breathe for a specified interval to get the nitrogen balance in equilibrium with the ambient air. Astronauts currently have to do this prior to spacewalks becuase the current Extravehicular Mobility Unit (EMU) operates at 4.3 psi with a near pure oxygen atmosphere.
How extreme is the fire danger with 100% oxygen at those low psi?
I can note that smoking a cigarette above 18,000 feet while sucking on an oxygen tube is quite difficult. Got to be a real smoker to get it done. Unfortunately I did not burn up back in the day according to some.
But,
What about stuff that will self-combust on the ISS or Shuttle? Could the astronauts accidentally bring something in personal gear that would be way dangerous in 100% oxygen?