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Today 1 atm (101.3 kPa, 14.7 psi) with sea level composition is most commonly used on spacecraft to avoid fire hazards like in Apollo 1 (not that it would have helped the Apollo 1 crew as they couldn't exit the capsule, it would only have postponed their deaths I guess). But the Apollo spacecraft used 5 psi (34 kPa) onboard pressure. Space Shuttle spacesuits use 4.3 psi (29.6 kPa). U2 and Mercury spacesuits use(d) 3.7 psi (25 kPa) psi of pure oxygen. That's still more oxygen than at sea level.

If spacecraft, -stations and -suits were pressurised to 2.5 psi (17 kPa) psi of pure oxygen, wouldn't this both make them lighter and better prevent potential hazardous fires? It's an oxygen level similar to 4,500 ft (1,370 m) above sea level, so astronauts should be fine, shouldn't they?

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    $\begingroup$ See these questions and answers: space.stackexchange.com/questions/13331/… space.stackexchange.com/questions/23658/… $\endgroup$
    – Uwe
    Jan 3 at 16:04
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    $\begingroup$ Pure oxygen is seriously dangerous stuff. Even if Apollo 1 had a lower capsule pressure, the fire would have almost certainly still happened. $\endgroup$ Jan 4 at 15:12
  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – gerrit
    Jan 6 at 16:59
  • $\begingroup$ I have edited the question to include both SI and imperial units, and have moved a discussion about the relative merits of both systems to chat. $\endgroup$
    – gerrit
    Jan 6 at 17:12
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    $\begingroup$ @gerrit The spacecraft/-suit values are to be in psi. You can convert them, but the first value should be the psi value because that's the one in which the pressure was determined, whether you like it or not. My own suggestion is 2.5 psi, not 17 kPa, so one can convert it but 2.5 psi should go first. I replaced the units. $\endgroup$
    – user46063
    Jan 6 at 17:31
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The sum of all partial pressures in the lungs must add up to the ambient pressure.

The composition of gas inside the lungs includes the vapor pressure of H₂O at body temperature which is about 0.9 psi. This is independent of ambient pressure.

Inhaled air is diluted by this H₂O vapor. It is also diluted by the dead space gases from the previous breath, which is about 150ml out of every 500ml breath.

If you tried to breathe pure O₂ at 1 psi, your lungs would be full of H2O vapor (steam). Your respiratory efforts would be useless as the H2O vapor would shuttle back and forth in the dead space and no O₂ would get to the alveoli. However, you would feel only mildly short of breath until you lost consciousness. This is because respiration is driven primarily by blood CO₂ levels, not blood O₂ saturation. Your respiratory efforts would continue to eliminate CO₂ despite being unable to oxygenate your blood.

To compensate for these effects, space suits use ~4 psi O₂. Higher pressure could have some advantages for respiration, but would decrease the flexibility of the suit. Volume changes with joint movement are opposed by suit pressure.

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More detail is provided by the excellent answer to Why is the EMU space suit pressurized to 4.3 psi specifically?

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    $\begingroup$ I have never thought about the vapor pressure of water in lungs and its effects. $\endgroup$ Jan 13 at 18:16
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A mix of 79 % nitrogen and 21 % oxygen at a pressure of 1 atm, 14.7 psi or 1.01 bar is more fireproof than pure oxygen at a pressure of 0.21 atm, 3.087 psi or 0.212 bar. The partial pressure of oxygen is the same for both gases.

The nitrogen content cools the fire. There are poorly flammable substances that would sustain a fire in the pure oxygen at the low pressure but extinguish by itself in air.

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    $\begingroup$ @Giovanni you would save 300 kg from a spacecraft at 440000 kg. (ISS with a volume of 916 m³). That's a 0.07 % saving, sure astronauts are very fit, but no need to risk altitude sickness for 0.07 % savings I'd guess. $\endgroup$
    – Arsenal
    Jan 5 at 9:03
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    $\begingroup$ @Giovanni Why even risk insomnia for the kind of people whose time literally worth more than almost anyone else? $\endgroup$
    – Martheen
    Jan 5 at 11:36
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    $\begingroup$ "should ensure things work well and prevent fire-causing disasters in the first place" including not using pure oxygen in the first place. $\endgroup$
    – Martheen
    Jan 5 at 12:16
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    $\begingroup$ Why are you hung up on Apollo 1? The decision was made after the incident, applied on modern spacecraft with various configuration. $\endgroup$
    – Martheen
    Jan 5 at 12:25
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    $\begingroup$ The reality is a little more complicated than this answer claims, but for a reference, here is a paper published by NASA on the topic : Oxygen Partial Pressure and Oxygen Concentration Flammability: Can They Be Correlated? $\endgroup$
    – J...
    Jan 6 at 22:27

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