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I am writing a space sci-fi short story series, and I am trying to figure out what the range in pressure a human can survive AND breathe. I am trying to find the lowest, safest air pressure a human can breathe in where the oxygen isn't leaving the blood. The atmosphere is the same as Earth's.

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    $\begingroup$ This may have already been answered in this site. I recall some discussion in posts and comments about the minimum pressure for a pure oxygen environment, oxygen toxicity, scuba diving, etc., but you haven't mentioned the composition of the atmosphere in your question. Are you asking about a standard composition with something like 21% oxygen and just a loss of pressure, or an enhanced oxygen atmosphere at lower total pressure. $\endgroup$ – uhoh Apr 24 '18 at 2:41
  • $\begingroup$ The minimum pressure depends on the gas mixture. For pure oxygen it is much lower than for air with only 21 % oxygen. $\endgroup$ – Uwe Apr 24 '18 at 8:30
  • $\begingroup$ Also you must describe how long the guy must survive in that conditions: 1 minute, 1 hour, 1 year? $\endgroup$ – jean Apr 24 '18 at 10:51
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    $\begingroup$ @Norm, The pressure in an EVA suit can be less than that if the gas is pure oxygen. $\endgroup$ – Solomon Slow Apr 24 '18 at 20:05
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    $\begingroup$ 4 psi of pure O2 is used in the US Shuttle/Station suits. space.stackexchange.com/questions/13331/… $\endgroup$ – Organic Marble Apr 28 '18 at 22:20
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If the atmosphere on your world has the same composition as Earth’s, then your answer probably is “as high as people can live on Earth”. That seems to be up to 5,000 meters: According to Wikipedia there are multiple villages and even the city of La Rinconada, Peru (pop. 50k) around that altitude.

At that altitude, the pressure is about 50% of sea level.

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  • $\begingroup$ 5000 m is possible only for very healthy and adapted persons. The pressure in airplanes is adjusted to about 0.75 bar, a value tolerated by most healthy passengers. But altitude sickness may occur above 1500 m. $\endgroup$ – Uwe Apr 29 '18 at 18:21
  • $\begingroup$ @uwe I think the question-author didn't really make the criteria clear. People can "survive AND breathe" even higher; it's extraordinary that Everest has been climbed without extra oxygen. There are villages and even a mid-size town at 5000m; I edited the answer to make that clearer. And most people can live at 2000m just fine. Not sure what the question-author was looking for. $\endgroup$ – Bob Jacobsen Apr 29 '18 at 20:15
  • $\begingroup$ For not adapted climbers in heights of 4500 to 5500 m, there will be 50 to 85 % affected by altitude sickness. $\endgroup$ – Uwe Apr 29 '18 at 20:50

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