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I know that more oxygen in the air can lead to oxygen toxicity, but what if we took a room or area and lowered the pressure in it down to about 21% and then filled it with pure oxygen? Would the low pressure and the high oxygen cancel out, or is this a ridiculous proposal? Could this ever work, and I just have the proportions wrong?

(Lets assume these people have no adverse effects due to low pressure or anything else - purely testing the oxygen.)

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Low pressure and high oxygen do indeed cancel out. For most purposes (flammability, breathing, etc.), what matters is the partial pressure of oxygen: what the atmospheric pressure would be if you removed everything except the oxygen.

This means that on a planet (or spaceship) with a low atmospheric pressure, you'll need to increase the fraction of the atmosphere that's oxygen to compensate (NASA used to do this to save weight on their space capsules), while on a high-pressure planet, the oxygen fraction needs to be reduced to avoid oxygen toxicity.

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  • $\begingroup$ How far can this be taken before problems happen due to less pressure (assuming gradual depressurization) or too high of a percentage of oxygen in the air? Or is about .2 atm of Oxygen and no other gases completely survivable. $\endgroup$ – Jarred Allen Dec 15 '16 at 4:41
  • $\begingroup$ @JarredAllen, you can take it all the way to 0.2 atm of pure oxygen. $\endgroup$ – Mark Dec 15 '16 at 6:06

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