If we put humans on planets with atmospheres different from Earth's atmosphere, but still containing oxygen, would humans be able to breathe?

There are three separate issues here:

• Too much oxygen, and the human body (as well as any other organic material) will be highly flammable. You don't really want to live there.
• Too high or too low oxygen partial pressure, and the metabolism won't work correctly.
• The other gases, or lack of them, may also cause problems

So a 100% oxygen atmosphere will not work, due to point 1. Also your 90% CO2 atmosphere won't work because the high CO2 concentration will kill you.

Whether the other 10% atmospheres will work depends on the total pressure. If the total pressure is twice as high as on earth, the partial pressure is the same as on earth, and since the other gases are inert (either nitrogen like here on earth, or noble gases), they should not make too much trouble on their own. However, with the exception of helium, they have narcotic effects, therefore the nitrogen, argon and xenon atmospheres would also not be the best to live in, although temporary visits should be less of a problem (thanks to Rob Watts for making me aware of this problem). Moreover, there should also be some CO2 for respiration to work.

With the 2% atmosphere, you'll need 10 times as much pressure to get the same partial pressure for oxygen as on earth. I'm not sure whether the human body could tolerate that for an extended time (for a limited time it certainly can, as divers prove; it's the pressure you find on earth under water in a depth of 90 meters). At those concentrations, you really should have helium as inert gas, however.

Another point: In all of the above I've assumed that the percentage is the one on the ground (because that's also where the earth atmosphere values are valid). On earth the oxygen fraction goes down with height because nitrogen is slightly lighter than oxygen. Of the other gases you mention, only helium is lighter than oxygen; however that one is much lighter, so in the helium-oxygen atmosphere, the oxygen percentage will quickly go down as you go up (note, however, that the partial pressure will go down no quicker than in our atmosphere, assuming the same gravitation). So in the helium-oxygen atmosphere, already a small mountain will hinder you from making fire (but not yet from breathing).

For the gases which are heavier than oxygen, the fraction of oxygen will grow as you go up. However, the partial pressure will still go down. Which gives the seemingly paradox situation that things burn better at greater height, while your body gets less oxygen.

posted about 9 years ago

celtschk‭

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