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As I understand it, the important part of Earth's atmosphere that we breathe is the oxygen. However, Earth's air is only about 21% oxygen with the rest made up of about 78% nitrogen and 1% other gases, mostly argon.

Could we safely breathe the air of another planet as long as it still had the requisite 20% oxygen even if the other 80% was helium or some other stable gas that was not otherwise harmful to humans?

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    $\begingroup$ @SnakeDoc names of chemical elements are not proper nouns nor are they capitalized. I find myself doing it all the time as well. $\endgroup$
    – uhoh
    Jul 6, 2018 at 18:08
  • $\begingroup$ There's no way that deep sea divers could survive on a mixture of helium and oxygen. (Is there?) $\endgroup$
    – Dan
    Jul 6, 2018 at 23:11
  • $\begingroup$ There was a way for divers using heliox too. $\endgroup$
    – Uwe
    Jul 6, 2018 at 23:37

4 Answers 4

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We can breathe pure oxygen for unlimited time if the pressure is not too high; about 0.4 bar is okay. Breathing pure oxygen at 1 bar is possible for some hours, but a longer time may damage the lungs.

A mix of oxygen and helium is also breathable and is used for deep diving. Xenon cannot be used due to its narcotic effect on the body. Argon is less narcotic and may be used at a pressure less than about 24 bar. Krypton is also narcotic at pressures above 3.9 bar. Neon is narcotic at very high pressure above 110 bar. Radon, the heaviest noble gas should not be breathed due to its radioactivity. A mixture of several noble gases is possible if containing oxygen too.

Even a mixture of hydrogen and oxygen may be breathed. For security the mixture should not be ignitable or explosive. A mixture of 4 % oxygen and 96 % hydrogen could be used at a pressure of 5 bar or more, see hydrox.

The partial pressure of oxygen is important, it should be neither too low (less than 0.2 bar, hypoxic) nor too high (more than 0.4 bar, oxygen toxicity). This means that the percentage of oxygen might well be very different than 20%; what is important is that each lungful have neither too much nor too little oxygen, with the other gases causing no harm. A mixture with only 5% oxygen is breathable if the total pressure is 4 bar or more, but less than 8 bar.

An atmosphere with 21 % oxygen and 79 % nitrogen at a pressure of only 0.75 bar will be breathable too. That is what many aircraft passengers and personnel experience every day, also humans living in heights of about 2500 m.

Of course the content of toxic or harmful gases like chlorine, fluorine, carbon monoxide or dioxide and many others should be so low to be not harmful. Gaseous chemical compounds may be part of the mixture if they are inert for the human body and thus not toxic or harmful.

More information about breathable gas mixtures in Wikipedia.

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    $\begingroup$ This is the best answer since it mentions the partial pressure limits. $\endgroup$
    – called2voyage ♦
    Jul 6, 2018 at 15:16
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    $\begingroup$ The problem of a breathing gas mixture at different pressure could not be explained without partial pressure. $\endgroup$
    – Uwe
    Jul 6, 2018 at 15:23
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    $\begingroup$ 110 bar? Seems like the you'd be squashed by the neon long before you had a chance to experience the narcotic effects. $\endgroup$
    – Jennifer
    Jul 6, 2018 at 17:50
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    $\begingroup$ @Jennifer So long as the change isn't too rapid the body doesn't seem to care--you simply don't notice the ambient pressure. The limits are how fast dissolved gases go out the lungs (lower the pressure too fast and they come out elsewhere--commonly called the bends) and having nothing toxic (which is the limit to the pressure humans can experience. Everything has a point beyond which it becomes dangerous, once you can no longer make a mix that's safe to breathe you've reached the pressure limit (and therefore the limit of how deep you can dive.) $\endgroup$
    – Loren Pechtel
    Jul 7, 2018 at 4:24
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    $\begingroup$ @Jennifer: The maximum pressure for human deep diving experiments was about 70 bar. The 110 bar with Neon would be from experiments with animals. $\endgroup$
    – Uwe
    Jul 7, 2018 at 20:16
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Yes, we don't require nitrogen to breathe. For example, NASA astronauts used to use a pure oxygen environment. The complication with this environment was the risk of fire.

For more information:

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    $\begingroup$ And still do, in the EVA suits. $\endgroup$
    – Organic Marble
    Jul 6, 2018 at 15:38
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    $\begingroup$ Breathing pure oxygen at 1 bar pressure is possible for some hours. Longer use is damaging the lungs. For extended use the partial pressure of oxygen must be lower than 0.5 bar. $\endgroup$
    – Uwe
    Jul 6, 2018 at 16:03
  • $\begingroup$ @OrganicMarble is there a 'NO SMOKING' label on the suits? $\endgroup$
    – tedder42
    Jul 11, 2018 at 18:33
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    $\begingroup$ @tedder42 At least one suit got smoked: tested.com/science/space/… $\endgroup$
    – Organic Marble
    Jul 11, 2018 at 19:19
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Yes, provided that the inert gas is not toxic and contains the requisite proportions and pressures of oxygen, humans should have no trouble breathing a gas mixture comprised of helium or other gasses. In fact, many scuba divers will use helium based trimix specifically to reduce the nitrogen content of their breathable gas mixture.

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    $\begingroup$ Reducing the nitrogen content using trimix is necessary at higher pressures only, above about 5 bar. Nitrogen is narcotic at higher pressure. $\endgroup$
    – Uwe
    Jul 6, 2018 at 15:21
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    $\begingroup$ In addition, Helium can escape the body's tissues more quickly than can Nitrogen, which means your decompression time can be reduced by switching to Helium if you're deep diving. Yet another good reason to switch. $\endgroup$
    – dgnuff
    Jul 7, 2018 at 8:12
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There are several breathing gas mixtures that are either in use, or have been tested, amoung them being Helium + Oxygen (helox), Hydrogen + oxygen (hydrox) and argon + oxygen (argox).

https://en.wikipedia.org/wiki/Heliox

Note the "See Also" section.

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  • $\begingroup$ Hydrox mixture should be not explosive, the oxygen content must be low. To be non explosive and breathable pressure should be very high. Using hydrox at 1 bar is impossible. $\endgroup$
    – Uwe
    Jul 6, 2018 at 16:00

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