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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?

uhoh
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Rozgonyi
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4 Answers4

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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.

Uwe
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    This is the best answer since it mentions the partial pressure limits. – called2voyage Jul 06 '18 at 15:16
  • Don't heavy gasses pose a problem in that they'll separate out and stay in the lungs, so you'll end up with increasing sections of the lungs that don't see oxygen? – Hobbes Jul 06 '18 at 15:22
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    The problem of a breathing gas mixture at different pressure could not be explained without partial pressure. – Uwe Jul 06 '18 at 15:23
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    Heavy gases like Xenon or sulfur hexafluoride do mix with the other gases in the lungs due to the currents of breathing. – Uwe Jul 06 '18 at 15:41
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    110 bar? Seems like the you'd be squashed by the neon long before you had a chance to experience the narcotic effects. – Jennifer Jul 06 '18 at 17:50
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    @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.) – Loren Pechtel Jul 07 '18 at 04:24
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    @Loren Pechtel - yes, but 110 bar? That's 110 times normal atmospheric pressure. I have difficultly believing anyone could survive that. (But I could be wrong ... .) – Jennifer Jul 07 '18 at 18:49
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    @Jennifer: The maximum pressure for human deep diving experiments was about 70 bar. The 110 bar with Neon would be from experiments with animals. – Uwe Jul 07 '18 at 20:16
  • IIRC, hydrogen allows for pretty extreme pressure breathable mix - but it's obviously extremely dangerous due to risk of explosion. – SF. Jul 08 '18 at 01:46
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    In Earth's oceans, a pressure of 110 bar (11 MPa) corresponds to a depth of approximately 1.1 km. – Jeppe Stig Nielsen Jul 08 '18 at 13:45
  • @SF: A mixture of hydrogen and oxygen may be used only at very high pressure when the oxygen content is so low that the mix is neither explosive nor ignitable. – Uwe Jul 08 '18 at 14:21
  • @Uwe: I remember a documentary of an experiment where the mix was ignitable; the habitat was made with extreme care to have sparkless electric installation, no materials that would allow static buildup, generally making sure the ignition would not happen. – SF. Jul 08 '18 at 14:24
  • @SF: Do your remember pressure and mixture of that experiment? If pressure is 50 bar, you need only 0.4 % of oxygen and 99.6 % hydrogen. This mixture is not ignitable.
    See this publication: Fife, William P. (1979). "The use of Non-Explosive mixtures of hydrogen and oxygen for diving". Texas A&M University Sea Grant. TAMU-SG-79-201.     – Uwe Jul 09 '18 at 08:40
  • @Uwe: Sorry, that was sometime in the 80's and try as you might my memory isn't that good. – SF. Jul 09 '18 at 08:42
  • Does it "feel" exactly the same when you are breathing these unusual mixtures, i wonder ?! – Fattie Jul 10 '18 at 02:16
  • You don't feel any difference breathing air or pure oxygen (personal experience at 1 bar). Breathing at very high pressures increases the flow resistance in the respiratory system and body heat loss. This may change the feeling of breathing. From personal experience in a recompression chamber feeling at 5 bar was not different. But your voice sounds different for others. – Uwe Jul 11 '18 at 20:04
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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:

called2voyage
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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.

Justin Braun
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    Reducing the nitrogen content using trimix is necessary at higher pressures only, above about 5 bar. Nitrogen is narcotic at higher pressure. – Uwe Jul 06 '18 at 15:21
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    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. – dgnuff Jul 07 '18 at 08:12
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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.

Petro
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  • 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. – Uwe Jul 06 '18 at 16:00