So my question is about what the acceptable envelop of a planet's atmosphere would need to have to enable humans to live there.

As we all know, earth is comprised of a mix of 21% Oxygen, 78% Nitrogen and 1% other stuff AND the specific pressure is 29.92 at sea level. So, we have to talk not just about the varied composition of the atmosphere, but also the pressure of said atmosphere.

What would you say are "doable" for humans?

  • 18% Oxygen; 81% Nitrogen; 1% other; 29.92 @ sea level
  • 25% Oxygen; 70% Nitrogen; 5% other; 29.92 @ sea level
  • 15% Oxygen; 84% Nitrogen; 1% other; 34.00 @ sea level

These are just three examples of different parameters of an atmosphere. What I'm looking for is what is an acceptable envelope. Can that be mapped?

  • $\begingroup$ What is specific pressure? Total atmospheric pressure? Also, "other" can be inert, or can be toxic - that would make a huge difference. $\endgroup$
    – Alexander
    Commented Aug 16, 2018 at 18:58
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    $\begingroup$ Possible duplicate of What is necessary for a breathable artificial atmosphere? $\endgroup$
    – dot_Sp0T
    Commented Aug 16, 2018 at 19:02
  • $\begingroup$ This question may be helpful as well. It was limited to oxygen, though, we didn't cover the rest in as much detail: worldbuilding.stackexchange.com/questions/21079/… $\endgroup$
    – James
    Commented Aug 16, 2018 at 19:20
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    $\begingroup$ What are those 29.92? They are not SI pascals (1 atm is about 101 kPa), not CGS barye (1 atm is about 1 MBa), not American medieval pounds-force per square inch (1 atm is about 14.7 psi). $\endgroup$
    – AlexP
    Commented Aug 16, 2018 at 20:06
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    $\begingroup$ @AlexP Ceaseth thou thy renaissance bluster! Besteth thou the imperial system? Wouldst thy maiden not be impressed by our honoring the motherland? Keepest thou thy metric system! It behooveth us to remain difficult! (And we, the people, understand... you should trying fixing cars with imperial frames and metric engines....) $\endgroup$
    – JBH
    Commented Aug 16, 2018 at 22:07

2 Answers 2


The 'formula' you need basically relates to what we call Partial Pressure. In terms of human survive-ability, this is the primary 'measure' you need to consider when planning your atmosphere.

The O2 at sea level represents about 0.21 bar of pressure (being 1 bar pressure, O2 being 21% of that mix). The minimum that humans can safely handle is around 0.16 bar, and any higher than 0.30 bar over sustained periods is going to cause issues to human biology.

SO - If your planet has a LOW pressure atmosphere, then a pure O2 atmosphere is fine. This is why the Apollo missions could safely occur in a pure oxygen environment; the cabins of the spacecraft were not pressurised to sea level. They pressurised them to around 0.3 ATM (0.3 bar) so that the pressure differential between the inside of the ship and the vacuum of space was not so great, meaning that they didn't need to be as strong (read as heavy).

Of course, your natural atmosphere isn't going to be pure O2, and if it is, you don't want to live there anyway because it means there's nothing consuming O2, which probably means there's nothing there to produce it either. Definitely a 'bring your own plants' scenario.

As the pressure increases, so too must your inert (or at least poorly reactive) gasses. On Earth, that's nitrogen. It's not inert, but at sea level pressures it doesn't react with our systems. As you increase the pressure however, nitrogen becomes a narcotic at high partial pressures (divers know all about this) meaning that you can't use it to dilute O2 if your pressure is much higher than Earth's.

Deep see divers mix other inert gases into their breathing mix to cater for this when going deep where the air they breathe is under considerable pressure.

That only leaves toxic gases. Carbon Dioxide, Chlorine, and others can't exist in any more than trace amounts for humans to function normally. Better to exclude them as much as possible.

So with all that said, your formula (based on atmospheric pressure) looks like this...

0.16 to 0.3 bar Oxygen (Necessary)
0.30 to 0.8 bar Nitrogen (Optional)

Inert gases to taste (up to a threshold pressure that makes it hard for humans to function, whatever that may be)

Trace amounts of CO2, Chlorine etc. (Maximums, preferably absent)

Working on this formula, you can basically mix and match an ideal atmosphere by first deciding on your atmospheric pressure, adding in your O2 and then adding in other gases according to the proportions above to suit your taste until your pressure is reached.

  • $\begingroup$ This is a great start. You totally are working this answer in the direction I was looking for. Very interesting. Thank you! $\endgroup$ Commented Aug 17, 2018 at 14:11

So lets look at some separate requirements:

Pressure: fortunately for us humans regularly go into extreme pressure environments for recreational and professional purposes I am talking of course about diving and mountain climbing. For our lower bound I would take earth highest human habitation at 6000 m or about 0.8 atmospheres and for our upper bound i'd take the low diving depth of 30 m were Nitrogen Narcosis sets in which is roughly 4 atmospheres, bear in mind these values will vary depending on your atmospheric composition.

Oxygen: for this quite simply i'll take the OSHA acceptable levels of between 19.5% and 23.5% Oxygen concentration as "safe". Though bare in mind whats important here is the Partial Pressure of the oxygen, so you actually want 0.195-0.235 atmospheres of oxygen. This means if your atmosphere is at 3 atmospheres of pressure you want it to be made of about 7 percent oxygen (give or take.)

What is the 'other'?: This final percentage is hugely dependant on what the other stuff actually is. Argon for instance you could go up to 50% with since it is inert while Chlorine shouldn't even register as a percentage. But in-short as long as the 'other' is nontoxic in the concentration that it is at it can be as high as it likes

N2?: you shouldn't actually need that much nitrogen to be perfectly honest. it's almost completely inert humans flat out don't need it. The Russian space program initially used a atmosphere without nitrogen, simply because it was cheaper to carry into space. That being said plants do need nitrogen and humans need plants so a complete absence isn't advisable keep a few percent of it.

Edit length of stay: It's worth noting that this will vary on your definitions of "survive" and "human", you can push the boundaries a bit more for shorter stays or multi-generational stays if your willing to let humans adapt to your new atmosphere. If you count cyborgs and bio-forming then you can basically change your answer to "everything"

This is also all likely somewhat wrong as, needless to say, no one wants to do extensive testing on which atmospheres would be harmful to long term human habitation so take my estimates with a large grain of salt.

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    $\begingroup$ The thing is the lower pressure bound that you stated is due to the lack of oxygen, not the lack of atmosphere itself. Theoretically, humans could survive at a lower pressure if the percentage of oxygen was adjusted accordingly. So, I would put the lower limit at "when does the water in the body start boiling off" $\endgroup$ Commented Aug 16, 2018 at 19:48
  • $\begingroup$ I did say theese would vary depending on atmoshperic composistion yeah. to put an absolute limit on it it would be about 0.16 atm of pure oxygen however as nasa found out that is quite hazardous, unhealthy and not ideal for prolonged human habitation space.stackexchange.com/questions/5690/… $\endgroup$
    – Ummdustry
    Commented Aug 16, 2018 at 19:53
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    $\begingroup$ Pure oxygen atmosphere is a real bad fire hazard while on earth. Once in zero gravity, fire is much, much less dangerous because there is no convection. Of course with 75% nitrogen, the oxygen depletion near a flame is much stronger, so it still improves safety quite a bit. Also preparing lunch at 0.25 atmospheres is a real pain. Boiling point of any food is much reduced, your package of instant pasta blows up in your face as it reaches 45°C. Any kind of (bio-)chemical experiment becomes very cumbersome. Etc. $\endgroup$
    – Karl
    Commented Aug 16, 2018 at 20:22
  • $\begingroup$ I think this has already been answered here: $\endgroup$
    – Slarty
    Commented Aug 17, 2018 at 0:52
  • $\begingroup$ worldbuilding.stackexchange.com/questions/45529/… $\endgroup$
    – Slarty
    Commented Aug 17, 2018 at 0:52

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