From Wikipedia:

By volume, dry air contains 78.09% nitrogen, 20.95% oxygen, 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases.

For example, would humans be able to survive in these atmospheres? :

  • 100% oxygen, nothing else
  • 10% oxygen, 90% nitrogen
  • 10% oxygen, 90% argon
  • 10% oxygen, 90% carbon dioxide
  • 10% oxygen, 90% helium
  • 10% oxygen, 90% xenon
  • 2% oxygen, 98% of other elements which are not toxic
  • 1
    $\begingroup$ This is already mostly answered between worldbuilding.stackexchange.com/questions/4600/… and worldbuilding.stackexchange.com/questions/4567/… $\endgroup$
    – Tim B
    Commented Feb 17, 2015 at 18:36
  • 45
    $\begingroup$ You would respectively burn, suffocate, suffocate, suffocate, suffocate, suffocate, and suffocate. There. $\endgroup$ Commented Feb 17, 2015 at 21:51
  • 5
    $\begingroup$ The question and answers are mostly implicitly assuming standard pressure (and temperature). As @2012rcampion implied, if you play with overall pressure you get more options. $\endgroup$
    – Keith
    Commented Feb 18, 2015 at 4:40
  • 1
    $\begingroup$ Not to say the answers here are bad, but I would think this question, as a scientific question, could be more authoritatively answered on one of the science Stack Exchanges. I would think it would mostly fit under the Biology SE, given that other questions about oxygen levels and respiration are asked there. $\endgroup$
    – trlkly
    Commented Feb 18, 2015 at 14:42
  • 1
    $\begingroup$ @SerbanTanasa. No. #1 is slowly lethal beyond the fire danger, but tolerable at lower pressures. #4 is lethal, #6 I think is lethal. #7 is lethal at standard pressure but I believe 2% oxygen/98% hydrogen is safe at sufficient pressure (and, no, it won't go boom.) The others are breathable. $\endgroup$ Commented Feb 19, 2015 at 7:46

6 Answers 6


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.

  • 1
    $\begingroup$ Si. Me gusta. The OP didn't specify a pressure range, good of you to do it for them. $\endgroup$ Commented Feb 17, 2015 at 22:12
  • 7
    $\begingroup$ I think you'd also have to worry about nitrogen narcosis in the 2% scenario. That might be the fastest reason why 10x pressure would kill you. $\endgroup$
    – Rob Watts
    Commented Feb 18, 2015 at 18:23
  • $\begingroup$ Ah, I didn't know about that, thank you. I'll amend the answer accordingly (and also add some other information). $\endgroup$
    – celtschk
    Commented Feb 18, 2015 at 19:46
  • $\begingroup$ 10% oxygen, 90% helium. Sounds like Mickey Mouse universe :) $\endgroup$
    – Daerst
    Commented Feb 19, 2015 at 11:08
  • 3
    $\begingroup$ @Daerst but nevertheless in common use. Divers who breathe it often use an electronic pitch filter on their radios so that the boat crew can better understand them (...and perhaps so they don't laugh their butts off all day) : en.wikipedia.org/wiki/Heliox $\endgroup$
    – J...
    Commented Feb 19, 2015 at 17:45

100% oxygen is dangerous - stuff will burn, oxygenation can damage tissues. Apollo 1 had pure oxygen atmosphere, did not ended up well. oxygen is toxic

90% CO2 will kill you (not enough partial pressure of oxygen in hemoglobin)

2% of oxygen is not enough.

10% of oxygen with other inert gasses (nitrogen (not inert, but not reactive either), helium) might be survivable. See trimix breathing gas for scuba diving. Different mixes for different usage, and pro/com for helium and nitrogen, and lowering partial pressure of oxygen.

Seems than xenon is used for anesthesia, so it will do you no good to have it in such concentration. Thanks @Twelfth

Argon enhances change of gas embolism, but it was used to replace nitrogen to speed up decompression, so it seems NOT be immediately poisonous.

Apparently, well trained and adapted humans (with more hemoglobine in blood) can survive 12% of oxygen

Image at this page about limits of human survival shows dangerouns levels of hypoxia (low oxygen) and hyperoxia (too much oxygen).

Twelfth in his answer raised interesting aspect: relative density of different gasses, and possibility of separate layers. Looking at gas density table, air density (1.2 kg/m3) is close to oxygen (1.3) and nitrogen (1.16) - no surprise there.

Helium density (0.16) is 10% of oxygen, so likely atmosphere with mix of oxygen and helium would after a short time formed layers, with more oxygen at the lower layers - leading to oxygen poisoning. Not good. Even worse: @Marky Mark is right helium does escape from Earth atmosphere.

Argon is bit heavier then oxygen (1.66), but not much. But if such layer of argon was possible and stable, all lowlands would be uninhabitable. So you would need to add some winds and turbulence to mix stuff up. Even then, mines and cellars would not be possible without active ventilation, because heavier argon would accumulate there, causing asphyxiation.

  • 3
    $\begingroup$ 20% oxygen / xenon would knock you out close to instantly. Xenon enters the blood brain barrier and has an effect on us substantially greater than Nitrous oxide does. $\endgroup$
    – Twelfth
    Commented Feb 17, 2015 at 22:42
  • $\begingroup$ The density of helium is so low that in an Earth like atmospheric pressure and gravity it achieves escape velocity and is vented into space. So I believe an oxygen (O2) / helium (He) atmosphere would quickly become 100% oxygen. Assuming you start with 10% O2 / 90% He, the end result would be a low pressure pure O2 atmosphere. You'd need to find some inert gas of around the same density as oxygen (like nitrogen) to increase the pressure to something survivable. $\endgroup$ Commented Feb 18, 2015 at 0:31
  • $\begingroup$ The earth's dry atmosphere has an essentially uniform composition from the ground to 100 km because mixing is a more powerful effect than separation due to density variation. At 100 km the changes mostly due to the increased mean free path length of molecules in the very thin air. Water vapor is not uniform because of other effects $\endgroup$ Commented Feb 18, 2015 at 6:08
  • 3
    $\begingroup$ There are a lot of errors in this post! The percentage is irrelevant, 100% Oxygen is fine at 20 kPa, for example, but 20% Oxygen at 1 MPa is too much. Also, CO2 kills you through respiratory acidosis even if you have enough Oxygen in your bloodstream. $\endgroup$ Commented Feb 18, 2015 at 7:30
  • 1
    $\begingroup$ Feel free to write more correct answer. My post is just a bit of google-fu. $\endgroup$ Commented Feb 18, 2015 at 14:15

100% O2

At normal atmospherical pressure (101.325 kPa) if your only gas is O2 you will have a partial pressure of 101.325 kPa of O2... lethal, deadly and painful.

Lets talk about consequences!

  • Oxygen toxicity: When O2 partial pressure is above 50 kPa oxygen become toxic.
    • Disorientation, breathing problems, vision changes such as myopia.
    • Prologed exposures of higher O2 PP or shorter exposure but very higher, can cause oxidative damage to cell membranes, collapse of the alveoli in the lungs, retinal detachment, and seizures.
    • A lot more, click in the link for more info.

enter image description here

10% O2 and 90% N2

$$ \left| \begin{array}{cc|ccc|c} \text{Gas}&\text{%}&\text{gr/mol}&\text{Mols}&\text{Fractal Mol}&\text{Partial Pressure (kPa)}\\ \text{O}_{2}&\text{10%}&31.9988&0.31&\text{8%}&8.98\\ \text{N}_{2}&\text{90%}&28.0134&3.21&\text{91%}&92.37\\ \text{Total}&\text{100%}&60.0122&3.52&\text{100%}&101.325 \end{array} \right| $$

In Wikipedia you can find another gr/mol values. This is because O2 and N2 are <2> atoms per molecule, so I multiply the value per atom by 2.


  • 10% O2: 8.98 kPa O2
  • 90% N2: 92.34 kPa N2


10% O2 and 90% Ar

$$ \left| \begin{array}{cc|ccc|c} \text{Gas}&\text{%}&\text{gr/mol}&\text{Mols}&\text{Fractal Mol}&\text{Partial Pressure (kPa)}\\ \text{O}_{2}&\text{10%}&31.9988&0.31&\text{12%}&12.34\\ \text{Ar}&\text{90%}&39.948&2.25&\text{87%}&88.98\\ \text{Total}&\text{100%}&71.9468&2.56&\text{100%}&101.325 \end{array} \right| $$

Argon is only ONE atom, it doesn't have any Ar<2>.


  • 10% O2: 12.34 kPa
  • 90% Ar: 88.98 kPa


  • Low value of oxygen - Hypoxia: Hypoxia is at 13.3 kPa and you have 12.34 kPa. (See above to read symptoms).
  • Argon Asphyxia: Although argon is non-toxic, it is 38% denser than air and therefore considered a dangerous asphyxiant gas in closed areas. It is difficult to detect because it is colorless, odorless, and tasteless.
  • Argon narcopsia: I don't know much about it but I think it can cause narcopsia like nitrogene.

10% O2 and 90% CO2

$$ \left| \begin{array}{cc|ccc|c} \text{Gas}&\text{%}&\text{gr/mol}&\text{Mols}&\text{Fractal Mol}&\text{Partial Pressure (kPa)}\\ \text{O}_{2}&\text{10%}&31.9988&0.31&\text{13%}&13.44\\ \text{CO}_{2}&\text{90%}&44.01&2.04&\text{86%}&87.89\\ \text{Total}&\text{100%}&76.0088&2.35&\text{100%}&101.325 \end{array} \right| $$

There are several atoms, you can find the value in wikipedia or calculate by yourself (sum of all atoms * quantity values).


  • 10% O2: 13.43 kPa
  • 90% CO2: 87.89 kPa


  • Almost low value of oxygen - Not Hypoxia: Hypoxia is at 13.3 kPa and you have 13.43 kPa. You can survive (unconscious I think) but not live, you will suffer a lot (and will afect your health).
  • Too high value of CO2 - Hypercapnia (Carbon dioxide poisoning): Severe hypercapnia is cause by an increment of 10 kPa CO2...mmm... you have 87.89 kPa, quite more... than the lethal.
    Be careful about carbon dioxide toxicity, concentrations of 7% to 10% may cause suffocation, even in the presence of sufficient oxygen, manifesting as dizziness, headache, visual and hearing dysfunction, and unconsciousness within a few minutes to an hour. (You have 90%) Symptoms and signs of early hypercapnia include flushed skin, full pulse, tachypnea, dyspnea, extrasystoles, muscle twitches, hand flaps, reduced neural activity, and possibly a raised blood pressure.
    Other symptoms of mild hypercapnia might include headache, confusion and lethargy. Hypercapnia can induce increased cardiac output, an elevation in arterial blood pressure, and a propensity toward arrhythmias.
    In severe hypercapnia, symptomatology progresses to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.
    Also you will increase the carbonic acid of your blood in a process called respiratory acidosis.
    Also, do you remember what I said about asphyxiant gas, well, CO2 have the same results.

You can see this interesting table that I found:

%CO2 in air   Duration       Major limitation
0.04 %           lifetime    normal atmosphere
0.5  %           lifetime    no detectable limitations
1.0  %           lifetime                 "
1.5  %        > 1    month   mild respiratory stimulation
2.0  %        > 1    month                "
2.5  %        > 1    month                "
3.0  %        > 1    month   moderate respiratory stimulation
3.5  %        > 1    week                 "
4.0  %        > 1    week    moderate respiratory stimulation, exaggerated respiratory response to exercise
4.5  %        > 8    hours                "
5.0  %        > 4    hours   prominent respiratory stimulus, exaggerated respiratory response to exercise
5.5  %        > 1    hours                "
6.0  %        > 0.5  hours   prominent respiratory stimulus, exaggerated respiratory response to exercise, beginnings of mental confusion
6.5  %        > 0.25 hours             "
7.0  %        > 0.1  hours   limitation by dyspnea and mental confusion

And this photo:
enter image description here

Well, 6 minutes at 7% CO2, think about at 90%...

10% O2, 90% He

$$ \left| \begin{array}{cc|ccc|c} \text{Gas}&\text{%}&\text{gr/mol}&\text{Mols}&\text{Fractal Mol}&\text{Partial Pressure (kPa)}\\ \text{O}_{2}&\text{10%}&31.9988&0.31&\text{1.37%}&1.38\\ \text{He}&\text{90%}&28.0134&22.48&\text{98.63%}&99.93\\ \text{Total}&\text{100%}&36.001402&22.79&\text{100%}&101.325 \end{array} \right| $$


  • 10% O2: 1.38 kPa
  • 90% He: 99.93 kPa


  • Extremely low value of oxygen - Hypoxia: Hypoxia is at 13.3 kPa and you have 1.38 kPa. (See above to read symptoms).
  • Helium and voice: The higher resonant frequencies caused by the gas (see the link to know how) cause a change in timbre, resulting in a reedy, duck-like vocal quality.
  • Helium asphyxiant gas: Inhaling helium can be dangerous if done to excess, since helium is a simple asphyxiant and so displaces oxygen needed for normal respiration. At 90% He it's obvious that it's lethal.
  • Helium narcopsia: I don't know much about it but I think it can cause narcopsia like nitrogene.

10% O2, 90% Xe

$$ \left| \begin{array}{cc|ccc|c} \text{Gas}&\text{%}&\text{gr/mol}&\text{Mols}&\text{Fractal Mol}&\text{Partial Pressure (kPa)}\\ \text{O}_{2}&\text{10%}&31.9988&0.31&\text{31%}&31.72\\ \text{Xe}&\text{90%}&131.293&0.68&\text{68%}&69.59\\ \text{Total}&\text{100%}&163.29182&0.99&\text{100%}&101.325 \end{array} \right| $$


  • 10% O2: 31.72 kPa
  • 90% Xe: 69.59 kPa


  • High but no lethal oxygen - Oxygen toxicity: Oxygen toxicity is above 50 kPa and you have 31.72 kPa so you won't have this but maybe you would have some other health problems.
    High values of oxygen produce oxygen stress in your cells, also increase the oxydation of your body (more free radical -reactive oxygen species -) and this would increase your aging speed, tumors, cancer and cell death. See free radical theory of aging.
  • Xenon and voice: Xenon vibrates more slowly in the vocal cords when exhaled and produces lowered voice tones, an effect opposite to the high-toned voice produced in helium.
  • Xenon isn't toxic by itself: Xenon is non-toxic, although it does dissolve in blood and belongs to a select group of substances that penetrate the blood–brain barrier, causing mild to full surgical anesthesia when inhaled in high concentrations with oxygen.
  • Xenon anesthesia: Xenon has been used as a general anesthetic, I think (quote needed) that a 30% Xe is used like anesthetic.
    Dense gases such as xenon can be breathed safely when mixed with at least 20% oxygen, sadly you have only 10% of oxygen. Xenon at 80% concentration along with 20% oxygen rapidly produces the unconsciousness of general anesthesia.
  • Xenon asphyxiant gas: Like argon and other inert gasses it can be a gas asphyxiant because it has more weight than O2 and can fill the surface of a planet with 100% inert gas (O2 will fly upper).

2% O2, 98% of other elements which are not toxic

Well, sadly I can make calculations because I don't know the molecular mass of other elements but at least I can safestly say that obviously you would die of Hypoxia.

I hope this help you, sorry for the long post, I hate long post but you ask too much atmospheres. If you want more information check my others answers: this and this.

  • $\begingroup$ I did not expect such a well written and proofed answer to such an old question. Lookit that math! You have my +1 good sir. $\endgroup$ Commented Jun 21, 2017 at 17:43
  • $\begingroup$ @Draco18s, Thanks! I am so glad that someone apreciate my post. $\endgroup$
    – Ender Look
    Commented Jun 21, 2017 at 18:38

100% oxygen, nothing else - no the human body would go up like a gallon of gasoline with a static electricity spark, on top of it being toxic.

10% oxygen, 90% - carbon dioxide carbon dioxide of %7 can kill, long term exposure to %.5 can cause health issues.

for the rest humans to work OK need ~20 oxygen Less than 19.5% and you start feel severe effects and in General %10 is likely to lead to severe health issues including death.

  • 10% oxygen, 90% nitrogen
  • 10% oxygen, 90% argon
  • 10% oxygen, 90% helium
  • 10% oxygen, 90% xenon
  • 2% oxygen, 98% of other elements which are not toxic
  • $\begingroup$ you beat me by 10 seconds :-) $\endgroup$ Commented Feb 17, 2015 at 18:47
  • $\begingroup$ @PeterMasiar but you added in the trimix. Which ultimately the correct partial pressure is the most important as long as the other gases aren't toxic $\endgroup$
    – bowlturner
    Commented Feb 17, 2015 at 18:50
  • $\begingroup$ I know. Hopefully OP will be willing to do own reading, because as always, answer is "it depends" -- on other factors not considered by original question :-) $\endgroup$ Commented Feb 17, 2015 at 18:56
  • 5
    $\begingroup$ Lower percentages of oxygen will work if the partial pressure is the same as standard air. For example, Mercury astronauts breathed a pure oxygen atmosphere at 5.5 psi (0.4 bar) and divers might breathe 10% oxygen at 300 ft/140 psi (100 m/10 bar). $\endgroup$ Commented Feb 17, 2015 at 22:15
  • $\begingroup$ Does the extra partial pressure of the inert gas make that much difference? 10% oxygen in 14.7 psi (sea level) is 1.47 psi of oxygen. In my teens I was in about 1.2 psi partial pressure of oxygen. Effort was difficult, that's all. (I had had a few days of acclimatization, though.) In my 40s I was in about 1.4 psi partial pressure, I was breathing harder than normal for what I was doing but that was all. $\endgroup$ Commented Feb 19, 2015 at 7:56

10% is too little and wouldn't work. If you increased it to 20%:

100% oxygen:

  • I think we'd spontaneously combust...this is toxic as well.

20% oxygen, 80% nitrogen

  • pretty close to what we live in now. Oxygen and nitrogen are around the same weight and mix really well.

20% oxygen, 80% argon

  • Where nitrogen is around the same weight and mixes well with oxygen, Argon is several times heavier than oxygen and therefore tends to sink to the bottom. On an atmosphere scale, this would end up in death as the surface would see 100% argon with all the oxygen relegated to the higher levels of the atmosphere.
  • We can breathe argon and oxygen mixtures...infact argon is considered doping for Olympic purposes.
  • The pressure of this atmosphere will be significantly higher...argon weighs a lot.

20% oxygen, 80% carbon dioxide

-Carbon dioxide is toxic at this level, we'd die.

20% oxygen, 80% helium

  • Sorta funny, but this might end up as the 100% oxygen scenario again. Helium is light enough that the two gasses would layer, near 100% oxygen at the surface and 100% helium in the upper atmosphere.

20% oxygen, 80% xenon

  • Xenon is heavy and will have the same issues argon does...we'd asphyxiate pretty quickly and probably be crushed by the weight of the atmosphere.
  • We can breath it, but it knocks us out pretty quickly...it works as a pretty heavy anesthetic (breathing 20% oxygen 80% xenon will knock you out quicker than the general anesthetics used in hospitals) and fits into the doping category for the Olympics.
  • Xenon is considered toxic as well...it dissolves into plastics and rubber

Edit in a caveat - I'm unsure how water (air humidity) changes in these environments. FOr example, water diffuses much quicker in Helium, which should mean humidity spreads out quicker and further making it harder for clouds to form.

  • 3
    $\begingroup$ Argon is not several times heavier than oxygen. Using round numbers, molecular weight of O2 is 32, Ar is 40, so density of Ar is about 25% heavier than O2. Also you would not be crushed by a Xenon atmosphere, the pressure in your body matches the environmental pressure -- assuming there is time to adjust to pressure changes, no crushing effect. Breathing would be difficult because your lungs would have to work much harder to pump the heavy atmosphere -- deep sea oxy/helium workers adapt to the extra lung work because the atmos is dense due to the working pressure. $\endgroup$ Commented Feb 18, 2015 at 5:55
  • 5
    $\begingroup$ Atmospheric gasses mix a lot better than you seem to think they do, which is why plants above sea level can still breathe even though the CO2 they need is ~50% heavier than the nitrogen/oxygen mix that makes up the vast majority of the atmosphere. It's also why CFCs, of which the lightest one that was in common use - Dichlorodifluoromethane (CCl2F2 aka R-12) - with a molecular mass of ~121 can reach the stratosphere to destroy Ozone despite being 4x as heavy as the majority of atmospheric gas. $\endgroup$ Commented Feb 18, 2015 at 11:30
  • 2
    $\begingroup$ As well as the points discussed in the two comments above, "Xenon is considered toxic as well...it dissolves into plastics and rubber" is a total non-sequitur. Toxicity of a substance has nothing at all to do with whether it dissolves into plastics and rubber. $\endgroup$ Commented Feb 18, 2015 at 11:57

You basically need oxygen to breathe. Everything else is not strictly necessary. The limits you are willing to take depend on how much of a mountaineer and how much of a diver you are.

These two articles explain the upper and lower limits for oxygen partial pressure for humans: https://en.wikipedia.org/wiki/High-altitude_adaptation_in_humans https://en.wikipedia.org/wiki/Maximum_operating_depth

Where the Tibetan highlanders live, the oxygen level is only about 60% of that at sea level. Most people will tolerate this level as a minimum, so you will have ~100mbar/10kpa of pure oxygen as a "minimum atmosphere". Breathing pure oxygen works well, as every tech diver will tell you. Still, everything that's not wet or otherwise inflammable will burn VERY easily. A little oil and a spark and you have a BIG fire.

Then you need to make sure that the amount of a lot of gases that are more or less poisonous are not too high. There's a partial pressure for each gas that you'll have to keep below a threshold.

Maximum partial pressures for some gases

Some effects may be additive. You could set up an atmosphere e.g. consisting of 1 part O2, 40 parts H2, 5 parts He and 5 parts N2 at a total pressure of fifty times the pressure on earth and you could breathe it, although you will feel the air being very "think" and you will need to get accustomed to it. You could also live in a 500 times thinner atmosphere made up of pure oxygen. That's about the limits for humans.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .