I got some great feedback on this atmosphere already, so I decided to lower the oxygen levels as a result of the feedback.

I am creating an atmosphere for a planet in a science fiction book I am writing and want to make sure that this atmosphere is sustainable for intelligent aliens while also being relatively breathable for humans but they have to resort to using gas masks at first before they acclimate over a long period of time. I also want the gas masks because I know xenon settling into valleys and depressions creates a death zone when there is no wind and want to include that in the book. I know there are a lot of noble gases comprising my atmosphere, is that too unrealistic? The reason I have this is because I want voice to be distorted in an unusual way via the helium and xenon combination. Here is the edited atmosphere in question.

  • Atmospheric Pressure at sea level: 0.81 atm
  • Atmospheric composition:
  • 58.02% nitrogen
  • 14.72% oxygen
  • 12.79% argon
  • 7.41% helium
  • 6.18% xenon
  • 0.88% other gases (carbon dioxide, water vapor, etc.)

Extra info: Size is similar to earth although has slightly less land mass, one moon orbits planet as well

Goal: I want to make it so that normal humans could acclimate with time but initially it is unsafe and use gas masks. (I fear I may have overdone it with reducing oxygen levels past the point of acclimation)

Thank you for great feedback last time!

  • 2
    $\begingroup$ My answer to your last question stands for this one. You're a long way away from needing gas masks... but it would help if you explained what you mean by "gas mask." Traditionally, a "gas mask" exists to filter out something harmful from the atmosphere (e.g., mustard gas). Is that what you're trying to achieve? Or by "gas mask" are you thinking of an oxygen mask or a non-rebreather mask, meaning a supplemental oxygen supply? Those two masks have two very different purposes. $\endgroup$
    – JBH
    Sep 27, 2022 at 19:06
  • 1
    $\begingroup$ RESPONDENTS! Before you answer this question, it would be helpful if you reviewed the OP's previous question and its answers. We're already getting some of the same answers here that were given there, which will quickly require this question to be closed as a duplicate. $\endgroup$
    – JBH
    Sep 27, 2022 at 20:32
  • 2
    $\begingroup$ Fog, fair warning, asking "Is this good enough?" in increments will likely get those incremental questions closed as duplicates of the first. If this Q devolves into a duplicate of the first Q, my suggestion is to not ask duplicates, but to not present an atmospheric content at all and instead ask, "I want a world where humans can acclimate to the atmosphere at sea level, but upon first arrival must wear oxygen masks. What's the highest oxygen level I can use to do that? My target atmospheric pressure is X." $\endgroup$
    – JBH
    Sep 27, 2022 at 20:37
  • $\begingroup$ @JBH OK makes sense, I can see how this is a duplicate question. I am new to the site so I am still learning the rules. I also mean oxygen supplemental masks. The book is set in the future (2100s) so the mask would be pretty advanced and filter air when necessary as well. $\endgroup$ Sep 28, 2022 at 2:33

3 Answers 3


You are about at the limits on lack of oxygen. Earth's sea-level atmosphere has about 0.2 bars of oxygen. Humans in Tibet and South America have adapted over generations to 4000-5000 metres, the lower end of which matches your atmosphere with about 0.12 bars of oxygen. Fit people can acclimatise in a few weeks to Everest southern base camp, which is at 5,364 metres, but unfit or unhealthy people usually cannot. A stable population contains many people who aren't fully fit and healthy, who are going to need supplemental oxygen in your atmosphere.

Also, your planet isn't heavy enough to hang onto helium: it will escape reasonably rapidly on geological timescales, so finding so much in a natural atmosphere is implausible.

Having that much xenon in the atmosphere is also unlikely. It's only formed in supernova explosions, not in ordinary stellar nucleosynthesis. If you want to keep it, you also ought to have large amounts of other heavy elements around, which may be hard on your biosphere. Xenon is also a powerful anesthetic. I doubt you have enough to put people to sleep, but it may well impair their judgement.

  • 2
    $\begingroup$ 0.12 bar partial pressure of oxygen corresponds to about 4000 meters above sea level on Earth. Most people can breathe just fine on top of Mont Blanc (4800 meters), and there are quite a few permanent settlements above 4000 meters elevation, including some towns with populations in the tens of thousands. $\endgroup$
    – AlexP
    Sep 27, 2022 at 20:35
  • $\begingroup$ @AlexP: Thanks, updated. $\endgroup$ Sep 28, 2022 at 9:32

People could mostly acclimate

The Death Zone (the altitude at which you need oxygen tanks to survive) is about 8000m. This is where you only get 35% as much oxygen as you do at sea level.

That said hikers often use oxygen tanks at elevations greater than 3000m to sustain physical exertion where oxygen is about 69% normal.

Altitude sickness for a person in average shape not exerting themselves too hard, can happen in altitudes as low as 2500m. 74% normal oxygen.

Considering these limits, you planet has about 56.8% normal oxygen. This gives you an equivalent amount of oxygen as you find on Earth at an elevation of 4523m.

While this is survivable it is far from ideal. El Alto-La Paz is the highest altitude major city at on Earth at about 3,870m (62% oxygen) suggesting that normal people don't adapt very well to elevations in excess of this however, smaller towns like La Rinconada can be found at elevations up to 5100 (53% oxygen), but hypoxia is a common heath problem, even among long term residents of this town.

Since your O2 levels are less than 69% your assumption that supplement O2 would be needed for most people at first is correct, and since your O2 levels are also lower than than 62%, it is unlikely the people in normal health could adapt to live thier comfortably without long term health concerns, but because your O2 levels are higher than 53%, we can assume life without a mask is sustainable if it absolutely had to be.



Wow, that's a low oxygen level!

OSHA (The USA's Occupational Safety and Health Administration) says that oxygen levels below 19.5% is low oxygen. In your proposed range of about 14%:

Concentrations of 12 to 16 percent oxygen cause tachypnea (increased breathing rates), tachycardia (accelerated heartbeat), and impaired attention, thinking, and coordination (e.g., Ex. 25-4), even in people who are resting.

Smarter Every Day has an interesting and terrifying video about hypoxia and air masks. We could expect these people to struggle with basic tasks and have severely impaired higher-level thinking ability. I would expect this situation to be unlivable over extended periods and for a normal society to not function.

This also isn't accounting for reduced atmospheric pressure, either! The lower pressure would reduce the effective equivalent concentration even lower to about 12%

More Realistic Oxygen Levels

I would go with OSHA's recommendation: 16-19%. That's enough that people can function, but not well. Even at 19%, this would require an acclimation time for people who just landed. It would be noticable but still allow people to perform higher cognitive tasks.

Factoring in the lower pressure, erring on the higher end of this percentage gives you an effective equivalent of about 16%, just in the border of this range.

Some Humans Have Adaptations

Genetic adaptations! Various groups of people have been living at high altitude (and therefore low oxygen) environments for a long time without complications. These groups would have a distinct advantage in a low-oxygen world! It would be certainly worth your time to investigate these adapted groups further.

  • $\begingroup$ Their sea level pressure is only 0.81 atm, so the equivalent under a full 1 atm would be more like 12%. $\endgroup$ Sep 27, 2022 at 20:20
  • $\begingroup$ @ChristopherJamesHuff good point, but still doesn't change the overall conclusion- it's just too low to get the sweet spot the author wants. $\endgroup$
    – PipperChip
    Sep 27, 2022 at 20:38
  • $\begingroup$ I wouldn't know what OSHA is, but it surely is not the Mountain Climbers' Association or the Community of People Who Live on Top of Hills. It is trivially easy to go below their "low oxygen" threshold -- just go on top of a thousand meters tall hill. Millions of people do it for fun. Millions live there all their lives. When you go to an alpine sky resort, the air you breathe is below that value! $\endgroup$
    – AlexP
    Sep 27, 2022 at 20:40
  • $\begingroup$ @AlexP I forget that not everyone knows about the US Department of Labor's Occupational Safety and Health Administration. I will make that more clear that this is considered, in spite of what people may think of US law, minimum safety standards for a developed nation. $\endgroup$
    – PipperChip
    Sep 27, 2022 at 20:44
  • 1
    $\begingroup$ OSHA's purpose is to keep people as safe as possible in their work environments. It's not a credible source for whether or not people can acclimate and live at lower percentages of oxygen. In fact, it might not have any meaning at all other than, "if your employer ensures this much oxygen, you can't sue them for that reason." If you pop over to the OP's previous question you'll find answers about an entire population living their lives for generations at 13% and the death zone being at 7%. $\endgroup$
    – JBH
    Sep 27, 2022 at 20:44

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