I have been creating a world, but I'm not sure if the atmosphere would be breathable for humans. (I'm kind of worried about the xenon levels)

The planet is made of

  • 62% Nitrogen (N)
  • 21% Oxygen (O2)
  • 15.5% Argon (Ar)
  • 1% Xenon (Xe)
  • 0.49% Neon
  • 0.01% Trace Gases (like Carbon dioxide and water vapor)

The atmospheric pressure is 1.42 atm.

Since I am not very good at math, can you tell me if it is habitable, and if it isn't, can you suggest some improvements?

  • $\begingroup$ Hello @Neillyer, welcome to Worldbuilding. When you have a moment, it's worth reading through our tour and the following two Help Center pages to know the limits of this site: help center and help center. We look forward to helping you build worlds! $\endgroup$
    – JBH
    Apr 21, 2023 at 23:34
  • $\begingroup$ You can get high on Xenon, though probably not with 1%. xenonclinic.com/en/xenon-inhalation-therapy $\endgroup$ Apr 22, 2023 at 9:30
  • $\begingroup$ Yes usually 70% xenon is used in Anesthesia, so 1% shouldn't be an immediate problem in that respect, but who knows what long term effects might build up? $\endgroup$
    – Slarty
    Apr 22, 2023 at 10:05
  • $\begingroup$ @Slarty Building up of Xe in the lungs is the problem. $\endgroup$
    – user86462
    Apr 22, 2023 at 15:54
  • $\begingroup$ I'm also guessing that the lightning would excite the xenon in the atmosphere and create a brilliant blue/purple color. $\endgroup$
    – Neil Iyer
    Apr 23, 2023 at 22:15

5 Answers 5


From Source concerning noble gases (xenon, neon, argon, etc.):

These gases are not very toxic but they can be harmful in large amounts. Noble gases are asphyxiants. Asphyxiant gases replace oxygen in the air so there is less of it to breathe. Without oxygen you can die.

However, helium is also an asphyxiant. All that means is that it's a gas humans can't do anything with. Asphyxiate... drown... fundamentally the same thing: no oxygen. Anything not oxygen is an asphyxiant. However... those radioactive bounders, if they could be gasses, they'd cause a world of hurt in a different way. Let's ignore that. There are other gases that are very toxic, but they tend not to be in atmospheres that don't belong on Venus, so let's ignore them, too.

Since you have the same percentage of oxygen, your atmosphere is fine. The pressure difference isn't that big of a deal. It would take a bit of acclimation, but it's not so great that we can't handle it.

Based on saturation diving operations, it looks like the limits are as follows:

  • Compressed air: Nitrogen narcosis limits you to around four times Earth's atmospheric pressure.
  • Any gas mix: Hydreliox was used for the current depth record; insomnia and fatigue issues appear to limit you to around 65 times Earth's pressure regardless of gas mix.

The oxygen partial pressure of 0.3 bar is on the threshold of risk of mild oxygen toxicity, especially if one hyperventilates. Some diseases, especially cancers, will be more prevalent and harder to treat. Percentage of oxygen in the mixture is irrelevant; it's the absolute quantity of oxygen per lungful that matters. All else being equal, the humans will have superior athletic performance at low altitudes, but better long term health outcomes at high altitudes, where the pressure and hence partial pressure of oxygen is less.

I don't think there would be any direct health impacts from the other gases.

  • $\begingroup$ How can I make the oxygen not toxic at all? $\endgroup$
    – Neil Iyer
    Apr 22, 2023 at 3:31
  • $\begingroup$ Lower the partial pressure (multiply atmospheric pressure by percent oxygen) from 0.3 bar to about 0.28 bar. You'll still have more cancer, but no risk of hyperoxia. $\endgroup$
    – g s
    Apr 22, 2023 at 5:52
  • $\begingroup$ If I decreased the pressure to about 1.33 atmospheres and keep the same percent of oxygen, that would decrease the partial pressure to 0.2793 bar. $\endgroup$
    – Neil Iyer
    Apr 22, 2023 at 14:45

No, due to xenon asphyxiation

SF6 is an inert, non toxic gas. You still can't have it in a breathable atmosphere in significant quantities because it gradually builds up in the lungs; the density is such that it is preferentially retained in the lungs, and you eventually asphyxiate.

Note that that the Xe% will probably be much higher than 1% at surface level, although it depends a bit on atmospheric circulation. You certainly wouldn't want to dig a pit there.

SF6 has molar mass 146, Xe has 131, so they should be directly comparable re: asphyxiation.

The rest looks OK, especially after adaptation.


Xenon is heavy, and will sink to the bottom of the atmosphere, so the concentration is greater there. Despite being noble, it is also an anasthetic, the mechanism of it's operation is unknown. It's not generally used due to its price, though some very high people have demanded it's use due to having fewer after effects than conventional operations. The asphyxiation mentioned in the other answer is not correct, the motion of your lungs will get any gas out and in.

Although the 1% you mention is less than the 63+% used in anasthesia, I think the 1% will almost certainly put you to sleep if you are breathing it non stop. This has never been tried.

  • $\begingroup$ I don't know about this, but if the animals and humans have been living on the planet for centuries, would they learn to evolve a mechanism to suppress the worst of the effects of xenon? $\endgroup$
    – Neil Iyer
    Apr 22, 2023 at 18:31
  • $\begingroup$ well, you can make up whatever biology you like. If you took current humans and put them in this atmosphere, I'd be surprised if they could function properly. They might end up permanently high, or be unable to function. It would likely be a significant change in the biology to make this breathable. There are a lot of halocarbons that are hallucinogens as well. $\endgroup$
    – camelccc
    Apr 22, 2023 at 22:02
  • $\begingroup$ Yeah, that makes sense because normal humans have adapted to conditions with very little xenon in the atmosphere, but I guess this planet's inhabitants would be able to adapt after thousands of years and the suppress the effects of the xenon. $\endgroup$
    – Neil Iyer
    Apr 23, 2023 at 14:39

According to Habitable Planets for Man, Stephen H. Dole, 1964, Table 2 on page sixteen, the approximate upper limits of various gases given as partial pressure in millimeters of mercury (mmHg) are:

Helium 61,000 (?)

Neon 3,900 (?)

Nitrogen 2,330

Argon 1,220

Krypton 350

Xenon 160

Carbon Dioxide 7

According to Dole on page 15 the upper limit for oxygen is about 400 millimeters of mercury (mmhg).


The atmospheric pressure is 1.42 atm.

One atmosphere pressure is defined as 760 millimeters of mercury, so 1.42 atmospheres is 1,079.2 millimeter of mercury, and each percent of a gas in the atmosphere of that planet would be equal to 10.792 millimeters of mercury (mmHg).

Nitrogen 62 % equals 669.1 mmHg - within limits.

Oxygen 21 % equals 226.6 mmHg - within limits.

Argon 15.5 % equals 167.2 mmHg - within limits.

Xenon 1 % equals 10.792 mmHg - within limits.

Neon 0.49 % equals 5.28808 mmHg - within limits.

So every atmospheric gas listed seems to be at a safe pressure level, according to the data available to Dole in the 1960s.

Some answers indicated the oxygen and Xenon levels would be too high. So you might want to find out if any recent research has changed the acceptable levels of those gases since 1964, and ask the answerers what evidence they have for their statements.

Since the planet is supposed to be naturally habitable, it has to have liquid surface water in some locations at some times. And that means there will be some water vapor in the atmosphere. According to Dole in Table 4 on page 21 the maximum pressure of water vapor is 25 mmHg.

So don't forget to have some water vapor in the atmosphere.


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