Could an atmosphere of 50% O₂ 25% Neon 23% Nitrogen and 3% trace gasses be survivable for a human being at the similar or slightly less atmospheric pressures as Earth?

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    $\begingroup$ You might want to read about the Apollo 1 fire where a 100% oxygen atmo lead to a disaster. $\endgroup$
    – ghellquist
    Dec 9, 2018 at 8:53
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    $\begingroup$ 50% + 25% + 23% + 3% = 101% $\endgroup$
    – fabian
    Dec 9, 2018 at 15:27
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    $\begingroup$ @fabian: It might simply come from displaying rounded values. If you round $1.5 + 1.5 = 3$ to the nearest integers it looks like $2 + 2 = 3$. $\endgroup$ Dec 9, 2018 at 17:40
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    $\begingroup$ Note that your trace gasses need to include a certain amount of CO2. $\endgroup$
    – TLW
    Dec 10, 2018 at 1:17
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    $\begingroup$ I notice a distinct lack of CO2. How does your flora work? No flora, no humans. $\endgroup$
    – Mast
    Dec 10, 2018 at 6:57

5 Answers 5


Some of the information in this thread is questionable, so I'm adding my 2 cents (as a qualified rebreather diver):

  • rebreathers use soda lime absorbent material to remove the CO2 in their breathing loop. The CO2 passing through the scrubber absorbent is removed when it reacts with the absorbent in the canister; this chemical reaction is exothermic, which results in heat and moisture.
  • we target a PPO2 of 1.2 for most dives. Going above 1.6 is where toxicity starts to become an issue. That said, many divers report blurry vision after long (4+ hours) dives at 1.3.




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    $\begingroup$ Greetings! I certainly appreciate your qualifications and desire to address points made in various answers, but on Stack Exchange this is rather a no-no! WB.SE is a Q&A forum. Answers are not available spaces for you to comment on the question or on other answers. Kindly review the help center and tour so you can get a better idea of what is expected in an answer. That said, I am certain your particular expertise and knowledge will come in handy around here! And I hope you'll find places to exercise it properly! $\endgroup$
    – elemtilas
    Dec 10, 2018 at 5:29

This environment is almost certainly fatal, due to Oxygen Toxicity:

When the exposure to oxygen above 0.5 bar (50 kPa) is intermittent, it permits the lungs to recover and delays the onset of toxicity.

0.5 bar would be your 50% O2 level at atmospheric pressures, so current medical knowledge says exposure to this environment must be intermittent.

Oxygen toxicity occurs at points above 0.3 bar (30% at 1atm). It's more trouble as one progresses to higher levels, but that shows that such high oxygen levels are going to cause problems.

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    $\begingroup$ Thank you, do you think if I lessened the atmospheric pressures or increased elevation that this would improve survival conditions? $\endgroup$ Dec 9, 2018 at 2:19
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    $\begingroup$ If you've got 1 atm pressure but 50% Oxygen could you use e.g. a filter mask of some sort to reduce oxygen content in some way to safe levels ? $\endgroup$ Dec 9, 2018 at 5:27
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    $\begingroup$ Fatal, yes, but not quickly so. Extrapolating from the charts on the Wikipedia article, I'd guess you could function for days or possibly weeks without serious harm. $\endgroup$
    – Mark
    Dec 9, 2018 at 8:17
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    $\begingroup$ @StephenG That's an interesting question. My instinct is yes, you could pull that off. SCUBA divers that use rebreathers use chemical reactions to strip the carbon off of the CO2 to make O2. Going the other way seems easy, since so many things want to react with oxygen. You might even have a fire "pre-breathe" it for you! $\endgroup$
    – Cort Ammon
    Dec 9, 2018 at 17:21
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    $\begingroup$ That's all depends on definition of "Fatal". Wiki's article does not seem be compelling that permanent 0.5 bar of oxygen will be fatal. It will cause adverse health effects for sure, but the question "how quickly it will kill?" is still wide open. $\endgroup$
    – Alexander
    Dec 9, 2018 at 18:17

Even if the toxicity didn't do them in, this atmosphere would turn a small firecracker into a grenade. Higher concentrations of oxygen make things burn and/or explode more intensely.

Don't believe just because I'm saying this. Watch this video (you may jump to 0:50).

Humans are dumb, and explosions are a recurring theme in the Darwin Awards. Any large group of people wouldn't survive in that atmosphere even if they were breathing an Earth-like mix of gases from a scuba gear.

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    $\begingroup$ One of the funny things i found when researching my answer was Hydrox, a breathing gas made of hydrogen and oxygen, as dumb as that sounds. It cannot be made with a O2 partial pressure above 5% to avoid fire/explosions. However, 5% O2 is not enough to keep a person conscious at 1atm, so you have to breath a different gas mix until you get deep enough to where it is safe to switch to hydrox. I think that's hillarious! $\endgroup$
    – Cort Ammon
    Dec 9, 2018 at 20:21
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    $\begingroup$ Na, your firecracker won't turn into a grenade, it already comes with it's own oxidizer. All explosives do, the only explosions where air oxygen is relevant are gas explosions (mixtures of flammable gases with oxygen or air). But the remains of your firecracker will burn quite cheerfully in an oxygen enriched environment... $\endgroup$ Dec 9, 2018 at 22:36
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    $\begingroup$ @cmaster: Not just gases, unfortunately. Dust explosions are a well-known industrial risk. Flour can explode, to name a seemingly harmless compound. $\endgroup$
    – MSalters
    Dec 10, 2018 at 9:21
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    $\begingroup$ @MSalters Grind it fine enough, and disperse it in an oxygen-rich atmosphere, and pretty much anything will explode. So, the good news is "no more hayfever". The bad news is that "seasonal allergies" are replaced by "seasonal explosions" $\endgroup$ Dec 10, 2018 at 10:07
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    $\begingroup$ The Darwin Awards are called that for a (tongue-in-cheek) reason: Unfit individuals are de-selected from the gene (or rather meme) pool. The individuals surviving the initial time needed to establish safe procedures for handling fires and flammables would have normal survival chances. $\endgroup$ Dec 10, 2018 at 11:45

If 3% traces are not a problem. Then the only thing that rises concern is an oxygen levels. Oxygen toxicity, as stated, will be part of your problems, as there are more, like production of oxygen species, sight defects and blindness in infants and more.

  • Oxygen at 50% is around levels that have no prominent toxicity for humans. So any healthy adult human will be able to live in such an atmosphere.
  • Such levels will be a problem for the weaker ones: children, elderly, sick. Your population will age faster and live less, your child mortality and birth defects most likely will push them to extinction.

Good options are:

  • We live high. With altitude there is less air, so less oxygen partial pressure. Living 4000 m - will deal with most of problems, around 6000 meters will be Earth like. Do not forget your pressure cooker.
  • Balance atmosphere pressure and composition. You would like your oxygen partial pressure to be less than 30 kPa.

Of note such an atmosphere is a fire hazard and promotes corrosion.

  • $\begingroup$ +1 for lowering partial pressure $\endgroup$
    – Pere
    Dec 9, 2018 at 21:26
  • $\begingroup$ You may end up needing to boost the partial pressure of CO2 if you're living high, depending on how much of that trace gas is CO2. $\endgroup$
    – TLW
    Dec 10, 2018 at 1:19
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    $\begingroup$ If it is same as Earth's ~ 0.04%, it won't be a problem for humans, decrease in crop yields. But you can just make it higher > more crops. Just hope that carbon dioxide is in 0.02-0.5% range so it doesn't cause problems: too much bad for humans, too little will cripple your flora. $\endgroup$ Dec 10, 2018 at 2:35
  • $\begingroup$ Crop yields are fixable with greenhouses; already on Earth we run greenhouses at elevated CO2 levels. $\endgroup$
    – MSalters
    Dec 10, 2018 at 9:22

It would be survivable for moderate durations, but probably not for many days at a time.

A partial pressure of oxygen (PPO2) of 0.5 (50% of O2 multiplied by 1atm of pressure) is considered the cusp of noticeable effects of oxygen toxicity and the equations used in diving for computing the allowable time limit works from that, so a PPO2 of 0.5 wouldn't return a valid result. The closest data point is the first entry in diving tables at a PPO2 of 0.6, which recommends a time limit of 720 minutes.

If you wanted to increase the atmospheric pressure to 2atm, the new PPO2 would be 1, as 0.5*2. Using that you should be able to find the allowable time for any combination of oxygen percentages and atmospheric pressures.

  • $\begingroup$ Hi Roco, welcome to Worldbuilding.SE. Please consider taking the tour. Your answer would profit from adding some more details. What does PPO2 stand for, for example? $\endgroup$ Dec 10, 2018 at 10:35
  • $\begingroup$ Adjusted, should be more clear now $\endgroup$
    – Roco
    Dec 10, 2018 at 11:11

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