Sometime in the near future, a sizable population of humans has transported, through unspecified means, to a habitable super earth that has a much higher oxygen content than on Earth. Now, to the lay man that may sound like a good thing, the issue is however that too much oxygen can result in Oxygen Toxicity.

Fortunately, the unspecified means in which the humans have been transported to this world have also modified them to be able to survive in such an environment. With that said:

What possible modifications could be done to human physiology to live in a high oxygen world?

P.S. Ignore the super earth bit and other possible (fun) gasses that may be present. Those are for later

EDIT: For those wondering what the total atmospheric composition is, it's 70.1% Nitrogen, 24.9% Oxygen, 2% Argon, 1.2% Carbon Dioxide, 0.7% Ammonia, 0.01% Carbon Monoxide, 0.0001% Hydrogen Sulfide and trace amounts of other gases, mostly the noble gases, with pinch of Chlorine maintained through a small amount of metabolic process that also allow Ammonia and Hydrogen Sulfide. The atmosphere at sea level is also 1.55 atm.

Please note: the other gasses are irrelevant in this question, they may be getting their own questions and this iteration of the atmosphere may change as I flesh out my world even more.

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    $\begingroup$ Unspecified: how much more oxygen is present? $\endgroup$
    – L.Dutch
    Commented Oct 2, 2021 at 7:05
  • $\begingroup$ some space missions had atmosphere of pure oxygen, and were doing just fine. So your world needs to just have lesser or equal to Earth's atmospheric pressure. $\endgroup$ Commented Oct 2, 2021 at 13:35
  • 1
    $\begingroup$ Just burn some candles in front of your mouth ;) $\endgroup$ Commented Oct 2, 2021 at 13:51
  • $\begingroup$ @L.Dutch would the specification of oxygen amount change the answers? I would have expected a request for more clarity on how the answers are judged as currently there doesnt seem to be a way to differentiate between any of the (bio-altering) answers and pick the best. $\endgroup$
    – Demigan
    Commented Oct 2, 2021 at 17:49
  • 1
    $\begingroup$ @FuzzyChef Oxygen toxicity kicks in at an O2 partial pressure of about 0.3bar. Being a super-earth with a 1.55bar atmosphere (denser than Earth), it's 4% of the composition plus and additional 55% due to the higher ambient pressure. Total O2 partial pressure on OP's planet is 0.249*1.55=0.386bar, which is almost 30% higher than the oxygen toxicity threshold for humans. The question should probably include these details. $\endgroup$
    – J...
    Commented Oct 4, 2021 at 13:43

7 Answers 7


70.1% Nitrogen, 24.9% Oxygen, 2% Argon, 1.2% Carbon Dioxide, 0.7% Ammonia, 0.01% Carbon Monoxide, 0.0001% Hydrogen Sulfide and trace amounts of other gases, mostly the noble gases, with pinch of Chlorine maintained through a small amount of metabolic process that also allow Ammonia and Hydrogen Sulfide. The atmosphere at sea level is also 1.55 atm.

  • Your air is totally safe for unaltered humans in regard to oxygen. Your people will quickly adapt by lowering their hemoglobin levels (the adaptation will need probably months as we know of mountain climbing here on Earth).

  • 1.2% CO2 is livable (the exhaled air is ~4% CO2). Not pleasant, but livable - just like in a room with a bad ventilation. Be warned that most modern plants can't use it at concentration this high. You may want to start with reversed greenhouses.

  • 0.7% ammonia is outright deadly and you will need to handwave quite an adaptation to this - it will crash few homeostatic values at once. 1/100 of this has disastrous smell. I think it will be deadly even only by skin exposure. On the other hand, given chance, terrestrial algae and plants will eat it in no time. It is not really long-term compatible with oxygen, either.

  • Chlorine - 30 years ago I used to live in a city routinely polluted with chlorine. Bad, but survivable up to some quite unpleasant concentration. Not really stable in contact with vegetation.

  • Carbon monoxide has no chance in an oxygen-rich atmosphere. Other than that, the concentration is somewhat safe. It will also offset the oxygen toxicity, somewhat. (Oxygen therapy is used for CO poisoning).

  • Hydrogen sulfide is not even smellable at this concentration.

  • $\begingroup$ Thanks fir the input. Time to go back to the spreadsheet to get the desired results! $\endgroup$
    – Seraphim
    Commented Oct 2, 2021 at 21:39
  • $\begingroup$ One correction - 0.0001% is 1 ppm if I counted the zeroes correctly, That is enough for smell to be noticeable to most people, even if after a while you'll adapt and no longer notice it. $\endgroup$
    – LSerni
    Commented Oct 2, 2021 at 22:43
  • $\begingroup$ This, exactly this, only people who already have compromised lungs will suffer oxygen toxicity at your current concentrations. $\endgroup$
    – Ash
    Commented Oct 3, 2021 at 5:25
  • $\begingroup$ "Carbon monoxide...will also offset the oxygen toxicity, somewhat." Ooh, i did not think of that! That CO level is high and dangerous but still sub-lethal. It very well might work to reduce hemoglobin efficiency, without completely bombing your oxygen transport system. $\endgroup$
    – PcMan
    Commented Oct 3, 2021 at 7:08

Our own world has had a higher oxygen content, which let certain animals grow much larger, like the Meganeura*. So a higher oxygen content may not be too much of a problem and even beneficial.

If you look in your own link at "mechanism" you can also see a link to antioxidants♤ that the body already employs to prevent damage from current oxygen levels, and that oxygen toxicity occurs when these antioxidants and the cell repair functions are overwhelmed. I assume that creatures like the meganeura have had the capability to step up their antioxidant production to protect their cells since higher cell repair seems unlikely. If humans have similar capabilities they would be able to survive on your superearth.




You could use a tool like a Rebreather to recycle air being breathed out, and mix the air being breathed out with a small amount of fresh oxygen-rich air. Using the outside air means that you don’t need a gas tank.

Hell, you could create a simple temporary solution with a paper bag. Just breath in oxygen high air and then breath in and out in the paper bag a few times, lowering the oxygen content in the bag. If you make a small hole in the bag, it could mix the oxygen-arm air with some fresh air from the outside.

As for living there, I think a controllably ventilated closed off space would be safest. Ensuring that the oxygen levels never become too high in there.
You could initially lower the oxygen levels by building a small fire in the room, but that would also carbon monoxide most likely. (I expect it to produce way less than here on earth since there is more oxygen to easily burn).

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    $\begingroup$ Not really a physiological change, but I really liked the angle of using our existing capacity to convert oxygen to carbon dioxide. Then again, won't the carbon dioxide become toxic before the oxygen stops being toxic? We can't make Nitrogen. $\endgroup$ Commented Oct 2, 2021 at 11:43
  • $\begingroup$ @EmilioMBumachar The question assumes 25% of O2, while humans are adapted to 20%. You'd replace O2 with CO2 in the breathing mix, giving you 5% CO2. It's not toxic per se AFAICT, but breating reflexes will make you hyperventilate and push the O2 level in your blood up. You can't go up much more, toxic effects of CO2 start at 6% (mental confusion). So it's a nice idea but probably won't work for the gas mix in question. $\endgroup$
    – toolforger
    Commented Oct 2, 2021 at 21:20
  • $\begingroup$ @toolforger this answer was posted before the edit which added the exact gas composition, a composition the OP seems to want to adapt already. $\endgroup$
    – Demigan
    Commented Oct 3, 2021 at 9:59
  • $\begingroup$ I think in a 25% oxygen world fire would be treated like an unstable explosive, something extremely dangerous and to be only handled by the most qualified of personnel. Nobody's having barbecues on this planet! $\endgroup$ Commented Oct 3, 2021 at 14:28

First thing that comes to mind is that they can reduce the oxygen permeability of the lung tissue in the pulmonary alveoli, where the gas exchange happens.

enter image description here

In this way the amount of oxygen which enters the bloodstream, both by direct solution in the blood and by binding with hemoglobin, can be kept at the same level our organism is accustomed here on Earth.

Advantage of this approach is that it can work for any oxygen concentration in the atmosphere.

  • 2
    $\begingroup$ Looking at the link of oxygen toxicity this answer does not actually solve the problem. It solves higher pressures, but not higher oxygen content. You need something to specifically protect the cells of the alveoli which take oxygen directly from the higher oxygen content (and potential higher pressures of superearth causing them to collapse). Also things like the retinas would still be exposed. $\endgroup$
    – Demigan
    Commented Oct 2, 2021 at 10:54

24.9% Oxygen, and 1.55Bar pressure?

I would stop worrying about safely breathing it, and panic about a fire breaking out.

With 184% of the Oxygen partial pressure, fires will ignite a bit easier and burn several times faster, and a bit hotter, than on Earth.

P.S. That CO2 at 1.2%, at 1.55 bar, will cause some level of respiratory difficulty. It's not in the lethal range, but your people will have permanent headaches, tiredness and shortness of breath.

And let's please not think about that 0.7% ammonia. That will kill you in minutes! 0.5% (at 1 bar) is the "immediate respiratory arrest" level.

Similarly the 0.01% Carbon Monoxide is right on the knife edge between "will hurt you badly" and "will kill you immediately"

My advice, is to use mechanically augmented breathing filter/recirculator. A filter to keep out the really nasty stuff, partial recirculation of the old air to reduce the oxygen concentration, and consumable CO2 scrubbers to handle both the high atmospheric CO2 and the excess that gets recirculated by the rebreather system.
Think something like the Sohki Mask as portrayed in Nausicaa.
enter image description here

  • $\begingroup$ Fire is not fun enough on our world, so I thought of upping it little. You know, for the extra spicy flavour! Also, the CO2 levels only cause head aches? Hmm, time to up it up! But I’m all seriousness, I will write questions on what changes are required to breath in such an atmosphere. The drama of what happens if the system fails due a disease or the difficulty of having conventional technology is too good to pass. $\endgroup$
    – Seraphim
    Commented Oct 2, 2021 at 20:22
  • $\begingroup$ @Seraphim well, your in will burn steel territory. $\endgroup$
    – joojaa
    Commented Oct 4, 2021 at 5:24
  • $\begingroup$ @Seraphim CO2 is somewhat harmless to us. OSHA is happy to allow you to work at 5000ppm for the whole day, every day. 30000ppm is ok for half an hour. 100000ppm is survivable for 10 minutes, but leads to some nasty lung irritation, mostly due to the carbonic acid forming in the lungs and mucus membranes. That's 10% of the air already! The biggest danger of high ambient CO2 is that it confuses the heck out of your breathing reflex, leading to hyperventilation. $\endgroup$
    – PcMan
    Commented Oct 4, 2021 at 14:42

You can avoid oxygen toxicity by reducing pressure, so my suggestion would be to live high in the mountains, where the atmosphere is thinner. From their base up there they could develop new technology that allows them to progressively move further down.

But after seeing the edit, I don't think the increase in oxygen levels will cause any serious issues to a present-day human from Earth

  • 3
    $\begingroup$ OP is explicitly asking about modifications to human physiology $\endgroup$
    – L.Dutch
    Commented Oct 2, 2021 at 7:57
  • $\begingroup$ Welcome David, Please take our tour and refer to the help center for guidance as to our ways. Also worth carefully reading questions before answering. $\endgroup$ Commented Oct 2, 2021 at 9:23

Interestingly our breathing is controlled by the amount of carbon dioxide in the bloodstream rather than the oxygen level, which is why patients under general anaesthetic are given air with increased CO2 - it makes the patient want to keep breathing, which is rather useful.

An adjustment to this mechanism so it depends on oxygen levels instead of CO2 would result in lower breathing rates that would keep the oxygen concentration in the blood correct. CO2 could build up to higher levels than normal for humans as breathing also removes CO2, but I do not think that would have bad effects as it is essentially neutral (like nitrogen) but perhaps someone knowledgeable than me might comment.


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