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Suppose that humanity arrives on a super-Earth. Surface gravity = 1,24 g. Atmospheric pressure = 1,5 atm. Atmospheric composition includes 38,4% oxygen + 1,2% carbon dioxide + 0,5% hydrogen

How would this environment affect the physical condition (such as stamina, endurance, fertility, or cancer risk) of ordinary humans? (The humans have a prolonged period to acclimate in advance of their arrival, if that helps, but are not genetically modified for the environment)

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  • $\begingroup$ Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. $\endgroup$
    – Community Bot
    Apr 13, 2022 at 3:21
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    $\begingroup$ You're not going to have 0.5% free hydrogen in an atmosphere that's 38% oxygen. (To be clear, you're not going to have more than a statistically insignificant amount of free hydrogen.) Also, "super habitable" is not a reasonable classification, as that would be a situation more ideal for earth life, not something marginally more hostile, as specified. $\endgroup$
    – jdunlop
    Apr 13, 2022 at 7:14
  • $\begingroup$ Why won't I be able to get that much free hydrogen? What if water vapour molecules would split due to solar winds, couldn't I have the hydrogen in the atmosphere? $\endgroup$
    – Yulian
    Apr 13, 2022 at 13:41
  • $\begingroup$ Hydrogen bonds with everything that is even a little bit more electronegative, which is all the non-noble gasses. Chlorine-Hydrogen? Hydrochloric acid. Flourine? Flourine acid. $\endgroup$
    – Trish
    Apr 13, 2022 at 13:42
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    $\begingroup$ In addition to @Trish's correct point, water vapour split into its components by solar wind is the chief means by which "water" escapes the atmosphere, as it happens so close to the edge of the atmosphere that gravity cannot hold onto the hydrogen. Any hydrogen that circulates in the atmosphere will bond with anything electronegative, even without a combustion catalyst. 0.5% would mean 1 part of every 200 parts of atmospheric gases would be hydrogen, which, in the presence of oxidizers, is impossible. $\endgroup$
    – jdunlop
    Apr 13, 2022 at 17:12

3 Answers 3

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Both Would Die Without Protective Equipment

The big problem with oxygen toxicity isn't just absolute percentages, but also partial pressures. It's why divers have to adjust their oxygen mix to be more meagre under higher pressure. In this case, you've got 1.5 atm and a richer mix, so the partial pressure of oxygen is 1.5 atm * 0.384 = 0.576 effective bar of O2.

Oxygen toxicity sets in after prolonged exposures anywhere over 0.5 bar.

So it would be a prolonged period of suffering followed by death. The individual who was "acclimatizing" would simply die first.

Edit: The idea that someone can "acclimatize" to a toxic environment is popular and, often, wrong. Discomforts can be adjusted to, and certain biologically active poisons can have immunities developed by incremental exposure, but most toxins either cause accumulating damage or (as in the case of heavy metals, for instance) bioaccumulate, gradually making the subject less and less well until they die. Our metabolism cannot adapt to an environment that is consistently and immediately (in a metabolic sense) toxic.

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  • $\begingroup$ @EveninginGethsemane - not really. Unlike metabolic processes, combustion has been demonstrated to be mostly dependent on concentration, not partial pressure. 38% is high, but not so high that things will spontaneously burst into flame at the slightest spark. Hyperbaric oxygen therapy has those problems, but at that point we're talking concentrations approaching 100%. $\endgroup$
    – jdunlop
    Apr 13, 2022 at 8:49
  • $\begingroup$ Thanks, the paper there is more apt than my ageing memory. $\endgroup$
    – Angry Muppet
    Apr 13, 2022 at 8:52
  • $\begingroup$ +1 especially for mentioning the common misconceptions about acclimatization. One can acclimatize to pressure and temperature and humidity differences (within reason), but not to toxins. (and mechanically acting toxins are different from pathogens, and poisons acting like pathogens, the latter can be fought by the immune system, within reason, the former not) $\endgroup$
    – vsz
    Apr 13, 2022 at 12:21
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Your word is super not habitable

1.5 times atmospheric pressure and double oxygen concentration mean that we have people with a toxic level of oxygen. The carbon dioxide likewise is at a dangerous level.

Due to the high pressure, the lungs will not be able to get rid of the Carbondioxyde the body produces, so people will suffocate in a theoretically breathable atmosphere!

Surface gravity of 1.25 earth gravity is neglectable.

The Experiment is murder

Without a pressure reduction, the air will be toxic and kill them. Both. There is no acclimatization to that sort of pressure environment.

The gear needed is surprisingly simple

To breath on this planet, it would be enough to wear a rebreather that allows air to leak in at a slow rate one one side while an active pump removes the same amount of air on the other, keeping the pressure in the mask at about 1 atmosphere. This will keep the partial pressure of the oxygen below toxic levels and allow the human to live.

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  • $\begingroup$ Even at 1 atm, the partial pressure of O2 will be 291 mmHg and of CO2 will be 9 mmHG because of their increased concentrations in the air. This will increase intake of O2 a lot and decrease exhale of CO2 a lot. Also CO2 is 12000 ppm. This will result in severe headaches and intoxication because of both O2 and CO2. $\endgroup$
    – imtaar
    Apr 13, 2022 at 11:29
  • $\begingroup$ @imtaar It might need to lower the pressure more, but the gist is, with that gas, you could get away with just a pressure reduction. $\endgroup$
    – Trish
    Apr 13, 2022 at 21:41
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Gas Exchange

Gas exchange in lungs occur due to difference in partial pressures of gases.

Normally, the differences in partial pressures are as follows:

O2 CO2
Inhaled Air Concentration 21% 0.04%
Inhaled Air Partial Pressure 159 mmHg 0.3 mmHg
Alveolar Air Partial Pressure 104 mmHg 40 mmHg

Therefore O2 goes in and CO2 goes out.

In your world,

O2 CO2
Inhaled Air Concentration at 1.5 atm 38.4% 1.2%
Inhaled Air Partial Pressure 436.5 mmHg 13.5 mmHg
Alveolar Air Partial Pressure 104 mmHg 40 mmHg

Therefore O2 intake will increase but exhale of CO2 will decrease.

Bad health effects

Normally, CO2 in air is 0.04% = 400 ppm which has no health effects

In your world, CO2 in air is 1.2% = 12000 ppm

This will cause severe headaches, slight intoxication depending on the exposure time.

The person will get severe headaches and intoxication because of both O2 and CO2.

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  • $\begingroup$ @L.Dutch Thanks for edits. Is | for a table? $\endgroup$
    – imtaar
    Apr 13, 2022 at 11:32
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    $\begingroup$ meta.stackexchange.com/q/356997/644344 $\endgroup$
    – L.Dutch ♦
    Apr 13, 2022 at 12:06
  • $\begingroup$ Isn't exhaled air containing 4% CO2? If you're providing first aid to an unconcious person, your exhaled air is what they'll breathe. $\endgroup$
    – Yulian
    Apr 13, 2022 at 13:43
  • $\begingroup$ @Yulian - this is true, but if you're providing artificial respiration, headaches are the least of the side effects the recipient is going to experience. Assuming they survive, which isn't that likely "In cities such as Seattle where CPR training is widespread and defibrillation by EMS personnel follows quickly, the survival rate is about 20 percent for all causes and as high as 57 percent if a witnessed "shockable" arrest." (And that's a good situation.) $\endgroup$
    – jdunlop
    Apr 13, 2022 at 21:21
  • $\begingroup$ There's a reason why there's the trope of CPR standing for "Clean, Pretty, and Reliable" in media - the likelihood of someone in respiratory or cardiac arrest outside of a medical facility being revived by CPR is extremely low, so the risks involved in oversaturation of CO2 and the ribs broken during chest compressions are acceptable risks to provide any hope at all. $\endgroup$
    – jdunlop
    Apr 13, 2022 at 21:22

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