This is the first question in a series on the physical limits of the human body.

The intent of this series is to, illustrate the upper and lower limits of human biology for the purpose of building worlds on which they can (or cannot) survive.

Other Questions: (obviously this will get updated as other questions are asked in the series.)


What are the maximum possible and minimum required for the human body to function properly, in percentage of the atmosphere near the sea level.

  1. Oxygen

Future Questions will cover:

  1. Nitrogen
  2. Hydrogen


  • Humans must be able to function at least efficiently enough to provide food, water, and shelter for themselves without the aid of hydrocarbon fuels (basically pre-industrial, pre-gunpowder technology).

This can be in the form of farming or hunting. Humans still need to be able to exert themselves physically without the atmosphere causing undue limitation on their ability to do so.

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    $\begingroup$ This seems like a question for Biology SE rather than Worldbuilding... $\endgroup$ – Frostfyre Jul 22 '15 at 20:25
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    $\begingroup$ @Frostfyre Hmmm...I feel like it has a lot of use here...especially with the planned series I have in mind but I see your point...ideas to make it better for world-building specifically or should we just drop it? Have had this series in mind for the challenge topic... $\endgroup$ – James Jul 22 '15 at 20:27
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    $\begingroup$ I have something similar in mind, which is why I haven't voted to close it (yet). I don't really see a way to make this more about worldbuilding. I'll wait to see what the community thinks. $\endgroup$ – Frostfyre Jul 22 '15 at 20:37
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    $\begingroup$ My feeling is that a series of questions like this would be a great resource for sure...BUT we do have biology SE so...yeah...I see both sides. I flagged this to mods to put it on hold for me until we sort out what we decide. $\endgroup$ – James Jul 22 '15 at 20:39
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    $\begingroup$ I've already provided answers to this question even though the question asked might be different. Can I just link my answer to this question? $\endgroup$ – Jim2B Jul 22 '15 at 21:58

Just ask NASA, they have already studied it pretty carefully.

The key chart is
As long as you keep hydrogen concentration below about 4% (at normal earth atmosphere pressure and temperature) it is not flammable mix. Don't know where you can find flammability limits in chart form, but the Wikipedia article on flammability limits lists the formula for estimating this based on different atmospheric compositions. For low H2 concentrations (to avoid fire) you can see that lines are close to flat at the right edge of the chart, so you can probably ignore hydrogen pretty much for environmental tolerance.

Reading some specific values from the chart, for sea level pressures, an oxygen content from about 12% to 62% is considered safe, i.e., a human of normal health can survive indefinitely without negative consequences. In the range 9% to 12% people who have been acclimated to low oxygen environment will be OK as the body will adapt by changes in the blood and lungs to survive lower oxygen levels. Also note at low oxygen levels there may be some discomfort, (shortness of breath), but nothing of real consequence although your athletic abilities will be curtailed.

Above 62% the increased oxygen (at sea level pressure) is at least somewhat toxic, though at much higher pressures pure oxygen can also be quickly fatal. As the chart shows, 24 hours of even pure oxygen won't be a serious problem (though fire risks are severe).

Nitrogen is safe in any amount at sea level (assuming your still have enough oxygen). At higher pressures, narcosis becomes important, i.e., nitrogen (almost all inert gases too) puts you to sleep, makes you intoxicated, etc. The exact level of the inert gas required for narcosis varies by type.

A small amount of CO2 is also required for breathing regulation, etc.

  • $\begingroup$ Be advised that without fire or other ignition source, any $H_2$ or $CH_4$ in the atmosphere will slowly react with the $O_2$ and disappear without constant replenishment. There is a % of $H_2$ at which the mixture becomes explosive. I don't recall the % for $H_2$ but I think $CH_4$ is around 15%. $\endgroup$ – Jim2B Jul 22 '15 at 22:00
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    $\begingroup$ Above a certain % of $H_2$, the mixture is no longer flammable (not enough oxidant in the mixture). $\endgroup$ – Jim2B Jul 22 '15 at 22:04
  • $\begingroup$ The link to the picture appears to be broken. $\endgroup$ – Monty Wild Jul 22 '15 at 23:38
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    $\begingroup$ Gary, for those of us not terribly familiar with the science can you explain what the chart is showing? $\endgroup$ – James Aug 4 '15 at 15:49
  • $\begingroup$ Added verbal description of some parts of the chart. $\endgroup$ – Gary Walker Aug 5 '15 at 12:09

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