I recall (although, perhaps wrongly), from my PADI scuba diving books, that air is made up of 78% nitrogen, 0.8% CO2 and 1.2% O2.

I was wondering, what would happen to our lungs (or to that of any other living creature on Earth), if, that proportion changed.

For instance,


78% nitrogen, 0.9% CO2 and 1.1% O2.

I was wondering, hopefully, my question is a good one (I don't know if it's good or bad), but thank you for your answers, I think this would be intuitive, and shed light, on the workings and life of many living organisms.


  • 2
    $\begingroup$ The quoted numbers are incorrect; 1.2% O2 is way too low, should be ~20% and CO2 is way too high, should be ~0.04%. Nitrogen is about right. Check the source, try some others. Usually gas proportions are for dry air and leave out water vapor, that varies widely. $\endgroup$
    – Ken Fabian
    Commented May 21 at 22:27

2 Answers 2


The (dry) air in Earth's atmosphere is about 78.08 percent nitrogen, 20.95 percent oxygen, and 0.93 percent Ar. Roughly, 78% N2, 21% O2 and 1% Ar. CO2 is a mere 0.04%. If the oxygen content were 1.1-1.2%, we couldn't survive!


As a follow on to Riccardo's answer, dry-air oxygen levels are fairly stable at 20.95% outdoors. The info below is based on OSHA here and here:

  • Below 19.5% - air is oxygen deficient and one should leave the area

  • 16%–19.5% - decreased ability to work and for tissues to receive oxygen they need, impaired thinking while not at rest

  • 12%–16% - increased breathing rates, increased heartbeat, impaired thinking at rest

  • 10%–14% - faulty judgement, intermittent respiration

  • 6%–10% - nausea, vomiting, lethargy, loss of consciousness. Fatal in about 8 minutes

  • 0%-6% - convulsions, cardiac standstill, fatal in about 40 seconds

When you are exposed to oxygen poor air a body can rapidly release oxygen it has based on  Le Chatelier's principle. Which is why death can occur much quicker than humans can hold their breath.


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