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Referring to a chapter from Aircraft Maintenance And Repair

Normal air contains approximately 21 percent oxygen, and this provides adequate oxygen for the human body at lower altitudes. At 34 000 ft [10 363 m] altitude, a person must be breathing 100 percent oxygen to absorb the same amount of oxygen as when breathing air at sea level.>

Why is it required for us breathe 100% oxygen to absorb same 21 percent of oxygen we would breathe at ground/ lower altitudes?

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The answer is you don't need more oxygen. You need exactly the same AMOUNT of oxygen. But the density of the atmosphere decreases as you climb to higher altitude. (the density is created by the weight of the air above you). So the AMOUNT of oxygen in each cubic inch of atmosphere (or in each breath you take) decreases. To compensate for that, to keep the AMOUNT of oxygen in each breath the same, you must increase the percentage of each breath that is oxygen. So at Sea Level, the pressure is 14 lbs per sq inch, and the atmosphere is 20% oxygen. To get the same AMOUNT of oxygen at 17000 feet up, where the pressure is only 7 lbs per square inch, you would need to be breathing air that 40% oxygen. And at an altitude where the pressure was only 2.8 lbs per square inch, (this is somewhere around 38,000 feet as I recall), you would need to be breathing 100% oxygen.

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  • $\begingroup$ So in a way, we're just substituting the lost atmospheric pressure with oxygen? $\endgroup$ – Salmonbeing00 Sep 2 '20 at 11:28
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    $\begingroup$ The other gentleman's answer speaks in terms of what is called the Partial Pressure of oxygen (20% of 14.7 lbs per sq inch (psi) = 2.8 psi, So, yes, you could phrase it that way, although I would not use the concept of substitution. You just need to have enough oxygen in each breath you take. You need 2.8 psi of Oxygen to be equivalent to sea level.... But people can acclimate, some live and survive in the Andes at 16000, 17,000 feet, where the partial pressure of oxygen is only 1.4-1.5 psi.... $\endgroup$ – Charles Bretana Sep 2 '20 at 23:59
  • $\begingroup$ That just completes the whole answer, thanks! $\endgroup$ – Salmonbeing00 Sep 3 '20 at 12:53
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This question is best answered on https://biology.stackexchange.com, but in short it is the partial pressure that counts. This is (pressure in atmospheres) * (percentage).

At 0.5 atm (18 kfeet) you need twice the amount of oxygen in the air (aka 40%) for it to have the same effect on you. Above 38.5 kfeet the pressure is 0.2 atm, so even 100% oxygen will only provide 20% partial pressure of oxygen, which is the same as normal air at sea level

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    $\begingroup$ There's a good table on this at engineeringtoolbox.com/air-altitude-pressure-d_462.html. $\endgroup$ – Terry Aug 30 '20 at 20:14
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    $\begingroup$ The answer is you DON'T. You need exactly the same AMOUNT of oxygen. But the density of the atmosphere decreases as you climb to higher altitudes, so the AMOUNT of oxygen in each breath decreases. To compensate for that, to keep the AMOUNT of oxygen in each breath the same, you must increase the percentage of each breath that is oxygen... $\endgroup$ – Charles Bretana Aug 31 '20 at 16:52

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