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  • 400038

    Hi everyone,
    Try to figure out this question:

    Breathing 100% oxygen at 40000 ft is equivalent to breathing ambient air at:

    18 000 ft
    8 000 ft
    10 000 ft X
    14 000 ft

    However, if we do the calculation, breathing 100% O2 (unpressured) at 40000ft (=187hPa) is equivalent to 187hPa partial pressure of O2.

    With 21% of O2 in ambient air and a simple calculation, we can determine that 187hPa partial O2 pressure is reached when the ambient pressure is (187/0,21)=890hPa (roughly 3700ft).

    The answer is consistent with all the questions about that subject but I just don't understand why.

    If anyone can help,


  • #2
    Hi ska38,

    You're not factoring in the fact that there is also water and carbon dioxide in the alveoli of the lungs and the partial pressures for these two remain relatively constant. At 10,000ft the alveolar partial pressure is 55 mmHg. We then go onto oxy/air mix keeping us at sea level (103 mmHg alveolar partial pressure) up to 33,700 ft. Above this the partial pressure will reduce until it gets to 55 mmHg, the limit for human endurance. This occurs at 40,000 ft, hence the equivalent of being at 10,000 ft. As stated you shouldn't just think of this in ISA terms i.e. 21% O2 in the atmosphere. At sea level the alveolar partial pressure of 103 mmHg is 14%.


    • #3
      Thanks for this reply, OK understood.