Question

What is the partial pressure of oxygen in a commercial airplane at 30,000 feet?

Asked by: Timothy Doran

Answer

Airplanes are pressurised to maintain a comfortable living environment for human beings. A commercial airplane often flies at around 36,000ft (to put this in perspective the summit of Everest is 28,000ft).

Other than the extreme temperature at this altitude, there is very low Oxygen pressure (about a quarter of that at sea level). Humans would not be able to breathe without the pressurised environment.

Ideally an airplane would be pressurised to ground level pressure (760 Torr or mm of mercury). However this is not practical as the fuselage of a plane would have to be incredibly strong (and hence very heavy and expensive to fly) to withstand the outward force caused by 760 Torr while cruising at 36,000ft.

Therefore modern commercial jets compromise and are pressurised to an altitude of 5000-8000ft. This explains the phenomenon of 'ears popping' on take-off and landing as the air trapped in the ear is effected by the change in pressure from ground level to this effective altitude.

The actual equation for the change in partial pressure with altitude gives an exponential rather than linear decay. This is essentially because air is squashable so is compressed together at lower altitudes. Therefore pressure is decreased more rapidly near the ground than at higher altitudes. It is approximately halved by 18,000ft.

At ground level, the partial pressure of Oxygen is about 150 Torr (20% of atmospheric pressure at 760 Torr). Inside a plane at cruising altitude it is reduced to about 125 Torr, i.e. a negligible change. The blood in most people will remain fully saturated with oxygen at this pressure. However outside the plane it is around 25-30 Torr, a very hostile environment.

It is interesting to note that this altitude is about the highest at which a human can breathe, and requires a supply of pure oxygen. At greater altitudes insufficient oxygen can be provided (as the partial pressure is just too low) and consciousness is lost. This could be a problem on Concorde which flies at an altitude of about 45,000 ft...

Answered by: Jules Seeley, M.S., Physics graduate; Strategy Consultant, London.

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