VivaTheBeaver
Here’s how I understand the situation I’m a SCUBA diver (increased pressure breathing) and also a pilot (reduced pressure breathing)
The amount of oxygen the body can use from whatever gas fills the lungs depends on the partial pressure of the oxygen contained in that gas.
Under normal circumstances this lung-filling gas is air. Which is, to a first approximation, 20% oxygen and 80% nitrogen. For most of us this air fills our lungs at 1 atmosphere (1 bar) pressure. At this (sea level) pressure, the partial pressure of oxygen is around 0.2 bar. (ie. 20% - the percentage of oxygen in air, times the total pressure, 1 bar). That’s the partial pressure of oxygen most of us humans are accustomed to breath.
Obviously, if we live at 16,000 feet up in the Andes Alte Plano or in the Himalaya, we are accustomed to breathing a pressure somewhat less than this. The total air pressure, will be reduced and hence the oxygen partial pressure also. The body will acclimatise/adapt to this if we stay at this altitude for long enough.
But most of us live at sea level. When we ascend to (say) 10,000 ft, the total pressure is reduced to about 0.7 bar. The partial pressure of oxygen is about 0.14 bar. (0.7 x 0.2). This remains just enough for most of us (acclimatised to sea level pressures) to continue to function, Enough to just get by. If the partial pressure of oxygen falls much lower than this we get problems.
There is one other fact to consider. The human body can only breath in/out at the ambient pressure. There is no “holding your breath”, it cannot be done. The only pressure our lungs can work with is the ambient (cabin) pressure. For a scuba diver this “pressurisation” may be several bars pressure, for a pilot, it may be a fraction of a bar. This product of “cabin” (ambient) pressure, times the percentage oxygen in the gas (air) mixture gives the critical partial pressure of oxygen. The partial pressure is what keeps you alert and active.
If we humans want to continue to function adequately at altitudes much above 10,000ft ambient, we need to maintain a partial pressure of oxygen of (at least) 0.14 bar.
We get two choices:
We can either increase the “total pressure”, while continuing to breath air. This is called pressurisation.
Or we can breath gas mixtures enriched with extra oxygen (to a total of 100% oxygen) and still allow ambient pressure to fall. The sums are the same as far as the body is concerned.. We need to maintain a partial pressure of oxygen of a minimum of about 0.14 bar. The body does not care how it gets it. Total pressure and/or percentage oxygen in the gas mixture.
Looking at my Met notes, atmospheric pressure is about these values at various altitudes:
100 mb – 53,000’
200 mb – 38,000’
300 mb – 30,000’
400 mb – 24,000’
500 mb – 18,000’
700 mb – 10,000’
So you can see that when breathing 100% oxygen, one can go to somewhere between 30 to 45,000 ft and still maintain a partial pressure of around 0.14 mb oxygen. More or less enough to continue to function.
Current airliners cruise somewhere around 30,000 to 42,000 ft. You can do the sums yourself to see what is required.
Breathing 100% oxygen at 40.000ft cabin altitude may not be enough to remain conscious. (A cabin depressurisation without an emergency descent, even while breathing 100% oxygen)
However, a depressurisation at (say) 28,000 ft, while breathing 100% oxygen is not that bigger deal.
But you must also remember that there are large individual differences in reactions to reduced oxygen partial pressures. Caution is advised.
[This message has been edited by Lord Lucan (edited 28 January 2001).]
[This message has been edited by Lord Lucan (edited 28 January 2001).]