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The FAA's oxygen rules in §91.211 are based on pressure altitude. Assuming our lungs are like our engines and wings and dependent on density altitude, why are the oxygen requirements not based on density altitude?

If the answer is practicality, then would it not be wise to be more conservative with these requirements on hot days and more lenient on cold days?

Living in Colorado, the oxygen requirements are almost exactly what is usually required to transit the front range of the Rockies without an oxygen bottle, which leads me to believe practicality played a part. If density altitude were used instead, on a hot day you'd be unlikely to remain within the regulations and still transit the area without hitting anything made of rocks.

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    $\begingroup$ Our lungs take in oxygen based on partial pressure: How do people suffering from hypoxia not realize they aren't getting enough oxygen? $\endgroup$
    – fooot
    Jun 30, 2017 at 22:16
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    $\begingroup$ I believe partial pressure is dependent on density altitude, not pressure altitude. $\endgroup$
    – John
    Jun 30, 2017 at 22:20
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    $\begingroup$ @John, partial pressure is dependent on pressure altitude. That should be rather obvious since the partial pressure is just the total pressure times volume fraction of given compound. Temperature would only come into play if the thermal expansion of each compound was significantly different, but they are not. $\endgroup$
    – Jan Hudec
    Jul 1, 2017 at 21:43
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    $\begingroup$ Jan, I think the FAA has confused us by suggesting a redefinition of the term "pressure". Partial pressure is computed as you say, but the pressure you mention includes temperature. Pressure altitude also includes temperature, but it's at a temperature defined in the ISA, not the actual temperature. $\endgroup$
    – John
    Jul 2, 2017 at 19:13
  • $\begingroup$ I gotta believe that part of the reason is because airplanes file flight plans for a pressure altitude, and the altimeter reads pressure altitude. Do you really want to be calculating density altitude enroute to determine compliance and possibly squeeze just a little bit more out of the regs? KISS applies here... $\endgroup$
    – Michael Hall
    Dec 5, 2021 at 0:46

2 Answers 2

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The air in the lungs is in pressure equilibrium with the cabin, but not temperature equilibrium. It's always at body temperature. So the only variable that can affect how much oxygen is available to the body is the pressure of the outside air - thus pressure altitude, not density altitude, is physiologically relevant.

That said, if the density altitude is high it may mean you are hot and dehydrated, and the deleterious effects of those factors may be additive with those of hypoxia.

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    $\begingroup$ Even more importantly, the limiting factor is adsorption of oxygen on the hemoglobin and that process is governed by partial pressure. $\endgroup$
    – Jan Hudec
    Jul 1, 2017 at 21:46
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    $\begingroup$ I believe both pressure altitude and partial pressure necessarily must include a temperature, it's just not the one that's actually outside or in our lungs when using the ISA temp. So I think this answer could be restated: The temperature outside (and therefore density altitude) is not relevant because our lungs always heat or cool air to body temperature. This would still mean pressure altitude is not accurate as the ISA temp used in pressure altitude is usually not body temperature. en.wikipedia.org/wiki/Barometric_formula#Pressure_equations $\endgroup$
    – John
    Jul 2, 2017 at 19:15
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Assuming our lungs are like our engines

They are not. Engine (at least spark-ignited reciprocating one) uses most oxygen in each charge and its RPM is limited, so it can't be run faster to compensate for the lower air density.

However, our bodies only use small fraction of oxygen from each breath and we normally breathe much slower than we can if need be.

and wings

They are not. Wings depend on dynamic pressure, but dynamic pressure is not involved in any significant process in the lungs.

and dependent on density altitude

Well, they don't. The limiting factor for respiration is the rate of association of oxygen to the hemoglobin. That depends primarily on partial pressure of oxygen and since partial pressure is just pressure times volume fraction, on pressure altitude, not density one.

Temperature does affect the association rate, but since the temperature is that of blood, and we are warm-blooded, the temperature is fairly constant. And the effect is different from that on density anyway.

There are many other factors that affect the association rate that are physiological and have nothing to do with the weather and everything to do with the person.

Therefore there must be safety margin built into the rules. I can offer two anecdotes:

  • When I went up to 14,000 ft, we spent several days around 12,500 ft. Still, just around 14,000 ft, one of the 16 people there got, rather suddenly, symptoms of altitude sickness and had to turn back immediately. While the rest, most were fine and few (including me) were not able to exert much effort, but were otherwise also fine.

    Note that this shows the safety margin is not all that big. For some people, 14,000 ft is dangerous, though for most it is not.

  • During WWII, US Army was supplying China via airlift over Himalayas and the route had minimum safe altitude of 19,000 ft. They had some strange accidents where the aircraft failed to round out after apparently normal approach and crashed at the threshold that happened mainly on the Chinese side where airfields were around 7,000 ft (while they were almost sea level on the Indian side). Investigation showed that apparently some pilots didn't use oxygen. Obviously they didn't feel unwell, but the reduction in mental performance showed. Strict order to wear the masks did resolve the issue, proving the analysis [1].

[1]: Tunner, Lt. Gen. William H. (1955). Over the Hump; unfortunately the link on WP is dead, so I am linking the WP citation in the hope somebody will manage to fix it.

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