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In my experience, many people don't have serious problems camping, skiing, hiking or doing other physical activities at 12,000 feet (3,600m). We may get tired easily, and be out of breath, but we still function physically and mentally.

But for pilots (who aren't exerting themselves physically), hypoxia is a much more serious problem, e.g. see this article. Pilots aren't involved in heavy physical activity the same way mountaineers are, yet 91.211 requires O2 for periods as short as 30 minutes.

Is it accurate to say that hypoxia hits pilots harder than campers for some reason? Is there a clear reason why?

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  • $\begingroup$ Maybe because pilot usually "shoot" up there compared to the others and may not have time to adjust. $\endgroup$ – SMS von der Tann Sep 1 '16 at 15:38
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    $\begingroup$ The dangerous thing about hypoxia is that you are often not aware of how severely affected you are. Someone camping at high altitudes will probably not notice the extra fumbling needed to get the barbecue started or how finding direction takes a few seconds of extra thinking. In a cockpit, "fumbling" and "a few seconds" can mean the difference between life and death. $\endgroup$ – J. Hougaard Sep 1 '16 at 15:50
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    $\begingroup$ Hypoxia affects different people differently, and the FAA needed to be conservative with altitude restrictions for oxygen. People who are hiking/camping/skiing at 12k feet are usually active, athletic people who can adjust to that quickly. You need to take into account though the degradation in cognitive abilities, not just physical ones. There is also the problem of night/day. With oxygen deprivation, your eyesight degrades quickly at night. $\endgroup$ – Ron Beyer Sep 1 '16 at 15:51
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    $\begingroup$ Does the downvoter want to explain? I think the question is cromulent. $\endgroup$ – abelenky Sep 1 '16 at 15:53
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    $\begingroup$ @Simon: I'm using the Simpsons definition of cromulent. $\endgroup$ – abelenky Sep 1 '16 at 20:29
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The analogy that if someone can become acclimated to working at high altitudes and therefore should not need supplemental O2 in an airplane is flawed. While flying - at least straight and level - does not require strenuous exertion, it is very cognitively taxing. It's a constant process if taking in a deluge of incoming data, be it instrument read outs, navigation information, external stimuli, radio chatter, the grunt work of flying the aircraft etc.

Unfortunately, the conscious and cognitive centers of the brain as well as the eyes require an oxygen rich blood supply in order to operate fully; if this is denied or reduced, serious deterioration of these abilities will occur and can happen rapidly within seconds.

Night vision is also severely impaired by hypoxia as the rods in your eyes require a lot of oxygen as well. I did an experiment one night in C172 where I took off at sea level and climbed to 12,500 feet. Looking outside the aircraft at those altitudes I could see a few bright stars but after I had landed back at close to sea level, I looked up at the night sky while tying down the airplane and could see a sky filled with stars as well as the bright gaseous strip of the Milky Way. The effect was dramatic; you really are affected by hypoxia at night.

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  • $\begingroup$ I have heard of similar night experiments where a passenger at similar altitudes (12,500 feet) will then don an oxygen mask. Anecdotally, I have heard that it immediately increases brightness of stars and other lighted objects. $\endgroup$ – Chance Heath Sep 1 '16 at 17:35
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    $\begingroup$ As far as the anecdote in the last paragraph, I wouldn't be so quick to chalk that up to hypoxia. While flying the airplane, your eyes will be adjusted to the light levels from the lights that are allowing you to read your instruments. When you're outside tying the plane down, your eyes will likely be adjusted to a lower light level unless there are bright lights around the ramp. Also, glass is partially reflective. You won't be able to see light sources on the other side of the glass that are less bright than the light reflecting off the inside of the glass from the panel and such. $\endgroup$ – reirab Sep 1 '16 at 18:10
  • $\begingroup$ I turned the panel light rheostats to the lowest possible setting where I could read the gauges in addition to turning off the wingtip strobes and allowed by eyes to adjust to the light levels. The ramp was flooded with halogen light. No there was a noticeable difference directly attributed to hypoxia. $\endgroup$ – Carlo Felicione Sep 1 '16 at 23:18
  • $\begingroup$ @reirab Hypoxia begins to degrade your night vision at altitudes as low as 5000 feet. $\endgroup$ – Ron Beyer Sep 2 '16 at 1:54
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Most people can function at 12,000 ft. Pilots do as well and don't need oxygen there.

Above that altitude, however, it gets worse very quickly. By 14,000 ft (where oxygen is always required for crew) most people will be noticeably short of breath and between 16,000 – 17,000 ft they may pass out.

I was once that high in the mountains. We were hiking, so some acclimatisation was taking place, though we were climbing faster then would be ideal. We first noticed being weakened as we neared the 13,000 ft. Then we spent two days there, so it got a bit better, but as we climbed above 15,000 ft, one (of the 15 or so) got altitude sickness and somewhere below 16,000 ft one had acute symptoms of hypoxia and had to turn back I think two of us turned back as we were barely able to walk. That matches the altitudes where pressurisation or supplemental oxygen is needed quite well.

Now if you get acclimatised, you will be able to function normally at those altitudes. That, however, requires spending days to weeks at the altitude. You'll never manage to get acclimatised just by flying.

Pilots acclimatised because they live at higher altitudes do handle the altitude better. But the regulations need to be simple and work for the worst case with some safety margin. Also, Carlo is right that brain requires a lot of oxygen, so concentration suffers even if you are not doing any physical exercise and don't feel physical weakness.

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