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When flying at high altitudes in an unpressurized plane, you use portable oxygen tanks to prevent hypoxia and other altitude-related symptoms. but does this 100% pure oxygen cause health issues when flying at high altitude?

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    $\begingroup$ It should be noted that you are not breathing in 100% O2, you are supplementing the lack of O2 at altitude with additional O2. Very rarely would you breathe pure O2, it's typically mixed (using a rebreather, diluter, or cannula) with cabin air. $\endgroup$
    – Ron Beyer
    Jun 15 at 2:00
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    $\begingroup$ At high cabin/unpressurized altitudes "Diluter-demand" systems can provide up to 100% Oxygen when needed for a flight crew. $\endgroup$
    – 757toga
    Jun 15 at 2:44
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    $\begingroup$ "Love is like oxygen - you get too much, you get too high" $\endgroup$ Jun 17 at 18:57
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    $\begingroup$ If you consider a flash fire a health issue, then yes. However, it wasn't due to breathing the pure O2. $\endgroup$
    – FreeMan
    Jun 22 at 12:54

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When we discuss a mixture of gasses, it's often useful to compute the pressure of each component separately. This is called the "partial pressure" of that component. For instance, oxygen composes only 21% of our atmosphere, so at sea level (1013 hPa), the partial pressure of oxygen would be about 213 hPa.

If you have a semipermeable membrane (such as, for instance, the tissue of a human lung), it's the partial pressure of each component of the mixture that determines the flow of that particular component. Oxygen flows into the blood from the air because the partial pressure of oxygen in the blood is less than the partial pressure of oxygen in the air.

Thus, as the total air pressure goes down, the percentage of oxygen it contains can be increased without affecting the body. If you were breathing pure oxygen at 213 hPa, your blood would have the same amount of oxygen as if you were breathing normal sea-level air. Of course, it's dangerous to go that low for reasons other than lack of oxygen, but the human body is pretty resilient, so as long as you get close, you can safely breathe pure oxygen indefinitely.

For instance, astronauts who flew on Gemini VII spent two weeks breathing pure oxygen at a maximum of 6 psi (414 hPa) and were perfectly fine. That's roughly the ambient air pressure at 20,000 feet. However, Gemini missions had strict limits on how long the astronauts could stay in the capsule before launch, because at that point they were breathing pure oxygen at sea-level pressures, which can cause health concerns.

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    $\begingroup$ About the physiology of breathing: one thing lowering the partial pressure of oxygen while breathing in, is that as the air enters the lungs, it instantly becomes saturated with moisture. A 100% "shot" of oxygen will not be 100% anymore when it enters the lungs. This moisture saturation is one major factor in development of hypoxia at higher altitudes. $\endgroup$
    – Jpe61
    Jun 15 at 6:38
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    $\begingroup$ And going the other way, deep sea divers have to decrease the amount of oxygen in their breathing mixture to avoid oxygen toxicity and longer-term damage. $\endgroup$ Jun 15 at 13:44
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    $\begingroup$ So if you breath it at sea level pressures then it becomes a health risk? $\endgroup$
    – Boeing787
    Jun 15 at 16:05
  • $\begingroup$ @Boeing787 Yes, it does. $\endgroup$ Jun 15 at 22:35
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    $\begingroup$ @Boeing787 Breathing pure oxygen at sea level pressure is no health risk when done for 1 or 2 hours, but for 10 and more hours it is a health risk. $\endgroup$
    – Uwe
    Jun 16 at 21:31
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Yes...

but not predictably, and (almost certainly) not seriously. If you're worried for yourself and you're not already dead and not doing any crazy maneuvers, it is vanishingly unlikely that you need to worry about health effects of pure oxygen. Before focusing on composition control, focus on fixing your sleep, hydration and seated posture.


Specific to aviation, the biggest issue is that it exacerbates atelectasis. Your lungs are made up of many tiny air-sacks called alveoli. These are held open by the ambient air pressure in your lungs, while surface tension of the water lining your lungs naturally tries to squeeze them closed*. Oxygen goes through the walls of your lungs fairly readily; nitrogen does not. If you have actual pure O2 in an alveolus, it will collapse very easily. When a small fraction of alveoli collapse, it's called atelectasis (when most alveoli do it at the same time, it's a "collapsed lung"). You obviously won't get pure O2 after everything mixes together, but the closer you get, the less resistant your lungs are to atelectasis.

Atelectasis isn't that bad in isolation. It can be uncomfortable, and makes you work harder for a less effective result when you breath, but it's not usually a major problem. Also, it usually resolves itself with any brief, transient overpressure (i.e. it goes away if you cough). However, if you're already borderline on breathing, it's not a great addition to the mix.

That's probably fine for large passenger aircraft. For jets doing maneuvers, it is much less fine: another major contributor to atelectasis is g-force. F-22 pilots knew of a phenomenon they called "Raptor cough", where they would always need to cough after pulling positive-g maneuvers.

You can see where this is going, and indeed atelectasis was identified as a contributing factor on the US Navy's physiological episode epidemic a few years back.**

Although the NASA report highlighted the issue, it was a known design constraint for a while ahead of time. you can search "MIL-STD-3050" to get an idea of how the DoD plans to approach new designs. (3050 is a garbage spec because it's poorly written, but the underlying approaches are mostly solid).


The other potentially major issue is not about pure oxygen per se, and definitely not relevant to tanked oxygen: there is some evidence that rapid swings in breathing gas composition can induce physiologically Bad Things. Unless you are deliberately monkeying with the valves for a while, that's only going to be applicable to OBOGS, and only under some circumstances.


Otherwise, the dangers of pure O2 in aviation are the same as when you're sitting in your living room:

  • It's an oxidizer and thus promotes rapid combustion of flammable materials.
  • It can dry you out quickly.
  • There is at least one very rare condition where pure O2 will make you go hypercapnic and kill you.

For the more exotic dangers, you're going to want anaesthesiologists to answer. Outside of aviation, the only time you're likely to be on pure O2 is under general sedation, and, conveniently, that's also the time when there is a doctor very closely monitoring you (difficult to do in flight).


I am summarizing and simplifying at several points above, because the detailed interactions are complicated but don’t change the behavior. If you want a solid grounding in the details of how lungs work, find a copy of Guyton & Hall, Textbook of Medical Physiology. Very readable, and enough technical detail to engineer against.


*You have a special coating that floats on top of the water lining your lungs: "pulmonary surfactant" (i.e. "lung gunk"). It does a lot and is poorly understood. One major thing it does is reduce the surface tension which is trying to collapse your alveoli by about 60%. That collapsing force is proportional to surface tension and to the square of alveolar radius. Developing babies have smaller alveoli and start making surfactant around week 28. That's why week 28 is when premature birth goes from really, really bad to just regular bad.

** Search "NESC report on F/A-18, E/A-18 physiological episodes" to find NASA's report on the subject. I couldn't find a single authoritative document source, but there are copies of the thing all over.

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    $\begingroup$ "Oxygen goes through the walls of your lungs fairly readily; nitrogen does not." This is wrong, nitrogen does go from the lungs into the body and may cause decompression sickness. This is a problem for scuba divers but also for military pilots ascending very fast without a pressurized cabin. $\endgroup$
    – Uwe
    Jun 15 at 17:31
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    $\begingroup$ @Uwe I am comfortable describing equalizing on the order of tenths of a second as “fairly readily” and equalizing on the order of tens of minutes to hours as “not fairly readily”. $\endgroup$
    – fectin
    Jun 15 at 17:37
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    $\begingroup$ Is that equalizing difference really a lung surface effect? AFAICT the reason oxygen distributes so fast is because hemoglobin in blood is a far more effective carrier. And of course, the third important gas (CO2) is carried as H2CO3 $\endgroup$
    – MSalters
    Jun 16 at 8:00
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    $\begingroup$ @MSalters it’s a distinction without a difference here. The liquid side isn’t even a partial pressure anyway, because that only applies to gas. But the oxygen and nitrogen move bi-directionally through your lung surface based on chemical gradients, and oxygen flows out of the alveoli much more readily (largely because of what happens after, but still) $\endgroup$
    – fectin
    Jun 16 at 11:18
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    $\begingroup$ @MichaelHall agreed. That’s where it got a lot of publicity though (example: abcnews.go.com/Blotter/… ), which is why I pointed to it. $\endgroup$
    – fectin
    Jun 17 at 18:13
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Depending on environment pressure and duration, breathing pure oxygen may cause health issues. But health issues are more a problem during SCUBA diving and hyperbaric chambers than high altitude flying.

Oxygen partial pressures of above about 0.5 bar may cause lung problems after one to several days.

Above about 1.2 to 1.5 bar breathing of pure oxygen may cause spasm and loss of consciousness after some ten minutes. So this is not possible during aviation.

At partial pressures and durations possible when flying health issues caused by pure oxygen are impossible for persons healthy to fly as a passenger or a pilot.

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There is some evidence $^{[1]}$ to suggest that breathing high concentration oxygen can lead to lung damage over the long term as a result of the buildup of reactive oxygen species in lung tissues. This is linked to the development of lung cancer.

In terms of absolute risk this is likely minimal compared to the increased exposure to cosmic ionising radiation experienced at altitude. As such it's not really relevant, but it is interesting.

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