I've seen reports of flight passengers/crew suffering hypoxia, and many times without knowing it.

My question is, how could you not be aware of it? For example, at above 10000ft, if your body is suffering from a lack of oxygen, doesn't it make you breathe faster and harder? (which in turn, makes you aware that something is wrong in the air you're breathing)

Second question: if you're dealing with air containing half as much oxygen, is it not enough (sustainable) to inhale (faster) twice as much air when breathing (without oxygen mask of course) ?

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    $\begingroup$ One of the first things to go is your ability to reason. I've watched aircrew in decompression test chambers and one of the common side effects is that they appear drunk - they are not able to recognise the symptoms. This is also common to other forms of gas imbalances such as nitrogen narcosis in divers. $\endgroup$
    – Simon
    Commented Jul 22, 2016 at 12:15
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    $\begingroup$ On a very related note, Smarter Every Day on Youtube just (today) released a video showing the effects of hypoxia and why people require training before they realize it is happening to them. youtu.be/kUfF2MTnqAw $\endgroup$ Commented Jul 22, 2016 at 16:40
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    $\begingroup$ I once hiked up to 10300 feet. When I arrived at the summit, I felt a little euphoric and felt almost not tired. So it felt good. When I was back home, that made me think how I would reacted at higher peaks... $\endgroup$
    – orique
    Commented Jul 22, 2016 at 17:46
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    $\begingroup$ You may read about hypoxia in diving which is very well documented. When there is half the partial pressure of oxygen, you cannot just breath twice frequently. The problem is oxygen pressure, not oxygen quantity. That's why divers use gas compounds, rather than pure oxygen (which would be lighter and take less volume), to manage the partial pressure. Also remember that the heart would have to work quicker too, as well as blood pressure would have to increase, oxygen separation would need to be more efficient, etc. $\endgroup$
    – mins
    Commented Jul 22, 2016 at 19:24
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    $\begingroup$ Your brain needs oxygen to realize anything, including the fact that it's not getting enough oxygen. In short, biology makes it very hard for a person to realize that they need more oxygen. The brain's not equipped with a low-oxygen-sensor, it just stops working. Fairly rapidly, too. $\endgroup$
    – aroth
    Commented Jul 24, 2016 at 0:54

6 Answers 6


The key point here is that your body does not measure blood oxygen levels. Instead, the urge to breathe is caused by a surplus of carbon dioxide in your blood. At sea level, this is fine: the only way to get so much carbon dioxide in your blood is if you used up all the oxygen.

At higher altitudes, the pressure is lower, and consequently the oxygen partial pressure. Since the rate of diffusion in your lungs is a function of pressure, this means that oxygen will diffuse less readily into your blood. At the same time, carbon dioxide diffuses more readily out of your blood due to the low pressure. This reduces your urge to breathe; you do not feel 'out of breath'. See this Wikipedia page.

At even higher altitudes, the pressure is actually so low that breathing will cause oxygen to diffuse out of your blood; you can breathe as much as you want but the partial pressure of oxygen in your blood will never be higher than the partial oxygen pressure in the air. This is why oxygen masks work (even if they're not airtight): they replace all air near your nose and mouth with oxygen, so that the partial pressure of oxygen equals the total atmospheric pressure rather than the usual ~21%. See this and this Wikipedia page, or for example this YouTube video.

Somewhat related: at sea level, you can also run out of oxygen before you feel the urge to breathe. You might think that breathing deeply before having to hold your breath for a long time increases the amount of oxygen in your blood, increasing your endurance. This is only partially true: it also artificially reduces your carbon dioxide levels, reducing your urge to breathe. This way, you can black out before feeling out of breath, which can be dangerous under water!

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    $\begingroup$ Your answer is quite good and can reach excellency by adding a link to a study or any other reference to confirming your answer, allowing us to read mode about this specific subject. $\endgroup$
    – Manu H
    Commented Jul 22, 2016 at 16:02
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    $\begingroup$ Seems like a major "design flaw". Imagine determining an engine is out of fuel only by measuring a drop in the amount of exhaust! $\endgroup$ Commented Jul 22, 2016 at 17:53
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    $\begingroup$ That's why when breathing helium from baloons (or other helium source) you should make sure to breathe some air in-between helium breathes... it's not really funny to see someone with helium-pitched voice suddenly faint and fall to the ground (I saw that one happen once...) $\endgroup$
    – Bakuriu
    Commented Jul 22, 2016 at 21:09
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    $\begingroup$ @sanchises: The biological reason is rather straightforward. O2 doesn't dissolve in blood. It's carried by the red blood cells. This makes it hard to sense the concentraion. CO2 on the other hand dissolves, and subsequently forms H2CO3 (carbonic acid). As this is a weak acid, it causes a drop in the blood pH. This directly affects the part of your brain which regulates breathing. $\endgroup$
    – MSalters
    Commented Jul 22, 2016 at 22:23
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    $\begingroup$ Minor quibble: Anything that raises the acidity (lowers the pH) of the blood will cause an increase in respiration rate: large doses of aspirin, Vitamin C, and of course carbon dioxide... $\endgroup$
    – DJohnM
    Commented Jul 24, 2016 at 1:46

No, lack of oxygen, in an of itself does not trigger negative physiological responses; quite the opposite. Most people who are affected by hypoxia have a general feeling of elation or euphoria and think everything is fine. The body will, however, respond strongly to high levels of carbon dioxide in the blood and lungs but not low levels of oxygen; this is why you feel a strong need to begin breathing again after holding your breath.

Below is a graphic demonstration of what happens to a pilot in a pressurized aircraft should it suffer rapid decompression and the pilot does not access a pressure breathing/on demand oxygen supply very quickly. The person feels drunk or euphoric; they believe nothing is wrong with them and that they are able to fly the airplane and operate the onboard systems just fine. The casual observer, however, sees the same person's cognitive and motor abilities degrading to the point that they cannot even accomplish basic tasks like play with a child's toy for instance or do simple math. And this condition gets worse and worse, all without the affected person even realizing what's wrong, until unconsciousness, coma and death ensue. The man in the video below would have died, had it not been for the tech in the chamber with him who forced the oxygen mask back onto his face.

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    $\begingroup$ It's amazing how quickly he recovers once he has the mask on. $\endgroup$
    – Ordous
    Commented Jul 22, 2016 at 18:19
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    $\begingroup$ Similar video by Smarter Every Day. $\endgroup$
    – E.P.
    Commented Jul 23, 2016 at 20:15
  • $\begingroup$ As I understand it, all mammals do have a physiological response to hypoxic conditions, it's just that humans aren't consciously aware of it outside of some mild light-headedness. But behind the scenes, your cells are well aware of the lack of oxygen and are going into a panic mode. The hormone "Hypoxia Inducible Factor" is responsible for regulating your response to hypoxia and when you do eventually die it's because your cells have decided to give up (i.e. cell death occurs through senescence and apoptosis rather than necrosis). $\endgroup$
    – Taedrin
    Commented Dec 14, 2023 at 16:36

As someone who has experienced hypoxia numerous times, the simple answer is that as you climb at 500-1000fpm into thinner air, the effects are very subtle, and, having compared them with other people, very personal.

For me, at about 86% O2 saturation, measured on a pulse oximeter, the first thing I start to feel is a very slight decline of mental function, such as lack of attention span and difficulty with words. the next effect at about 82% is a headache, and below 80% my mental capacity goes down. I have never had blue lips or nails, or euphoria, as the aeromedical factors in the pilot training tell me to look out for, nor have I blacked out down at 74%.

When I fly in my unpressurized mooney with built-in oxygen, I have the pulse ox on, and I tune the O2 valve to it at every altitude. But I have gotten good enough at recognizing the symptoms for myself, that I can tune my O2 valve by feel.

  • $\begingroup$ Wow. Never realised that the effects can be so different from person to person. I guess I should have expected it but I didn't expect it to the extent you're describing. $\endgroup$
    – slebetman
    Commented Jul 25, 2016 at 5:51

Apart from the points raised in other answers about breathing response being triggered by carbon dioxide in the blood, and the feeling of euphoria when hypoxic, there is also the fact that low blood oxygen impairs brain function. So not only do you feel giddy, but your ability to perform mental tasks degrades - you don't just feel fine, but you can't think straight to process the hazard.

For what it's worth: movies and TV probably have a lot to do with the misconception about human response to hypoxia. It's pretty standard for "the air is getting thin / running out of oxygen" to be portrayed by characters breathing more heavily. Whether or not it started out as an error which has never been corrected, it likely persists in part because it comes across in a more obvious and dramatic way on screen. And so the misconception likely persists in the general population because of how it is depicted on screen.


The 'breathe faster and harder' is a cinema trope.

One of the first things to go with hypoxia is judgement. Do you feel good? You cannot trust your assessment.

As an impecunious glider pilot (not able to afford oxygen gear), the drill my syndicate stick to is this. You don't go above 13000ft. If you're above 10000ft, and you yawn, that's it, you descend to sub 10000, regardless of how good the wave is, regardless of how you feel.


A simple explanation is that, if you reach the point when the Oxygen in blood is not high enough, then your alertness is reduced, and you may faint, and even die. This is why warnings are to put the oxygen mask yourself, before helping the one in the seat next to yours.

There are devices at a very low price, that measure Oxygen in Blood and Pulse Rate just by putting your finger inside it. 'Pulse-oxymetry' is the name, although it's a medical device, not intended for aviation use. If you begin to feeling your breath is racing, then you may be too close to losing consciousness for any additional steps to be taken.


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