How do people suffering from hypoxia not realize they aren't getting enough oxygen?

Question

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) ?

Answer 1

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!

Answer 2

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.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.