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This answer says at FL500 (50,000 ft; 15,250 m) the time of useful consciousness is only 6 to 9 seconds.

But if I just stop breathing at any arbitrary time, without any preparation, I can easily continue without air for about 20 seconds. How can these times be so short?

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    $\begingroup$ It's worth noting that the time of "useful consciousness" is much shorter than the time of "consciousness", because the effects that appear in the situation make the consciousness much less useful to solve practical problems: Tunnel vision and euphoria may be very distracting, while still being conscious. $\endgroup$ – Volker Siegel Sep 29 at 3:46
  • $\begingroup$ I the graphics and details in this answer - when you are fully exhaled at sea level, you have more leftover air in your lungs than you have when you are fully inhaled up at that altitude. $\endgroup$ – Džuris Sep 29 at 20:54
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When you are breathing, oxygen ($\mathrm{O}_2$) and carbon dioxide ($\mathrm{CO}_2$) are exchanged between the alveoli in your lungs and the environment. This gas exchange is based on diffusion, which means the partial pressures of each gas involved will move towards equalization:

Henry’s law states that the amount of a specific gas that dissolves in a liquid is a function of its partial pressure. The greater the partial pressure of a gas, the more of that gas will dissolve in a liquid, as the gas moves toward equilibrium.

(source)

As long as the oxygen partial pressure is higher in the environment, your blood will gain oxygen from breathing. But if the oxygen partial pressure is lower, you lose oxygen from breathing. Therefore, holding your breath at sea level gives you more time until you run out of oxygen than breathing at 15km altitude, where total pressure is about 10 times lower than at sea level. Furthermore, as John K pointed out in the comments, holding your breath at sea level allows you to feel when you need to breathe again, because of increasing $\mathrm{CO}_2$ levels, where breathing at high altitude does not feel different because $\mathrm{CO}_2$ can still leave your system.

This principle also allows oxygen masks in aircraft to function without creating higher pressure. Since (more or less) pure oxygen is created, the oxygen partial pressure is much higher than in the surrounding air despite being at the same total pressure.

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    $\begingroup$ So if you could hold your breath, you could stay conscious for a minute or so as normal, but attempting to breath no only removes the remaining oxygen from your lungs, but also removes oxygen from your blood? $\endgroup$ – Robin Bennett Sep 27 at 11:50
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    $\begingroup$ @RobinBennett In principle yes. But note that during a rapid depressurization you cannot really hold your breath. The air will be forced out of your lungs anyway. Holding your breath at sea level pressure gives you the "minute or so". I clarified the answer. $\endgroup$ – Bianfable Sep 27 at 11:59
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    $\begingroup$ Other thing is that when holding your breath on the ground what forces you to breath again is the carbon dioxide buildup in your blood, not the lack of oxygen. If it wasn't for that, you could easily make yourself pass out. When cabin pressure is reduced slowly, you won't even be aware of the pressure drop because you are still able to expel CO2. When I took a High Altitude Indoctrination chamber ride back in the 80s, the biggest surprise when I started breathing 25000 ft ambient air was I couldn't tell the difference, just sitting still in my chamber seat, until the hypoxia symptoms set in. $\endgroup$ – John K Sep 27 at 12:24
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    $\begingroup$ @JohnK Good point, I added it to the answer. $\endgroup$ – Bianfable Sep 27 at 12:30
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    $\begingroup$ @PerlDuck the biggest problem is that for many people the initial hypoxia symptom is the euphoria stage, which is very similar to being on laughing gas at the dentist. My initial symptom, luckily for me, was tunnel vision. Meanwhile a seatmate went straight to euphoria and was cackling in laughter as he tried to draw his shapes. I reconnected when I felt a weird dunk sensation well up as I observed my tunnel vision, then all the symptoms vanished. You're sitting drawing characters on a clip board. As your characters become all lop sided, you are AWARE that they are lopsided, but don't care. $\endgroup$ – John K Sep 27 at 13:32
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The situation in a high altitude depressurization is different because:

  1. The air in your lungs is now "FL500 air" - i.e. the pressure is about 0.1 atmosphere. This means that the partial pressure of O2 (ppO2) is about 0.021 atm, instead of 0.21 atm. Oxygen will rapidly diffuse out of your blood and into your lungs, and your brain will very soon not have enough oxygen to function. (You can actually manage with a slightly lower partial pressure of O2 - scuba diving mixes are often considered "hypoxic" once they're below 16% O2, since your can breath a ppO2 of 0.16 atm and function normally. But note that this is 8 times greater than 0.02 atm).

  2. You cannot hold your last breath of cabin pressure air. To try do so would in fact be very dangerous, since your lungs would be trying to support a 0.9 atm pressure difference between the inside and outside. Scuba divers are taught to never hold their breath while ascending for exactly this reason - the pressure difference can rupture the alveoli in your the lung. This can result in gas passing directly into the blood and causing an embolism.

  3. But if you stick on an oxygen mask, that 0.1 atm of air (with a ppO2 of 0.02) has been replaced by 0.1 atm of O2. So your ppO2 is then 0.1. Still not ideal, but much better than 0.02 atm.

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    $\begingroup$ If you're wearing an oxygen mask at FL500, it's going to be a pressure-demand mask (which feeds oxygen to your lungs at higher-than-ambient pressure), providing your lungs with a ppO2 of 0.16-0.2 atm (this leaves your lungs holding a small pressure differential - 0.06-0.1 atm in our example, potentially up to 0.18-0.2 atm at higher altitudes - but they handle small pressure differentials like these without much trouble). $\endgroup$ – Sean Sep 27 at 21:34
  • $\begingroup$ 0.16atm ppO2 allows functioning normally if you're not exerting too much. You'll probably feel it if you try to run up a flight of stairs, or cycle over mountain passes (I've done both at around that pressure) $\endgroup$ – Chris H Sep 28 at 19:45
  • $\begingroup$ Ok so the idea is that the pressure in the bloodstream is controlled by the external air or water pressure, while the pressure of the gas in the alveoli can be raised well above the external air or water pressure by holding one's breath during decompression or rapid ascent-- $\endgroup$ – quiet flyer Sep 29 at 17:52
  • $\begingroup$ At FL500 your mask doesn't work very well. $\endgroup$ – Loren Pechtel Sep 30 at 2:46
  • $\begingroup$ @quietflyer I don't think I'd use the phrasing "well above" in that context. Alveoli start to rupture and create air emboli at very little above 0.2 atm, and it takes conscious effort to exhale at much lower pressure difference than that -- making just breathing a tiring task over a fairly short term. Most people cannot exhale at all against 0.2 atm. $\endgroup$ – Zeiss Ikon Sep 30 at 13:36
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It's also the starting gun.

When you hold your breath, you choose when to start. That is known.

In a hypoxia incident, you rarely discover the pressurization problem at the very start of the event. It may be well along before you notice it. So you don't know when you actually started "holding your breath".

It would be more of a fair comparison if the failing system went "loss of cabin pressure in 3, 2, 1...” But of course, it doesn't do that!

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  • $\begingroup$ There's explosive decompression and nonexplosive decompression. $\endgroup$ – Ben Crowell Sep 30 at 0:11

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