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Assumptions:

  1. The typical oxygen generator, on a plane, lasts twelve to twenty minutes. (Source: Gizmodo.com)

  2. Even in the event of cabin decompression, the oxygen in the cabin does not disappear or extinguish instantaneously.

  3. If 1 is correct, then after how long would the oxygen usually need to vanish?

  4. Waiting 2-5 minutes after the mask drops does not harm or injure; but obviously one must not wait too long, and should apply the mask sooner than later.

The question is motivated by wishing to maximise the use of the oxygen mask, whose oxygen supply is limited.

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    $\begingroup$ See Would failure to put on an oxygen mask during loss of cabin pressure result in death?. Airliners are required to be able to descend to a safe altitude before oxygen runs out. If you wait, hypoxia may set in and you may forget or not be able to put on your mask. $\endgroup$
    – fooot
    Commented Dec 25, 2016 at 19:38
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    $\begingroup$ When the masks are released, there has been already a drop of pressure, the cabin altitude is already around 14,000 ft, this is what triggers their release. In addition of the pressure reason explained in answers, why would you wait? It takes time to adjust a mask and get used to it. Pilots will immediately descent to a safe altitude, and unless the aircraft is very damaged, it will take 5 min to lose 25,000 ft at 5,000 fpm. 10 miin at 2,500. $\endgroup$
    – mins
    Commented Dec 25, 2016 at 20:09
  • $\begingroup$ Are you asking for crew or passengers? I would assume passengers, since crew oxygen is supposed to last much longer that 12-20 minutes. $\endgroup$
    – J W
    Commented Dec 25, 2016 at 20:12
  • $\begingroup$ In rapid decompression, the oxygen does disappear nearly instantaneously. Or at least, enough of it disappears that what remains is not sufficient for your body. $\endgroup$
    – Jan Hudec
    Commented Dec 26, 2016 at 0:15
  • $\begingroup$ @JonathanWalters Thanks; I clarified. $\endgroup$
    – user128
    Commented Dec 26, 2016 at 1:34

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The point is not that the oxygen is used up. Even after a depressurization, the air around you still contains so much oxygen that it would take your body (at rest) hours to burn it all. But this oxygen is spread out so thinly that a standard set of lungs will not be up to getting enough of it from the cabin air into your blood at the rate your brain needs. This goes into effect immediately when the pressure drops.

As soon as the oxygen masks fall down, the flight crew will know, and they will make it an immediate priority to get the aircraft down to an altitude where the masks are not needed anymore because the outside pressure is high enough to breathe by. This takes less than the time the oxygen generators last for.

(Usually descending will take much less time than the masks are good for -- the critical design factor for the oxygen mask duration is that if you happen to be over tall mountains when the pressure drops you may need to fly for a bit of time to reach terrain low enough to dispense with the masks. There are a few places, such as over the Tibetan plateau, where airline flights simply don't go because it's too far from sufficiently low terrain for the masks to last).

There is no realistic chance of the oxygen generators running dry while you still need them. If you delay putting your mask on, all you buy is a danger of fainting or otherwise being unable to don it (from low pressure, not depletion of oxygen in the air) before you decide to put it on -- and then you may end up in a low-oxygen state for long enough to risk permanent harm.

There's a reason why the safety briefing always instructs you to put your own mask on before helping others -- because the time it takes to help someone else may well be enough to incapacitate yourself.

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The problem with high altitudes is not the lack of oxygen, but the lack of pressure. At lower pressures, your lungs are less effective at getting the oxygen into the blood. Oxygen masks on airplanes increase the amount of oxygen in the air you breathe, which helps the lungs to absorb more even in low pressure.

In a typical decompression, there will be a gradual loss of pressure. However, by the time the masks drop, the pressure has already dropped a fair amount. There is no set time that it takes for pressure to drop. It could be very gradual, or it could be explosive decompression that happens extremely quickly. As a passenger, aside from the obvious explosive decompression, you will not be able to tell how fast the pressure is dropping. And because of how hypoxia works, you many not even be able to tell that you are not getting enough oxygen. So you have no way to tell how long you can safely wait before putting on your mask. Without enough oxygen, you eventually die.

So by not putting on the mask, you are risking death. What is the benefit of waiting to put on your mask?

Airlines operating under FAA regulations are required to carry certain amounts of oxygen, depending on the flight. FAR 121.333 covers the requirements for oxygen supply during an emergency descent.

For airliners certified to FL250 and below, they must carry 30 minutes of oxygen for 10 percent of passengers, but only if they can safely descend to 14,000 feet in 4 minutes.

If they cannot safely descend, or if the airplane is certified above FL250 (as most airliners are), there must be at least 10 minutes of oxygen for all passengers, and enough for 10 percent of passengers for the duration of flight with cabin altitude between 10,000 and 14,000 feet.

So the airline is required to provide at least 10 minutes for all passengers, more than enough for an emergency descent to a lower altitude. If they can't descend within that time, they must carry enough for the 10 percent of passengers that need oxygen the most for the duration of flight at those high cabin altitudes. There is no reason to wait to put on your mask.

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  • $\begingroup$ I think the first paragraph is more confusing than anything—after all, it is breathing pure oxygen that you are trying to fix the problem with (the masts don't, can't, and mustn't increase total pressure). $\endgroup$
    – Jan Hudec
    Commented Dec 26, 2016 at 0:14
  • $\begingroup$ @JanHudec: I agree that's confusing, but it's important to understand that 100% oxygen at 11 km is possible because pressure is still 22 kPa, greater than the physiological minimum of 16 kPa. If it was below 16 kPa (e.g. at 13 km), even if this 100% O2 supply was in excess for our body needs, it wouldn't be transferred to our blood without increasing its partial pressure to 16 kPa, either by providing more oxygen, or adding another gas to the same quantity of oxygen. Hence at 13 km a pressurized mask would be necessary. $\endgroup$
    – mins
    Commented Dec 27, 2016 at 2:23
  • $\begingroup$ @mins, yes, above certain altitude even 100% O₂ won't help, but adding other gasses won't change anything, because what matter is that partial pressure of oxygen is at least ~14 kPa (You need 11.6 kPa more O₂ than CO₂ to displace the later from the hemoglobin, plus a bit more for the process to be sufficiently efficient. Exhaled air has around 14.5% of oxygen and that is still plenty for mouth-to-mouth resuscitation.). Total pressure only matters to prevent excessive drying above the Armstrong limit, that pressure is 6.8 kPa (18–19 km; then you need full pressure suit). $\endgroup$
    – Jan Hudec
    Commented Dec 27, 2016 at 11:10
  • $\begingroup$ @JanHudec: Ahhh, indeed, thanks. I have mixed, if I may say, with the (diving) high pressure limit where O2 partial pressure needs to be decreased. $\endgroup$
    – mins
    Commented Dec 27, 2016 at 11:59
  • $\begingroup$ @mins, the reason there is also partial pressure. The total pressure limit is only because the diving mixture must have enough oxygen for breathing near surface (so at least 20%) and the partial pressure increases proportionally with total pressure, reaching the dangerous 160 kPa at 60–70 m. Divers can work (much) deeper by staying in an underwater habitat with suitably reduced oxygen fraction. $\endgroup$
    – Jan Hudec
    Commented Dec 27, 2016 at 12:41
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At high altitude, the time for useful consciousness is measured in seconds. Pilots are trained to take on the mask immediately when the alarm goes off.

The problem of your approach lies in two uncertainties:

  1. Uncertainty of cabin pressure. Unless you happen to have carried a altimeter with you, of course.
  2. Uncertainty of the oxygen level in your blood. Prolonged hypoxia may cause permanent brain damage.

For your reference, the time of useful consciousness at FL350 is 30 seconds only. If it was a rapid decompression, then the fog which formed instantaneously may have obscured your vision for a good 10 seconds. Granted, at FL150 you'd have 30 minutes. But you wouldn't know. And most passengers simply wouldn't have known the aircraft's attitude the moment it happens. It is simply too risky (for both pilots and passengers) to wait while oxygen supply is available.

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    $\begingroup$ As you mentioned: time of useful consciousness is 30 seconds and fog may cost you about 10 of those. Now, the question becomes: how regularly do you practice putting on your oxygen mask? I'm guessing: never, not once. How attentive were you during the safety briefing? I'm guessing: not very. So, putting on the mask may take you as long as 15 seconds. 10 seconds until you are even able to see the mask, 15 seconds to put it on … that leaves a margin of about 5 seconds. Not 2-5 minutes. 5 seconds. After that, your mental capacity is impaired, but you won't even notice it. $\endgroup$
    – Jörg W Mittag
    Commented Dec 26, 2016 at 3:33
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Always put your mask on immediately. Cabin and flight crew do for a good reason! You may not be aware if it's a slow or rapid decompression. A rapid decompression will result in much less time available for you to remain conscious and save yourself. This is the same reason you are advised to don your own mask before helping others, so you don't put yourself at risk. When you are starved of oxygen, you firstly get Hypoxia, a low level of oxygen in the blood. This, if it worsens, can lead to the brain feeling that everything is ok, and it is not. Those who are elderly and with lung issues will be first to be affected. Another reason you shouldn't wait is because in some aircraft, such as the B767, the oxygen flows immediately when the masks drop - pulling on them does not necessarily open the supply. So it is possible if you wait, you will have less oxygen available for you.

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