Why should passengers not delay donning emergency oxygen masks after they deploy from the overhead compartments?

A

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. //

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. //

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:

Uncertainty of cabin pressure. Unless you happen to have carried a altimeter with you, of course.
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. //

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.
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mins Commented Dec 27, 2016 at 2:23

@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).
– Jan Hudec Commented Dec 27, 2016 at 11:10