# How long will one remain conscious in the event of a sudden cabin depressurization?

I know the usual scenario is where an airliner suffers from an explosive decompression and the oxygen masks drop and the plane descends to a safer altitude but.....

This is a purely hypothetical situation but if a passenger plane was cruising at 39000ft+ and it suffered a cabin leak that somehow went unnoticed by the crew and passengers and the oxygen masks didn't drop, how long roughly would it be before the passengers were incapacitated, and how long before the lack of oxygen became fatal.

• The depends entirely on how fast the cabin is leaking e.g. how big the hole is. Mar 26, 2014 at 16:12
• Good question, but can we assume it takes 30 minutes to clear the cabin of something mid size like a 737, I am guessing a slow drop in pressure over that sort of time wouldn't be noticed.
– Ian
Mar 26, 2014 at 16:20
• Or if the cabin didn't pressurise correctly and was at outside pressure as it climbed to cruising altitude.
– Ian
Mar 26, 2014 at 16:22
• More relevant answers in Would failure to put on an oxygen mask during loss of cabin pressure result in death? Mar 27, 2014 at 6:57
• Worth mentioning is that the masks don't care how fast the pressure falls: as soon as the pressure drops below a set pressure altitude (usually 12000 feet), the masks drop.
– Mark
Mar 28, 2014 at 4:09

The question you're asking roughly translates to What is the time of useful consciousness at 39,000 feet?. The answer is "About 15-20 seconds, once the pressure bleeds off." The FAA has a handy table for this:

From Chapter 16 of the Pilot Handbook of Aeronautical Knowledge.
You can get a PDF of Chapter 16 here, or you can grab the whole thing from the FAA

The Time of Useful Consciousness will vary depending on personal physiological factors (e.g. if you're a smoker your blood doesn't oxygenate as well - you will probably have less time. If you're a mountain climber in excellent shape and used to breathing rarified air on your climbs you'll probably have a little more time).

In the case of a non-explosive decompression (say a pressurization system failure) you'll probably have more time than this from the start of the event to the point where everyone is unconscious: the cabin pressure will take time to bleed off. Whether that time is seconds or minutes depends on the nature of the leak.

Presumably while the pressure is bleeding off the aircraft's systems will complain about the loss of cabin pressure and give the pilots a chance to address the problem (fix the pressurization system or descend to a safe altitude). Unfortunately the effects of hypoxia can start setting in before the crew realizes what's going on (which is one of the contributing factors in the loss of Helios Flight 522, which suffered just this kind of gradual depressurization).

• Thank you, that was exactly what I was looking for. Thanks.
– Ian
Mar 26, 2014 at 16:28
• @Ian You may also find this article from AvWeb interesting. It discusses a lot of the other factors that I alluded to above. Mar 26, 2014 at 18:02
• Is the pressure the main factor in how long, or the fact that there is less oxygen? I ask because I have many times emptied my lungs and sat on the bottom of a swimming pool longer than two minutes. (I realize it is impossible to completely empty them, but I do as much as I can—otherwise, I would not be able to stay on the bottom.) Oct 3, 2015 at 16:44
• @WGroleau That is an aeromedical/physiology question above my pay grade :) I know the "time of useful consciousness" is predicated on breathing "normally" (because the decompression would burst your lungs like an uncontrolled ascent from a dive if you tried to hold your breath), and the time you have has to do with the partial pressure of oxygen in the air you're breathing after the pressure drop, but I'm afraid I don't know much more than that... Oct 5, 2015 at 17:22
• Is the loss of consciousness in this table due on rapid change in pressure? Certainly many people can remain conscious indefinitely at 20000 feet; there are even permanent settlements as high as 17000. Or are the figures just aimed at average or worst-case health conditions? Mar 5, 2017 at 2:22