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Anyone who flies knows by heart the fact that, although oxygen is flowing, the bag may not inflate.

So what purpose does the "bag" serve?

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@SantaC. The fact that a question is on-topic elsewhere doesn't mean it should be migrated there, as long as it's not off-topic here. –  tohecz Feb 24 at 18:51
    
I note that remark for future reference –  PERSONA NON GRATA Feb 24 at 18:55
    
It's come up before on meta –  Mark Mayo Feb 24 at 21:33
    
The wording I've usually heard is that the bag may not inflate. As Affable Geek's answer points out, in some cases it will. –  Nate Eldredge Feb 25 at 15:53

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up vote 10 down vote accepted

The bag is there to contain the oxygen that is flowing from a central source. The flow rate, however, is slower than the capacity of the bag. It's much like the warning, "package contents may settle during shipping" - the warning is more for people's fears than any practical purpose.

Furthermore, as this link shows, the bag is also there to catch exhaled (aka CO2 rich air) as much as it is to hold new stuff.

Cecil Adams (The Straight Dope) reports similar findings. He states:

Here's the deal. Passenger oxygen masks give you a continuous flow of oxygen (as opposed to oxygen on demand, which only flows when you inhale). The oxygen obviously can't flow into your lungs while you're exhaling, so if there weren't some way to store it temporarily it would have to be vented wastefully. The bag makes this unnecessary. When you start exhaling, your breath plus the incoming O2 flow into the bag. When a certain pressure is reached the bag stops filling and the rest of your exhaled breath, which contains more carbon dioxide, is vented through a port in the mask.

The flight attendants make a point of telling you the bag won't inflate (right away, that is) because of an incident years ago. An airplane lost cabin pressure, the oxygen masks dropped down, and the passengers put them on--but when they noticed the bags didn't inflate, they figured the masks weren't working and took them off. Bad idea.

So, is it working? Yes. Is it unsettling? To some people. Regardless, it works, even if not in the way some people might expect.

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So is it there to contain oxygen that is flowing from a central source, or to catch exhaled air? –  Flimzy Feb 24 at 16:34
    
Both. The CO2 that is captured helps to equalize the air pressure to keep the oxygen flowing, but any excess is actually released through a valve. –  Affable Geek Feb 24 at 16:47
    
I am trying to make sense of all this, would that mean that the bag inflates at least a little? –  Relaxed Feb 25 at 14:41
    
A little - but the inflation is due more to the exhalation than to the inflow of the oxygen trickle –  Affable Geek Feb 25 at 15:17
    
The quote makes all the difference.. thanks! :) –  Flimzy Feb 25 at 16:23

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