When you wear an airline oxygen mask, why doesn’t the plastic bag inflate?

Dear Cecil:

Here is one I have been trying to find the answer to for years. I have asked flight attendants on airplanes all over the world. No one knows. No one even hazards a wild guess.

Why doesn't the plastic bag inflate? Since it doesn't, what is it for? I am speaking, of course, of the oxygen mask that will drop in case of emergency and that you are supposed to tie securely around your face before attending to infants or children. The plastic bag attached to the mask never inflates, and what's more, they make a point of telling you it won't inflate. This to me is more perplexing than some of the early undeciphered scripts I study.

Cecil replies:

Doc, your prayers have been answered. First an inside secret: the bag does inflate, but only when you exhale.

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. Thus the warning. Simple, eh? Now, you want a hand with those Aegean scripts?

Send questions to Cecil via cecil@straightdope.com.

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