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Straight Dope Classic from Cecil's storehouse of human knowledge19-Apr-1991
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. --Thomas P., director, Program in Aegean Scripts and Prehistory, University of
Texas at Austin
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?
--CECIL ADAMS
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