June 21

We’ve all heard the pre-flight airplane safety briefing that says, “In case of cabin decompression, oxygen masks will drop from the ceiling. Pull a mask towards you and place it firmly over your mouth and nose, and breathe normally.  Please note that the bag may not inflate.”

I was flying to Chicago the other on business.  Curiosity finally got the best of me and I asked a flight attendant “Why don’t those little bags inflate?”  He didn’t know.  He asked the other flight attendants.  They didn’t know either.

Since my last flight, I did a bit of searching online.  It turns out that even some pilots in discussion forums were unsure.  But I finally pieced together the answer:  With every inhale, we breathe in quite a volume of air — about half a liter per inhale.   However, the oxygen-delivery system in an airplane can’t deliver that large volume as quickly as we consume it.  So the little bags mix our exhaled air (which still contains a substantial amount of oxygen) with the relative trickle of pure oxygen that’s coming into the bag from the airplane’s supply tube.  When we breathe in, the bag quickly compresses as we inhale all of the air in the little bag plus whatever oxygen has trickled in from the oxygen tube.

So the bag is really just a mixing device, combining the exhaled air from our lungs with that of the supplemental oxygen.  But because our inhales are so large (compared to the size of the bag), the bag will not inflate.

Mystery solved.

And regarding the “breathe normally” part of the safety briefing?  Yeah, right.  Under those circumstances, I think most of us would be sucking in air as fast as our lungs would work.

Image courtesy of London Heathrow Airport.

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