If oxygen masks on airlines have empty bags, where is the oxygen stored?

C Stuart Hardwick:

Humans and airplanes both need oxygen, but high speed airliners operate much more efficiently at high altitudes where the air is thinner. The lack of oxygen doesn’t bother them because jet engines compress their intake air anyway, but above about 8000 feet, the lack of oxygen would start making passengers woozy, and at higher altitudes still, would be fatal.

That’s why airliners carry emergency oxygen. The cabin is normally pressurized to about 8000 feet, but if pressurization ever fails above about 14,000 feet, some passengers would be unable to absorb sufficient oxygen from the rarefied air and would grow faint, even ill—especially those who are old or infirm, and those accustomed to near-sea level pressure, which is a third of the global population.

We can tolerate much higher altitudes, though, if we have pure oxygen to breath instead of the 21 percent oxygen in ordinary air. Breathing supplemental oxygen, we can go up to about 35,000 feet (40,000 with a full face mask ensuring 100 percent undiluted oxygen). Much higher than that and we risk passing out or asphyxiation even with 100 percent oxygen. That’s why few airliners fly higher—passengers would need pressure suits or at least full face masks as backup life support. Were it not for that, planes would happily fly at 50,000, 60,000, or even 70,000 feet.

So ... to answer the question.

If you’ve ever gone scuba diving (or seen it on TV) you know that divers only get air when they draw a breath—their rig doesn’t just continually belch out air (unless the regulator is busted) because that would waste air.

That’s what the bag is for. The oxygen generator over your seat in an airliner does just continually belch out oxygen. There is no tank or regulator—just a canister containing chemical reactants which, once started, produce a continuous stream of oxygen until the reactants are used up (a few minutes, long enough to descend to thicker air).

The flimsy little bag is there to catch the stream of oxygen in between your breaths so it isn’t wasted. That’s it. It only inflates while you are exhaling, provided you are breathing slowly enough. That’s why it might not inflate (you might not give it a chance you panicky rascal).

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