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Where Does the Oxygen Go If the Bags on Airline Oxygen Masks Are Empty?C Stuart Hardwick:
Both humans and aircraft need oxygen, but jets are most fuel-efficient at high altitudes where the air density is lower. Although the lack of oxygen doesn't affect them because jet engines pressurize the intake air, passengers begin to feel lightheaded above approximately 8,000 feet, and at even higher elevations, the absence of oxygen can be fatal.
This is why airplanes carry emergency oxygen. The cabin is usually pressurized to an altitude equivalent to 8,000 feet, but if the pressurization fails at altitudes over 14,000 feet, some passengers won't be able to get enough oxygen from the thin air, leading to dizziness or illness—especially for the elderly, those in poor health, or people accustomed to sea-level pressures, which make up about a third of the global population.
We can handle much higher altitudes if we're breathing pure oxygen instead of the 21 percent oxygen found in regular air. With supplemental oxygen, we can reach up to 35,000 feet (40,000 feet with a full-face mask supplying 100 percent oxygen). However, beyond that, we risk losing consciousness or suffocating, even with pure oxygen. This is why most airliners don’t fly higher—passengers would require pressure suits or full-face masks as backup life support. Without these limitations, planes could comfortably 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 receive air when they take a breath—their equipment doesn’t continuously release air (unless the regulator is broken), because that would waste precious air.
That’s the purpose of the bag. The oxygen generator above your seat on an airliner continually emits oxygen. There’s no tank or regulator—just a canister with chemical reactants that, once activated, produce a steady stream of oxygen until they’re exhausted (typically lasting long enough for the plane to descend to a safer altitude with more breathable air).
The small bag is simply there to catch the oxygen between your breaths so it isn’t wasted. It inflates only when you exhale, as long as you're breathing slowly enough. That’s why it might not inflate if you’re panicking and breathing too quickly.
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