Buzz
If you’ve watched the airplane safety demonstration enough times, you’re familiar with the routine for the oxygen masks. In case of a drop in cabin pressure, the masks will drop down. First, you place your mask over your face and secure the strap behind your head, then assist anyone else who needs help. It’s a sensible procedure because you can’t help others if you’re unconscious from lack of oxygen. But what about when they explain that 'while the bag on the oxygen mask might not inflate, oxygen is still flowing to the mask'? If the bag isn’t doing anything critical, why is it even there?
Passenger airplane masks are ‘continuous flow’ masks, which differ from the 'diluter demand' masks that the crew uses. A diluter demand mask only delivers oxygen when the user inhales. This helps avoid wasting oxygen and lets the mask’s instruments regulate the exact amount being provided. It ensures that the crew stays alert enough to navigate the aircraft into more breathable air.
A continuous flow mask constantly supplies oxygen, regardless of whether the user is inhaling. Passenger masks don’t need to be as efficient because their primary goal is simply to keep you alive. Even with some waste, there’s plenty of oxygen to sustain everyone until the airplane reaches air with higher oxygen levels.
The bag on the oxygen mask helps conserve some oxygen. When the mask is positioned properly on your face, the continuous flow of oxygen will fill the bag as you exhale, rather than leaking out the sides of the mask. (The airflow through any oxygen mask is one-way; exhaled air exits through valves on the mask and does not travel back into the tube.) However, if you’re breathing quickly—likely in a stressful situation—the bag may empty faster than it can fill. The safety briefing aims to reassure you that, even if the bag doesn’t inflate, the mask is still functioning properly. You don’t need to worry about the bag—your focus should be on getting oxygen, not on whether the bag inflates. There’s enough to deal with when oxygen masks deploy. The bag on your mask shouldn’t add to that stress.
