Why the mask’s bag may not inflate
On many passenger jets, the oxygen bag is a small reservoir, not a balloon meant to fill before you use the mask. Oxygen flows into the mask system, and each breath draws some of it toward your nose and mouth. If you are breathing steadily, the incoming oxygen may be used about as quickly as it arrives, leaving the bag limp or only slightly expanded.
The bag’s appearance is therefore a poor test of whether oxygen is flowing. The mask can still be supplying oxygen even when the reservoir does not visibly puff up.
What happens after the masks drop
On aircraft that use chemical oxygen generators, pulling a mask down starts a reaction that produces oxygen for a limited time. The oxygen travels through tubing to the masks; the bag can help hold some between breaths. Other aircraft systems may supply oxygen differently, but the same basic point applies: the bag is not a tank that must inflate for the mask to work.
Passenger masks are intended to provide oxygen while the pilots bring the airplane to a lower altitude where the cabin can be safely pressurized again. That is why the practical response is to put the mask on promptly and breathe normally—not to wait for the bag to fill. A limp bag can look like a failure while doing nothing to show whether oxygen is reaching you.
