What starts the oxygen supply
When cabin pressure drops, masks fall from the overhead panels. Pulling a mask toward you starts a chemical oxygen generator above the seats: a reaction inside the unit releases oxygen and can make it hot. The mask is not connected to a large tank of compressed air, and its small bag may not visibly inflate. Oxygen can still be flowing.
The supply is deliberately limited. It is meant to bridge the period when the cabin is too thinly pressurized to breathe comfortably—not to support passengers through an entire flight.
Why the airplane descends
At cruising height, the outside air is too thin for people to breathe safely without pressurization. The crew responds to a pressure loss by descending to an altitude where the air is breathable, while managing the airplane and following the conditions around it. How quickly that can happen depends on the aircraft, the situation, and the terrain below; the masks’ supply is a bridge to that safer air, not a guarantee that every emergency follows the same timetable.
Why adults are told to mask themselves first
A child who is frightened may resist the mask, but an adult who becomes oxygen-deprived can quickly lose the ability to help. That is why safety instructions tell adults to secure their own masks before assisting others. Once breathing, they can hold a mask to a child’s face and keep it in place during the descent. The practical point is simple: the first job is to stay capable of helping.
