What Is an Oxygen Mask?
QUICK ANSWER
An oxygen mask is the emergency mask that drops from the overhead panel if the cabin loses pressure at high altitude. It supplies oxygen so you can breathe during the emergency descent. You put your own on first because at cruising altitude you have only seconds of useful consciousness.
The oxygen mask demonstration is part of every safety briefing, yet most travelers never think about how the masks actually work or why the crew stresses putting yours on first. The reasons are rooted in simple physiology. Here is what an oxygen mask is, when it drops, why you help yourself first, and how long the oxygen lasts.
What is an oxygen mask?
An oxygen mask is the emergency breathing device stored in the panel above your seat, ready to drop down if the cabin loses air pressure. Airliners fly at altitudes where the outside air is far too thin to breathe, so the cabin is pressurized to keep the air comfortable. If that pressurization fails, the masks deploy automatically so passengers can keep breathing until the plane descends to a safe altitude. Each mask is a soft cup connected by a tube to an oxygen source, and pulling the mask toward you starts the flow. The masks are strictly for emergencies and are not the same as the medical oxygen some passengers arrange in advance. Their whole purpose is to buy you breathable air during a rare loss of cabin pressure.
When do oxygen masks drop?
The masks deploy automatically when the cabin loses pressure and the cabin altitude, the effective altitude of the air inside, rises above a set threshold, generally around 14,000 feet. This can happen if a door seal fails, a window or section of fuselage is damaged, or the pressurization system malfunctions. The moment the sensors detect the pressure drop, the overhead panels open and the masks fall so passengers can grab them without delay. Pilots also have their own masks and will begin an emergency descent to a lower, breathable altitude as quickly as it is safe to do so. Masks dropping is a dramatic but rare event, and the automatic system is designed so no one has to wait for a manual command in a moment when seconds count.
Why do you put your own mask on first?
Because at cruising altitude, you may have only seconds before a lack of oxygen clouds your judgment. When the cabin depressurizes at high altitude, the time of useful consciousness, the window in which you can still act effectively, can be as short as 15 to 30 seconds. Hypoxia, or oxygen starvation, sets in quickly and quietly: you may feel euphoric or confused rather than alarmed, and you can pass out before realizing anything is wrong. If you try to help a child or seatmate first and lose consciousness in the process, you cannot help anyone. By securing your own mask immediately, you stay alert and capable, and then you can assist children or others who need help. It is not selfish; it is the only way to be useful.
How long does the oxygen last?
Long enough to reach breathable air, which is the whole point. The passenger oxygen supply generally lasts around 12 to 15 minutes, depending on the aircraft and system. That may sound short, but it is designed to cover the time it takes the pilots to bring the plane down to about 10,000 feet, where the outside air is thick enough to breathe without a mask. Many systems generate the oxygen chemically at the moment of deployment rather than storing it in a tank, which is why the masks may feel warm and why they cannot be turned off once started. So while the supply is limited, it is matched to the emergency: keep your mask on, breathe normally, and it will last through the descent to safe altitude.
An oxygen mask drops from the overhead panel if the cabin loses pressure at high altitude, supplying air during the emergency descent. Put yours on first because you may have only seconds of useful consciousness before hypoxia sets in. The oxygen lasts around 12 to 15 minutes, enough to reach a breathable altitude.
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