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Normal commercial jets drop gas masks when cabin pressure drops, and supply oxygen, but, where does that oxygen come from? Do commercial jets store oxygen? If they do, isn't pressurised oxygen very flammable and too hazardous to carry?

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Perhaps I'm just splitting hairs, but a "gas mask" does not drop. A gas mask is essentially nothing more than an air filter (whose fit to the face forms a seal intending to prevent air from coming in from the sides). The mask that drops on aircraft is not a filter, it's a source of oxygen giving you something to breath. –  mah Jan 9 at 11:56
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On commercial aircraft the oxygen supply of the masks in the cabin comes either from a chemical oxygen generator or a centralized oxygen bottle system. The flight deck is always equipped with pressurized oxygen masks.

The chemical oxygen generators generate lots of heat when they are activated. When normally placed in their overhead compartments the heat is slowly dissipated. However, oxygens generators are hazardous when canisters are not in their appropriate place. The ValuJet Flight 592 crash into the Everglades on 11 May 1996 was caused by a fire after improperly packed oxygen generators in the cargo hold were accidentally activated.

Pressurized oxygen bottles also are hazardous. Not only because the oxygen can turn a small fire into a blaze but if the high pressure bottles explode they cause substantial damage. On 25 July 2008 an oxygen bottle installed in a Qantas Boeing 747 ruptured and propelled itself upward through the floor into the cabin.

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It's maybe useful to note that they're actually called "oxygen candles" in the industry... they are essentially burning chemicals which produce oxygen as the waste gas. –  egid Jan 8 at 15:52
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Just to add to your answer, on my commercial aircraft we do have a central oxygen bottle, which is very common with smaller jets. –  Lnafziger Jan 8 at 16:45
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@LNafziger You're right, and after some thinking, I realized that it is not only the case on smaller jets. The B747-400 has also a bottled oxygen system, and I remember an accident related to that as well :-) –  DeltaLima Jan 8 at 16:54
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Airliners do not carry tanks of oxygen for the "rubber jungle" when the ox masks are deployed. The oxygen is created by chemical reaction within canisters stored in the overhead that are not activated until the rubber jungle drops down.

This chemical reaction does create a lot of heat. That's what caused the Valuejet fire and crash in the Florida everglades a few years ago. The canisters were being shipped and were not clearly marked as hazardous cargo, and other safety steps were missed that may have inerted the canisters.

Given the many millions probably billions of canister-hours that these devices have been used over the years, I would conclude they are safe. Given the disaster that happened when they were handled improperly for shipping, I would also say the airlines cannot lower their guard on shipping these apparently harmless canisters at any time, ever.

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+1 for the rubber jungle (and the rest as well) –  yankeekilo Jan 8 at 16:08
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Wikipedia has an excellent article on the subject:

http://en.wikipedia.org/wiki/Emergency_oxygen_system

In some cases the oxygen masks are connected to a central oxygen supply (or more accurately, a supply of breathing gas, containing a mix of oxygen and other gas). The oxygen can also be generated using a chemical device, one for each oxygen mask or seat row.

Side note: contrary to what many people believe, oxygen is not required during normal flight. The outside air at high altitude still contains the same percentage of oxygen (21%) as on the ground. The outside air is pressurized and fed into the fuselage, allowing normal breathing.

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