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Home > Air Travel > Where does the oxygenated cabin air of commercial airliner come from?
Where does the oxygenated cabin air of commercial airliner come from?
Flying - Air Travel
Monday, 14 January 2013 16:11

Jumbo Jet Lands With Gaping Hole in Side
Dear Capt. Lim,

Can you explain where the breathable cabin air of a commercial airliner comes from?

I know that for passenger comfort, the cabin is pressurized to 5000-7000 feet atmosphere but cruising at high altitudes (FL180 to FL400) where oxygen is very thin, where is the adequate oxygen needed for breathing comes from?

Is it coming from oxygen tanks tucked away somewhere below deck?

Are any stale air jettisoned or do they just keep recycling?

Does cabin air come in from engine intake or through separate air intake?

Thanks for your answers.



Hi James

The breathable cabin air in all commercial airliners comes from air drawn in from the engine compressors. This compressed air is then regulated by valves in the plane and they are also responsible for the pressurization in the cabin.

Many believe that the oxygen available reduces as a plane climbs. That is only partly true. The percentage of oxygen in the air hardly changes. For instance, at the ground level, oxygen comprises 23 % of the air by weight and at 40,000 feet; it is still 23 % of the air.

What changes is that the higher you go the pressure of the atmosphere decreases and so in effect there is less air for the passengers to breathe. Less air means less oxygen. Don’t forget, the proportion of oxygen remains the same but for every breathe that you take you get a lesser proportion of oxygen delivered to your blood stream.

What pressurization does is to squeeze the rarefied high altitude air back together recreating the dense oxygen-rich condition at sea level so as to enable you to breathe normally again.

When a plane is cruising at high altitude, there is a differential pressure of between 5 to 8 pounds per square inch between the air outside and inside the plane – think of it as though the cabin is like the inside of a balloon.

Obviously if there is a leak in the cabin (”balloon”) for whatever reasons, the air will depressurise and the oxygen level will also drop. Hence, in such an emergency, the oxygen masks would drop at certain differential pressure. Pilots have been trained to handle such a situation and descend immediately to 10,000 feet, a safe level, in about 5 minutes (the emergency oxygen will last for about 20 minutes!)

As regards to the question as to where the oxygen comes from at high altitude, well they come naturally from the compressed air that you breathe from, not from any storage tanks! You only need oxygen when the plane depressurise during an emergency. They come from the emergency oxygen generators located at the ceiling, and as described above – they last for only about 20 minutes.

Any stale air in the cabin are filtered and recirculated.

To summarise, cabin air are compressed air from the engine air intake and not from any separate intake unless the plane is fully depressurised, then the pilot may open a ram air valve (normally during an emergency) below 10000 feet to allow natural ambient air from the outside.

As such, you are now unpressurised, uncomfortable and either very cold or very hot – depending on the outside conditions!

PS. To check for any latest updates or postings, you can follow my new Twitter at @CaptKHLim

Cabin Depressurization Causes Release of Oxygen Masks

Qantas Airliner Plunges Thousands of Feet

Flying and Hypoxia (Oxygen Starvation)


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There is something wrong in the equation: High Altitude-Normal Partial Pressure of Oxygen in the cabin, of the airliner=23,8%.This is the dry air plus pollutants as smoke leaked into the cabin, from the engine. This brings about: Surfactant dysfunction:i.e.Alveoli do not open properly. That results in incomplete gas exchange. This lead to hypoxemia, mixed with pollutants. How do you
compensate for that?
Chagai Dubrawsky , 25 Jun, 2015

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