Airplane cabin air is not stored in the aircraft. It is outside air, compressed by the engines, cooled, filtered and mixed with recirculated cabin air, then released overboard through a valve at a controlled rate. That valve keeps the cabin at a comfortable pressure while the aircraft cruises in air far too thin to breathe.
Here is how the system works, from the engine to your seat.
Where cabin air comes from
On most jet aircraft the air starts as bleed air: hot, high-pressure air tapped from the compressor section of the engines. According to skybrary.aero, bleed air leaves the engine pylon at roughly 200 to 250 degrees Celsius and around 40 psi, far too hot to use directly.
It then passes through the air conditioning packs. Heat exchangers reject most of that heat to cold outside ram air, and an air cycle machine cools the air further by expanding it through a turbine, with no chemical refrigerant involved. A water separator removes the moisture that condenses out, and control valves blend hot and cold air so that each cabin zone can be held at its own setting.
Some aircraft, including the Boeing 787, use electrically driven compressors instead of engine bleed air, though the cooling and pressurising principles are the same.
Why the cabin has to be pressurised
At cruising altitude the outside air pressure is a small fraction of that at sea level, and the oxygen available in each breath falls with it. Cabin pressure is therefore described as a cabin altitude: the altitude at which the atmosphere would exert the same pressure as the air inside the cabin.
US certification rules require that cabin altitude to stay at or below 8,000 feet under normal operating conditions, as set out in section 25.841 at ecfr.gov. Modern aircraft usually hold something closer to 6,000 to 8,000 feet in cruise.
Pressure is not created by sealing the cabin tight. Conditioned air is pushed in continuously and an outflow valve near the rear of the fuselage lets it escape at a metered rate. A controller moves that valve to follow a planned cabin altitude profile through climb, cruise and descent, which is why the pressure change you feel is gradual instead of abrupt.
Recirculation and filtration
Not all of the air in the cabin is fresh. Part of it is filtered and recirculated, which reduces the bleed air the engines have to supply. Recirculated air passes through HEPA filters, which, according to the review of the airliner cabin environment at ncbi.nlm.nih.gov, remove essentially all airborne particles at 99.97 percent efficiency for particles of 0.3 micrometres, the size range that includes bacteria and viruses. Gaseous contaminants are not removed by those filters.
Section 25.831 of the same standards sets a floor for fresh air at 0.55 pounds per minute for each occupant, and caps carbon dioxide at 0.5 percent by volume in occupied compartments.
Why the air feels so dry
Air at cruise altitude holds almost no moisture, and cabin air is not normally humidified. The same National Academies review notes relative humidity at cruise of typically 10 to 20 percent, well below the range used for comfortable buildings. Humidification is avoided because of the weight of the water that would have to be carried, condensation and corrosion inside the structure, and the maintenance humidifiers need.
So you lose water through breathing and through the skin faster than in a room at the same temperature. Drinking water at intervals on a long flight is reasonable general advice; if you have a heart, lung or kidney condition, or you are pregnant, ask a doctor before flying.
What passengers notice
- Ears that pop in climb and descent, because the pressure on the outside of the eardrum changes. Swallowing, yawning or chewing helps to equalise it.
- Dry eyes, nose and throat from the low humidity.
- A slightly faster breathing rate, since a cabin altitude of 6,000 to 8,000 feet holds less oxygen per breath than sea level.
- Cool moving air at seat level, because conditioned air is supplied cold and cabin air is exchanged many times an hour.
All of this is general information about how aircraft systems work, not medical advice.
The short version: cabin air is outside air that has been compressed, cooled, cleaned and mixed with filtered recirculated air, and cabin pressure is the balance between how fast that air is pumped in and how fast it leaves through a single valve.