At cruising altitude, the air outside an airliner is too thin to breathe and far too cold to survive in unprotected. Cabins solve this by creating an artificial, lower "cabin altitude" inside a sealed fuselage, using air drawn from the engines and controlled by a system of valves, with oxygen masks and strict regulations as backup if something goes wrong.
Where cabin air comes from
On most jets, pressurized air is "bled" from the compressor stage of the engines, according to flight-training resource Boldmethod. This bleed air leaves the engine hot — around 200°C, according to Wikipedia's entry on cabin pressurization — and is then cooled through heat exchangers and mixed with recirculated cabin air before being pushed into the cabin through ducts.
The incoming air is deliberately supplied faster than it is allowed to leave. The cabin empties through one or more outflow valves near the rear of the fuselage, which open and close to fine-tune pressure: they close slightly as the aircraft climbs to hold more pressure in, and open more as it descends, per Boldmethod.
Why cabins aren't pressurized to sea level
US Federal Aviation Administration (FAA) rules require that cabin pressure on transport-category airplanes stay at or below the equivalent of 8,000 feet of altitude, according to FAA regulatory text summarized via the eCFR database. Pressurizing all the way to sea level would demand a heavier fuselage able to withstand a far larger pressure difference between inside and outside — a tradeoff aircraft makers have generally avoided.
Most older widebodies cruise with a cabin altitude of around 7,000–8,000 feet, Wikipedia's cabin pressurization article notes, citing the Boeing 767 as an example near 7,000 feet. Newer composite aircraft go lower: Boeing states on its own website that the 787 Dreamliner holds cabin altitude at 6,000 feet — about 2,000 feet lower than conventional aluminum jets — because its composite airframe resists the fatigue and corrosion risk that higher pressure differentials pose to metal structures. Boeing also says the 787 raises cabin humidity, aiming to help passengers feel "more refreshed" after long flights.
What happens if pressure is lost
FAA rules under 14 CFR 121.333 require oxygen to be available to at least 10 percent of passenger cabin occupants at cabin pressure altitudes between 10,000 and 14,000 feet following a depressurization event, with additional first-aid oxygen required for a smaller share of occupants above 8,000 feet, according to the regulation's text via Cornell Law School's Legal Information Institute. In practice, airline training materials cited by AOPA describe automatic mask deployment once cabin altitude reaches roughly 13,000–14,000 feet.
For the flight crew, FAA rule 14 CFR 121.333 requires a two-hour oxygen supply per crewmember above 10,000 feet, covering a 10-minute descent from maximum operating altitude to 10,000 feet followed by 110 minutes at that altitude, according to a summary of the regulation via Cornell Law School's Legal Information Institute. Above flight level 250, at least one pilot must wear and use an oxygen mask at the controls unless quick-donning masks — required to be secured and delivering oxygen within five seconds — are available.
Why speed matters: time of useful consciousness
The urgency behind these rules comes down to how little time crews have to react. Aviation safety outlet AVweb publishes a commonly cited table of "time of useful consciousness" — the window in which a person can still act effectively before hypoxia impairs judgment. At 18,000 feet that window is roughly 20–30 minutes; at 25,000 feet it falls to about 3–5 minutes; at 30,000 feet it is roughly 1–2 minutes; and at 40,000 feet it shrinks to about 15–20 seconds, per AVweb's figures. A sudden, rapid decompression — rather than a slow leak — cuts these times further, AVweb notes, because the body loses oxygen outward through the lungs at the same time inhaled oxygen drops.
That is also why pilots are trained to don oxygen and begin an emergency descent immediately after any depressurization event rather than diagnosing the cause first.
Built-in redundancy
Pressurization systems are designed with backups. Wikipedia's cabin pressurization entry notes that at least two engines typically supply bleed air so the system tolerates an engine failure, and that most modern jets run duplicated electronic pressure controllers with a manual backup mode. The outflow valve itself also serves as a safety relief valve, preventing the pressure difference between cabin and outside air from exceeding structural limits — generally cited in the 7.8–9.4 psi range depending on aircraft type.
Historical lessons shaped these safeguards. The same Wikipedia entry points to the de Havilland Comet accidents of 1954, in which repeated pressurization cycles caused metal fatigue and fuselage failure, as a turning point that led to improved inspection protocols, radiographic testing methods, and design changes — including the oval cabin windows now standard on pressurized jets.
Key numbers at a glance
Pressurization and oxygen thresholds| Figure | Value | Source |
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| Max cabin altitude allowed (transport aircraft) | 8,000 ft | FAA rule via eCFR |
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| 787 Dreamliner cabin altitude | 6,000 ft | Boeing.com |
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| Passenger oxygen required between | 10,000–14,000 ft cabin altitude | 14 CFR 121.333 via Cornell LII |
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| Masks drop in practice around | 13,000–14,000 ft | AOPA |
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| Crew oxygen supply required above 10,000 ft | 2-hour supply per crewmember | 14 CFR 121.333 via Cornell LII |
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| TUC at 25,000 ft | ~3–5 minutes | AVweb |
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| TUC at 40,000 ft | ~15–20 seconds | AVweb |
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