Cabin pressurization
Cabin pressurization is the process used in aircraft and spacecraft to regulate and maintain internal cabin air pressure at a level higher than the surrounding atmospheric pressure during high-altitude flights or space operations, thereby protecting occupants from the physiological effects of low oxygen levels, such as hypoxia.[1] This system seals the passenger cabin, cockpit, and certain cargo areas, simulating sea-level or low-altitude conditions—typically limiting the effective cabin altitude to about 8,000 feet—while the aircraft operates at cruising altitudes of 30,000 to 40,000 feet or more, where external air pressure drops to less than half of sea-level values.[1] By preventing rapid decompression and ensuring breathable air, cabin pressurization enables efficient, comfortable, and safe long-distance travel above adverse weather and turbulence.[2] The development of cabin pressurization addressed the limitations of early aviation, where high-altitude flights were restricted by the need for supplemental oxygen and the risks of thin air.[3] Pioneered in the 1930s, the technology first appeared in military aircraft; the Lockheed XC-35, a modified Electra, became the first U.S. airplane with a pressurized cabin when it flew in May 1937, allowing sustained operations above 30,000 feet without oxygen masks for the crew.[3] By the 1950s, pressurization became standard in commercial airliners as jet aircraft enabled routine high-altitude cruising, with systems evolving to include advanced materials like composites in modern designs, such as the Boeing 787, which achieves a lower cabin altitude of around 6,000 feet for enhanced passenger comfort.[4][5] In operation, the system draws compressed air—known as bleed air—from the aircraft's engines or auxiliary power unit, conditions it through air cycle machines or vapor compression units for temperature and humidity control, and introduces it into the cabin while outflow valves automatically regulate exhaust to sustain the pressure differential, typically 7 to 9 pounds per square inch (psi) depending on the aircraft's structural limits.[2][1] Critical components include the cabin pressure controller, which schedules pressure changes during ascent and descent to minimize ear discomfort; safety and relief valves to prevent over-pressurization; and indicators for monitoring differential pressure, cabin altitude, and climb rate.[1] In the event of failure, rapid decompression can occur—classified as explosive if instantaneous or insidious if gradual—triggering automatic oxygen deployment and emergency descent protocols to restore safe pressure levels.[2] Regulatory standards, primarily under Federal Aviation Regulations (FAR) Part 25, mandate that pressurization systems maintain cabin altitudes below 8,000 feet during normal operations and limit exposure to higher altitudes in failures, such as no more than 2 minutes above 25,000 feet and no exposure above 40,000 feet.[6] These requirements are verified through ground tests, flight demonstrations, and analysis of decompression scenarios, ensuring redundancy in components to handle single-point failures with high reliability.[2] Modern advancements continue to focus on efficiency, with some aircraft using electric compressors instead of bleed air to reduce fuel consumption and improve air quality.[5]Physiological Need
Human Response to Low Pressure
As atmospheric pressure decreases with increasing altitude, the partial pressure of oxygen (PO₂) in inspired air diminishes, impairing oxygen uptake in the lungs and subsequent delivery to tissues. At sea level, PO₂ is approximately 160 mmHg, but it halves to about 80 mmHg at 18,000 feet (5,500 m), effectively reducing the oxygen fraction to an equivalent of 10.5% at sea-level pressure despite the constant 21% oxygen composition in air.[7][8] This hypobaric hypoxia triggers physiological responses such as hyperventilation and increased heart rate, but above 10,000 feet (3,000 m), symptoms emerge including euphoria, impaired judgment, headache, cyanosis, visual impairment, and drowsiness, which can progress to unconsciousness without intervention.[8][9] The severity of hypoxia is quantified by the time of useful consciousness (TUC), defined as the maximum period after sudden oxygen deprivation during which an individual can perform rational, life-saving actions. TUC shortens nonlinearly with altitude due to rapid arterial oxygen desaturation; for instance, it ranges from 3–5 minutes at 25,000 feet to mere 9–15 seconds at 45,000 feet. Representative TUC durations, based on unacclimatized individuals at rest, are summarized below:| Altitude (feet MSL) | TUC Duration |
|---|---|
| 25,000 | 3–5 minutes |
| 30,000 | 1–2 minutes |
| 35,000 | 30–60 seconds |
| 40,000 | 15–20 seconds |
| 45,000 | 9–15 seconds |