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Hypobaric decompression and altitude exposure

Aircrew, chamber subjects and suited crew members work below sea-level pressure. OpenDeco gathers what the published record says about altitude decompression sickness, prebreathe and the transitions a cabin, a chamber or a suit imposes.

Diving decompression ends at sea-level pressure. Altitude decompression begins there and continues to lower pressures, and the exposure lasts as long as the cabin, chamber or suit holds its working pressure.

Test volunteer and technician seen through the window of a hypobaric chamber
Hypobaric chamber preparation for an altitude exposure.NASA / Bill Stafford

Pressure environments

020406080100Ambient pressure, kPaExtravehicular suit29.6 kPa, 4.3 psiCabin altitude limit75.3 kPa, 8,000 ftSea level101.3 kPa, standard atmosphere
Ambient pressure, kPa020406080100Extravehicular suit29.6 kPa, 4.3 psiCabin altitude limit75.3 kPa, 8,000 ftSea level101.3 kPa, standard atmosphere
Three reference pressures for an aerospace exposure, shown on one scale. Sea level follows the standard atmosphere. The cabin figure is the 8,000 foot limit, and the suit figure is the United States extravehicular working pressure.

  • Flight and cabin exposure

    Transport aircraft hold cabin altitude at or below 8,000 feet, and most flights stay far below the altitudes where decompression symptoms are reported. Unpressurised flight, high-altitude operations and loss of cabin pressure move the exposure into the range where altitude decompression sickness is reported.

  • Hypobaric chambers

    Altitude, hold time, breathing gas and workload are fixed by protocol, and subjects are observed throughout. Chamber work is the one aerospace setting that yields a controlled profile.

  • Suited operations and extravehicular activity

    United States extravehicular suits run near 4.3 psi, close to 30 kPa, or under a third of sea-level pressure. Crew members breathe oxygen for a set period before the suit closes, then work at that pressure for hours.

Altitude and diving evidence

Altitude and diving decompression follow the same physics. Dissolved gas comes out of solution as ambient pressure falls. Before a model fitted to one can be tested against the other, both records have to state the same terms.

Terms both records have to state

  • Reference pressure, stated explicitly: sea level, cabin, chamber or suit.
  • Direction of the transition, its rate, and the breathing gas used during it.
  • Prebreathe, where used: oxygen fraction, duration and workload during the period.
  • Endpoint definition, including who observed the symptom and how long after ascent.

Where to look next

  • ResearchRead aerospace and hypobaric decompression work.
  • DataBrowse altitude and hypobaric exposure datasets.
  • ModelsInspect hypobaric models and procedures.
  • MethodsRead how cross-domain validation and pressure-unit normalisation are handled.