Module MOD-15 · 6 min · ACS PA.I.H

Pressure and Time: Scuba, Altitude and Decompression

← Aeromedical Factorsdraft — pending CFI review

Why this matters in flight: Altitude does not only thin the oxygen you breathe; it also lowers the pressure on gases already dissolved in your body. For a pilot who has recently gone scuba diving, that can turn a routine climb into a medical emergency, so the interaction is worth understanding.

Reduced pressure with altitude has two related effects. First, less oxygen is available to the lungs, which is the mechanism of hypoxic hypoxia and the reason symptoms of oxygen deficiency grow as you climb. Second, gases dissolved in the body come out of solution as pressure drops — the same physics that opens a shaken soda. This matters greatly after scuba diving, when the body has absorbed extra nitrogen. Climbing too soon lets that nitrogen form bubbles, causing decompression sickness ("the bends"). The recommended protection is time: wait at least 12 hours after a dive that did not require a controlled ascent, and at least 24 hours after a dive that did, before flying to cabin altitudes above 8,000 feet. Combined with recognizing the subtle early symptoms of hypoxia, respecting these waiting periods keeps altitude from turning against you.

Key terms

Decompression sickness
Nitrogen bubbles forming in the body ("the bends") when pressure drops too soon after diving.
12/24-hour rule
Recommended wait after diving before flying above 8,000 ft cabin altitude, depending on the dive.
Cabin altitude
The effective pressure altitude experienced inside the aircraft.

Summary

Falling pressure with altitude both starves you of oxygen and lets dissolved nitrogen bubble out. Wait at least 12 or 24 hours after diving before high-altitude flight, and stay alert to hypoxia.

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Which form of hypoxia is most common during unpressurized flight at altitude?

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Sources

Every claim traces to a source — paraphrased knowledge elements pointing at the governing FAA publication; not yet verified against a retrieved source.

  • AIM 8-1 / scuba diving and flight — Aeronautical Information Manual unverified
  • PHAK Ch. 17 / hypoxia — Pilot’s Handbook of Aeronautical Knowledge unverified

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