Definition
Underwater diving in which the diver breathes compressed air or another gas mixture from a tank carried on the body. Because the diver breathes gas at higher-than-surface pressure, nitrogen dissolves into the bloodstream and body tissues. If the diver then flies before that excess nitrogen has fully cleared, the reduced cabin or cockpit pressure at altitude can cause the nitrogen to come out of solution and form bubbles, producing decompression sickness ("the bends").
Plain English
Diving underwater while breathing compressed air from a tank. After diving, the body holds extra nitrogen for hours. Flying too soon afterwards can let that nitrogen form bubbles inside the body, which can make a pilot seriously ill.
Context Anchor
Seen in pilot health and physiological factors guidance, especially in recommendations about how long to wait before flying after diving.
Derivation
SCUBA is originally an acronym for Self-Contained Underwater Breathing Apparatus. It became so common it is now used as an ordinary word. Knowing this reminds you that the diver carries their own pressurised air supply, which is exactly what causes the nitrogen-loading problem relevant to flying afterwards.
Why Pilots Care
Flying after scuba diving risks decompression sickness as trapped nitrogen expands at altitude.
Grounding Statement
A scuba diver has been breathing under higher pressure underwater, so the body may still be carrying extra dissolved gas after the dive.
Intuition Check
Do not treat scuba diving as just another workout before a flight. The issue is pressure: underwater pressure affects gases in the body, and flight changes that pressure again.
Example Sentence 1
She finished her scuba diving trip on Sunday afternoon and waited a full 24 hours before flying home Monday evening.
Example Sentence 2
Scuba diving on a layover required checking the handbook's surface interval guidelines before the next flight.