Definition
The practice of breathing 100% oxygen for a period of time before ascending to high altitude, in order to flush nitrogen out of the body's tissues and bloodstream and reduce the risk of decompression sickness during the flight.
Plain English
Breathing pure oxygen on the ground or at low altitude before climbing high, so the body has less nitrogen in it when cabin pressure drops.
Context Anchor
Seen in high-altitude flight, altitude chamber training, pressure-suit operations, and other situations where a pilot or crew member may be exposed to very low air pressure.
Derivation
From the prefix 'pre-' (Latin, meaning 'before') combined with 'oxygenation' (supplying oxygen). Literally 'oxygenating beforehand' — done in advance of the altitude exposure, not during it.
Why Pilots Care
It delays the onset of hypoxia and reduces the risk of decompression sickness if cabin pressure is lost.
Grounding Statement
The key idea is preparation: breathe oxygen before the pressure drops, not only after trouble starts.
Intuition Check
Preoxygenation is not the same as simply using oxygen during flight. It is oxygen use before the high-altitude exposure, to prepare the body in advance.
Example Sentence 1
The U-2 pilot completed an hour of preoxygenation in the ready room before walking out to the aircraft.
Example Sentence 2
Preoxygenation allowed the crew extra time to react after a rapid decompression at altitude.