October 25, 1999
Web posted at: 8:10 PM EDT (0010 GMT)

ATLANTA (CNN) -- Hypoxic hypoxia is the scientific term for a condition that occurs when there are not enough oxygen molecules available at sufficient pressure to supply one's respiratory system. It can happen suddenly, as occurs at high altitudes during a rapid decompression, or gradually, as occurs at lower altitudes when there is insufficient oxygen available.

The higher the altitude, the less oxygen is available - and the faster one can lose "useful consciousness." Without sufficient oxygen, even the simoplest takes -- such as talking on the radio -- may not be possible to complete.

The Lear Jet carrying Payne Stewart was reportedly at 39,000 feet - an appropriate altitude for its intended route of flight. At that altitude, the time of useful consciousness is between 15 and 20 seconds.

FAA rules require pilots flying above 35,000 feet in a pressurized craft to have oxygen masks that can be placed on the face with one hand in 5 seconds. The symptoms of hypoxia include:

• increased breathing rate

• lightheadedness, dizziness

• tingling or warm sensation

• sweating

• reduced field of vision

• sleepiness

• bluish tint to skin - fingertips and lips

• behavior changes - including a sense of euphoria