At ground level, the body tissues are saturated with nitrogen, the inert gas that makes up 80 percent of our atmosphere. During a rapid ascent, the rapid lowering of the external barometric pressure allows the nitrogen gas to form small bubbles (an example of this phenomenon is the bubbles formed when a bottle of pop is opened). The nitrogen bubbles form in and around blood vessels, joints and muscles, causing pain and cramps (known as the bends). They can also form under the skin, causing itching and tingling, or in the lung, causing chest pain and shortness of breath. Severe cases may result in a loss of consciousness. The risks associated with decompression sickness increase with high rates of climb, age, obesity, physical activity and low temperatures. Flight operations above a cabin altitude of 20,000 feet ASL should not be attempted unless crew members and passengers have completed specialized high-altitude indoctrination training. When decompression sickness is encountered, an immediate descent to a lower altitude is required.
Although normally decompression sickness does not occur below 20,000 feet ASL, people who fly after scuba diving may develop the symptoms at much lower altitudes. Atmospheric pressure beneath the water increases by one atmosphere for every 33 feet of descent. Divers who breathe pressurized air for more than a few minutes supersaturate their tissues with nitrogen. For this reason, as the aircraft ascends, nitrogen bubble formation may take place, causing the bends.