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Ebullism is the formation of gas bubbles in bodily fluids due to reduced environmental pressure, for example at high altitude. It occurs because liquids boil at a lower temperature when the pressure on them is reduced.

In the atmospheric pressure present at sea level (760 mm of mercury), water boils at 100°C (212°F). At an altitude of 61,000 feet (47 mm pressure) it boils at only 36.7°C (98°F). This is the normal body temperature of humans, so liquids and tissues containing water in the human body begin to vaporize at this pressure.

Symptoms of ebullism include bubbles in the membranes of the mouth and eyes, swelling of the skin, and bubbles in the blood. Blood circulation and breathing may be impaired or stopped. The brain tissue may be starved of oxygen because of blockage of arteries, and the lungs may swell and hemorrhage. Death results unless recompression is rapid enough to reduce the bubbles before tissue damage occurs.

To prevent ebullism, pilots and astronauts live in pressurized cabins or wear pressurized suits (such as space suits).

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