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 feet (47 mm pressure) it boils at only 36.7°C (98°F). This is the normal body temperature of humans, but in practice bodily fluids do not boil off at this altitude. This is because the skin and outer organs have enough strength to withstand this pressure, thus pressure inside the body would be significantly higher—however, bubbles starting to form is still an issue.

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 consume excessive amounts of salt to alter the boiling point of their blood, and increase their blood pressure. Some replace nitrogen in their atmosphere with other gasses such as argon or helium to prevent effects from the bends or decompression sickness. There has been some speculation as to the effectiveness of using techniques to lower body temperature however this is rarely used due to detrimental effects on the subject.