Ebullism is the formation of gas bubbles in bodily fluids due to reduced environmental pressure, for example at high altitude. It occurs because a system of liquid and gas at equilibrium will see a net conversion of liquid to gas as pressure lowers, for example, liquids reach their boiling point at lower temperatures when the pressure on them is lowered.
In the atmospheric pressure present at sea level, water boils at 100 °C (212 °F). At an altitude of 63,000 feet (18,900 m), it boils at only 37 °C (98.6 °F), the normal body temperature of humans. This altitude is known as Armstrong’s Line. 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, a pure oxygen (O2) atmosphere was used in early space flights to eliminate nitrogen in the blood. There are major fire hazards associated with using pure O2 as a breathing gas, which was central to the death of three astronauts in a fire during a ground test with Apollo 1. Nonetheless NASA continued to use a [nominally] pure oxygen atmosphere throughout the Apollo Program but switched to air for the follow-on Space Transport System "Space Shuttle". Russian cosmonauts used pure oxygen before changing to a higher-pressure nitrox mixture, leading to incompatibility problems in 1975 on the Apollo-Soyuz Test Project. Space suits are often pressurized to several psi lower than stations' capsules or shuttles and since they still use pure O2, an acclimation period is common in the airlock to remove nitrogen and other gases from the bloodstream.
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