Astronauts sealed in a spacecraft or space station need a continuous supply of oxygen. When they inhale, the oxygen in the air is absorbed by the blood and used by the body. When they exhale, nitrogen, water vapor, and carbon dioxide (CO₂) are expelled. During a flight, oxygen must be added to the air, while water vapor, CO₂, and other impurities must be removed.

Earth's atmosphere at sea level consists of 21 percent oxygen, 78 percent nitrogen, and 1 percent CO₂, water vapor, argon, methane, and traces of other gases, at a pressure of 14.7 pounds per square inch (psi). Pure oxygen is highly corrosive and reacts with most substances, sometimes violently, as in a fire or an explosion. Nitrogen in Earth's atmosphere dilutes the oxygen so that such violent reactions do not usually occur spontaneously.

In January 1967 three astronauts died while testing and practicing procedures...