The air humans breathe is made up of about 78 percent nitrogen, 21 percent oxygen, and 1 percent carbon dioxide and argon. Trace gases include krypton, methane, neon, helium and hydrogen. Air generally contains trace amounts of water vapor, as well, with greater concentrations occurring near sea level.
While the large amount of nitrogen in the air usually has no effect on humans, in some cases it can become extremely dangerous. When a diver breathes air deep underwater where pressures are high, the excess nitrogen forces its way into the bloodstream. When the diver ascends, the nitrogen can work its way back out slowly and be exhaled. If he ascends too rapidly, however, the nitrogen comes out of solution in his blood, causing intense pain and muscle spasms. Untreated, decompression sickness can be fatal.
The nitrogen in normal air can also act as a narcotic under pressure. For this reason, divers who descend into deep water typically use a carefully regulated mix of oxygen and helium to prevent dizziness, drowsiness and a loss of coordination.
During the early days of the Apollo program, NASA experimented with a pure oxygen atmosphere for its astronauts. Unfortunately, pure oxygen creates a highly flammable environment, and an accident during testing led to the deaths of the Apollo 1 crew. Since then, space agencies have used a mix of gases similar to the air found on Earth to prevent accidents.