Asphyxiation

Except for oxygen, all gases are asphyxiants. Even though a gas is completely nontoxic, it can easily cause suffocation unless it is mixed with sufficient oxygen to support life. Nitrogen, for example, is a harmless, inert gas which makes up about 78% of the air we normally breath. However, only a few breaths of pure nitrogen could render a person unconscious because nitrogen alone will not support life.