An asphyxiant is a gas that does not contain oxygen, or a chemical that prevents the body from using oxygen. Breathing an asphyxiant gas is toxic, because the body is starved of oxygen. Chemical asphyxiants, such as carbon monoxide or cyanide, prevent the body from using oxygen.

Asphyxiant inhalations can be life threatening, because they reduce the supply of oxygen to the heart and brain.