The most common gas people breathe from clean air is nitrogen. Air is a mixture of gases, and the air on Earth contains about 78 percent nitrogen and about 21 percent oxygen. Argon, carbon dioxide neon, helium, methane, krypton, hydrogen and xenon are in much smaller amounts.
If the air that is inhaled is impure, then it would have higher concentrations of carbon dioxide, carbon monoxide and sulphur dioxide. Some amount of water vapour and dust particles are also present in the air that is inhaled. The air in industrialized and urban areas could have higher amounts of pollutants due to industrial and vehicular pollution.