Everyday each of us breathes, on average, more than 3,000 gallons of air. The air we breathe is an odorless, colorless mixture of natural gases. It is composed of nitrogen, oxygen, argon, carbon dioxide, varying amounts of water vapor and trace amounts of neon, helium, ozone, xenon, hydrogen, methane, and krypton. When anything else is there, or there is too much of one of these elements – the air is polluted. Some air pollutants are asbestos, carbon monoxide, chlorofluorocarbons, ground level ozone, lead, mercury, nitrogen oxides, particulate matter, propellants, radon, sulfur oxides, and volatile organic compounds to name a few. The Clean Air Act requires the EPA to set limits on six common pollutants, the criteria pollutants, which are: particle pollution, ground-level ozone, carbon monoxide, sulfur oxides, nitrogen oxides, and lead.