EPA's mission is to protect human health and the environment. To achieve this mission, EPA implements a variety of programs under the Clean Air Act that focus on reducing outdoor, or ambient, concentrations of air pollutants that cause smog, haze, acid rain, and other problems; reducing emissions of toxic air pollutants that are known to, or are suspected of, causing cancer or other serious health effects; and phasing out production and use of chemicals that destroy stratospheric ozone.