Most national park units in the American Southwest are designated Class II air quality areas under the Clean Air Act Amendments of 1977. Nine parks (four in the Southern Colorado Plateau Network, three in the Chihuahuan Desert Network, and two in the Sonoran Desert Network) are designated Class I airsheds, requiring the most stringent air quality protection within and around their boundaries. Under the Clean Air Act, park managers have a responsibility to protect air quality and related values from the adverse effects of air pollution. Although most park units are some distance from cities and pollution sources, many experience occasional poor air quality from pollutants such as ozone, nitrogen oxides, sulfur dioxide, particulate matter, and volatile organic compounds. These air pollutants may adversely affect air quality and natural resources, including vegetation, wildlife, soils, water quality, and visibility. Air quality monitoring takes place in some American Southwest parks, but only nearby others.