The Environmental Protection Agency (EPA) is required to set national ambient air quality standards for the major pollutants that have an adverse impact on human health. The ambient air quality standards are to be set at two levels: 1) primary standards, which are designed to protect the public's health from harm, and 2) secondary standards, which are designed to protect vegetation, materials, climate, visibility, and economic values. Each state is required to develop-and obtain EPA approval for-a state implementation plan for meeting the national ambient air quality standards. This necessitates an inventory of the various sources of air pollution and their contribution to the total air pollution in the air quality region. The major emitters of pollutants are then required to reduce their emissions to a level that ensures that overall air quality will meet the national standards.