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The Clean Air Act requires the United States Environmental Protection Agency (U.S. EPA) to set National Ambient Air Quality Standards (NAAQS) for pollutants that cause or contribute to air pollution which may reasonably be anticipated to endanger public health and welfare. The U.S. EPA has set NAAQS for the six criteria pollutants (carbon monoxide, particulate matter, lead, nitrogen dioxide, ozone, and sulfur dioxide).
Nonattainment areas are regions within the country where measured concentrations of one or more criteria pollutants exceeds the NAAQS or contribute significant amounts of pollutants to an area that measures air quality that exceeds the standard. If a state has nonattainment areas within its borders, the state must submit a State Implementation Plan (SIP) to U.S. EPA detailing steps necessary to achieve the standard. Nonattainment areas which are later designated to attainment are considered maintenance areas. Specific nonattainment history for each of the criteria pollutants for the State of Indiana is provided below.