Regional haze is the result of a number of sources and activities located across a broad geographic area emitting fine particle pollution, known as PM2.5, and its precursors, including sulfur dioxide (SO2) and oxides of nitrogen (NOx). Sources of fine particles and their precursors can be natural, such as windblown dust and wildfires, as well as manmade, such as power plants, other industrial operations, motor vehicles and forest and agricultural burning.

When these tiny particles in the air encounter sunlight, haze occurs.  Some of the light is absorbed by the particles while other light is scattered. This absorption and scattering of light reduces the color and clarity of what we see, resulting in visibility impairment.  Sulfate particles scatter more light than other particles, especially during humid conditions.

Of particular concern is the impairment of visibility due to haze in mandatory Class I federal areas, including many national parks and wilderness areas, especially since PM2.5 and its precursors can be transported over long distances.

The Clean Air Act (CAA) defines the general concept of protecting visibility in each of the 156 mandatory Class I federal areas across the nation.  Section 169A of the CAA sets forth the following national visibility goal: “Congress hereby declares as a national goal the prevention of any future, and the remedying of any existing, impairment of visibility in mandatory Class I Federal areas which impairment results from man-made air pollution."

In 1999, EPA adopted the Regional Haze Rule, establishing a comprehensive visibility protection program for Class I areas.  Under this rule, each of the 50 states, as well as the District of Columbia and the Virgin Islands, was required to submit to EPA a regional haze State Implementation Plan (SIP) by December 2007, including determinations of Best Available Retrofit Technology (BART) for certain types of sources that emit pollutants that impair visibility and long-term strategies to ensure that reasonable progress is being made; these plans were to address the first 10-year implementation period ending in 2018. The Regional Haze Rule was amended in 2005 to include final BART guidelines for states to use in determining which facilities must install controls and the type of controls the facilities must use. In 2006, EPA adopted an alternative emissions trading program that gives flexibility to states in applying BART, provided the trading program meets or exceeds the visibility benefits resulting from BART. Further, as EPA moved forward with emissions trading programs to address the interstate transport of air pollution in the eastern portion of the country (the Clean Air Interstate Rule in 2005 and the Cross-State Air Pollution Rule in 2011), the agency revised its 1999 Regional Haze Rule to provide states in the transport region the opportunity to allow these trading programs to serve as an alternative to determining source-specific BART for SO2 and/or NOx emissions from power plants in the transport region.

On April 25, 2016, EPA announced additional proposed revisions to the Regional Haze Rule, which are intended to "streamline, strengthen, and clarify" various aspects of the program.  Among the specific revisions EPA proposed are ones to 1) extend by three years the State Implementation Plan (SIP) submittal deadline for the second regional haze planning period (covering 2019 through 2028) from July 31, 2018 to July 31, 2021 (SIPs for the first planning period, to cover 2008 through 2018, were to have been submitted in 2007); 2) adjust the deadlines for interim progress reports so that the report for the second planning period is due January 31, 2025, the report for the third planning period is due July 31, 2033 and subsequent reports are due every 10 years thereafter; 3) remove requirements that interim progress reports be in the form of a SIP revision; 4) replace the recurring process of assessing reasonably attributable visibility impairment (RAVI) with an as-needd approach and extend the applicability of RAVI to all states, rather than only those with Class I areas; and 5) require that states consult with Federal Land Managers earlier in the SIP development process.   

Because the pollutants that contribute to regional haze can originate from sources located across broad geographic areas, EPA encouraged the states to take a regional approach to addressing visibility impairment.  Five Regional Planning Organizations were established, in the Northeast and mid-Atlantic, Southeast, Central States, Lake Michigan area and West.