In 1999, the US EPA announced the Regional Haze Rule in an effort to improve air quality over a 60-year period in national parks and wildernesses designated as Class-1 areas. The rule requires reductions in fine particulate that contribute to visibility impairment – man-made sources of which include power plants, industrial and manufacturing processes, and motor vehicles.

The rule requires affected states to develop and adopt a state implementation plan or SIP that will improve the haziest days and protect the clearest days; comprehensive SIP revisions are required every 10 years. A key component of the Regional Haze Rule is the requirement to install and operate the best available retrofit technology (BART) for qualifying older, existing sources of pollutants.

Two major contributors to haze are NO_x and SO₂. AECOM Process Technologies offers several services and products that minimize NO_x and SO₂ emissions.