The Clean Air Act sets a national goal of reducing pollution that causes lowered visibility in national parks. Sec. 169  of the Act requires the Environmental Protection Agency (EPA) and land management Federal agencies to monitor air pollution-affected visibility in 156 national parks, wilderness areas, and wildlife refuges, known as "class I areas." These class I areas are affected by a variety of pollution sources, including natural sources like dust and wildfire smoke and manmade sources like motor vehicles, power plants, refineries, and manufacturing facilities. Pollution, especially particulate matter can be transported over a wide geographic area, affecting class I areas far from the source, even in nearby states. Haze develops when light bounces off this pollution, degrading air quality in national parks and obstructing views.
As part of the regional haze program, every state must submit a plan to reduce haze to EPA in coordination with the National Park Service, U.S. Fish and Wildlife Service, and U.S. Forest Service. These plans are submitted individually, though usually coordinated by regional, multi-state efforts. The long-term goal is to achieve natural visibility levels in class I areas by 2064.
These plans require emission sources to add control technologies, including retrofitting certain large facilities that have been operating since 1962 with the Best Available Retrofit Technology. If the plans fail to adequately clean the air quality in national parks, EPA may reject them and implement a federal haze plan.
The regional haze program was first mandated in the 1977 amendments to the Clean Air Act. The initial program required air quality monitoring in class I areas. Regulations implementing the program were promulgated in 1980, and monitoring began in 1988.
In the 1990 amendments to the Act, additional requirements were added to the program to reduce haze and increase visibility, leading to significant new regulations in 1999 that required the creation of state implementation plans. These plans were to have been filed by 2007; however, filing was delayed as states waited for finalization of the air transport rule. The air transport rule (the Clean Air Interstate Rule and its replacement Cross-State Air Pollution Rule ) added more stringent requirements on many of the same pollutants effected by the regional haze program, therefore states waited to consider the implications of that rule before adjusting their regional haze plans. Currently, only California and Delaware have EPA-approved plans in place, while 45 other states and territories have submitted a plan and are awaiting EPA action.
A 2011 lawsuit challenged the delay and asked EPA to begin accepting or rejecting the state plans. In November 2011, EPA reached a settlement agreement with plaintiffs and agreed to finalize acceptance of 37 state plans or implementation of a federal plan by November 2012. The result was a proposed rule  released on December 23, 2011 that allows Eastern U.S. states covered by the Cross-State Air Pollution Rule  to comply with regional haze requirements by implementing that rule, while consideration continues of Western U.S. state plans.
Read more from EPA  on regional haze.