EPA and other Agencies have been monitoring visibility in national parks and wilderness areas since 1988.

In 1999, the U.S. Environmental Protection Agency announced a major effort to improve air quality in national parks and wilderness areas. The Regional Haze Rule calls for state and federal agencies to work together to improve visibility in 156 national parks and wilderness areas such as the Grand Canyon, Yosemite, the Great Smokies and Shenandoah.

• List of 156 National Parks and Wilderness Areas (Class I Areas) Protected Under the Regional Haze Program.