Grand Canyon National Park enjoys some of the cleanest air in the United States. This clean air is a fragile resource, and although the air quality at Grand Canyon is generally good, it is increasingly threatened by human sources. These include metropolitan areas in Arizona, Nevada and California, and also development in northern Mexico. The net effect is a measurable impact on the visibility that is of paramount importance to visitors standing on the rim of the Grand Canyon.
Visibility in the park is often impaired by haze even though pollutant levels do not exceed National Ambient Air Quality Standards (NAAQS). Very small amounts of light-scattering pollutants can significantly reduce visibility under such clean conditions. Haze results in a reduction in clarity and brilliance of the natural features of the park and can eliminate distant views. Visibility at the Grand Canyon averages 106 miles, and can exceed 160 miles on the clearest days. Haze can reduce visibility to less than 50 miles, but visibility is still superior to many sections of the country.
Air quality monitoring at Grand Canyon began in 1958. Since then, many techniques have been used to measure visibility, aerosols, gases, particulates and acid deposition. The monitoring program is designed to identify existing air quality and trends, measure sensitivity of park resources to air quality, establish local and synoptic weather patterns affecting air quality, and identify sources and the nature of existing and potential pollutants. Grand Canyon National Park was designated a Class I area by the Clean Air Act as amended in 1977 (Public Law 95-217). The Act limits deterioration in air quality and gives added protection to uniquely scenic areas. Amendments to the Act in 1990 called for the creation of the Grand Canyon Visibility Transport Commission to study the interstate transport of air pollutants into the Grand Canyon area. The Commission made its recommendations to the Environmental Protection Agency on June 10, 1996. The recommendations addressed reducing air pollution emissions from industry and vehicles. They also addressed smoke from forest fires and agricultural burning. These recommendations are currently being put into place by the Western Regional Air Partnership and the Environmental Protection Agency. Some examples are the Regional Haze Rule and the voluntary "sulfur dioxide cap with a backstop" trading program.