The Clean Air Act Amendments of 1977 designated Yellowstone and Grand Teton among the 156 national parks and wilderness areas that are Class I airsheds, requiring the most stringent air quality protection within and around their boundaries. Although park visitation is far lower in winter than in summer, maintaining good air quality is more difficult in winter because of the emissions produced by oversnow vehicles. Bighorn Canyon National Recreation Area is a Class II airshed, in which increases in pollutants over baseline concentrations are restricted. All federal land managers are required to protect visibility, plants, soils, water quality, cultural resources, and visitor health from the adverse impacts of air pollution.