"INTRODUCTION Because of the need for protection against the adverse effects of air pollution on human health@ vegetation@ and materials@ the ambient air quality standards of the U.S. Environmental Protection Agency (EPA) were promulgated. The visual impact of air pollution is another concern; the aesthetic degradation caused by air pollution is familiar to most urban dwellers. Recently@ the intrusion of air pollution into national parks@ wilderness areas@ and other scenic areas has concerned many people. The experience of the scenic beauty of these areas can be impaired by haze@ discoloration@ and other visual effects caused by man and nature. A major impetus for the enactment of sections of the Clean Air Act Amendments of 1977 was the desire to prevent the visual impact caused by air pollution@ particularly in scenic areas. Presented with evidence that existing air quality regulations would be inadequate to protect visual values@ Congress stated in the 1977 amendments that one of the purposes of the prevention of significant deterioration section was ""to preserve@ protect@ and enhance the air quality in national parks@ national wilderness areas@ national monuments@ national seashores@ and other areas of special national or regional natural@ recreational@ scenic@ or historic value."" The protection of the scenic resource@ or ""visibility@"" is delineated in Sections 165 and 169A of the act. In Section 165@ federal land managers of Class I areas--national parks and wilderness areas--are given ""an affirmative responsibility to protect the air quality related values (including visibility)"" of these areas. In Section 169A@ Congress declared ""as a national goal the prevention of any future@ and the remedying of any existing@ impairment of visibility in mandatory Class I Federal areas@ which impairment results from man made air pollution."""