Pursuant to a congressional request, GAO reviewed federal and state efforts to maintain clean air in national parks and wilderness areas, focusing on: (1) the extent to which the Clean Air Act regulated stationary sources located near areas designated Class I, the highest protection level; (2) how federal land managers protected Class I areas from stationary source emissions; and (3) why states have not designated other federal lands as Class I. GAO found that: (1) Environmental Protection Agency (EPA) Prevention of Significant Deterioration (PSD) Program requirements covered very few air pollution sources around Class I areas; (2) 99 percent of the stationary sources near the Class I areas reviewed, accounting for 90 percent of local pollution, were exempt from PSD requirements because they either were in existence before the program went into effect or were considered minor sources and did not need permits; (3) although the act provided for installation of retrofit technology on exempt sources, the provision only applied in cases where the facilities adversely affected visibility in Class I areas; (4) although federal land managers improved the PSD permit review process, lack of information about the resources they tried to protect and the effects of air pollution on those resources continued to hamper reviews; and (5) although federal agencies recommended 14 states and territories with areas for Class I designation, state officials believed that the areas already had adequate protection, they lacked the resources to conduct the necessary studies prior to redesignation, and Class I designations hampered economic development in their states.