Wilderness ecosystems in the United States are federally mandated and set aside by the Wilderness Act. They are managed to minimize human impact using methods that leave these systems, to the extent possible, in their natural state uninfluenced by manipulation or disruption by humans. Management often involves controlling or minimizing visual impact by enforcing strict use rules such as no roads, motorized or mechanized equipment, structures, or harvesting. In addition to human-caused impacts, natural and anthropogenic environmental stresses affect the rates and magnitudes of natural dynamic processes in natural ecosystems. These stresses can have a large effect on ecosystem structure and function. The Clean Air Act designates federally mandated wildernesses and National Parks as Class I areas to be protected from deterioration from air pollutants. Data are not available to quantify and distinguish between natural and anthropogenic factors that cause changes in wilderness ecosystems.