Within the US's framework for air pollution management, Prevention of Significant Deterioration (PSD) Class I areas receive extra protection from the impacts of air pollution. The PSD classification was established under the Clean Air Act prevention of significant deterioration (PSD) program, to limit the amount of additional pollution to these areas resulting from construction or modification of major sources of air emissions. The PSD program establishes three thresholds of air quality and emissions to guide states in maintaining clean air. These thresholds are known as PSD Increments, Significant Impact Levels, and Significant Monitoring Concentrations. A PSD Increment is a measure of how much of a pollutant can be added to the ambient air before the air quality will significantly deteriorate. More specifically, PSD Increments are the maximum allowable increase in ambient air concentrations of a criteria pollutant from a baseline concentration.