The Acid Rain Program was established under Title IV of the 1990 Clean Air Act Amendments. The program requires major reductions of sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions, the pollutants that cause acid rain. Using an innovative market-based or cap and trade approach to environmental protection, the program sets a permanent cap on the total amount of SO2 that may be emitted by electric power plants nationwide. The cap is set at about one half of the amount of SO2 emitted in 1980, and the trading component allows flexibility for individual fossil fuel-fired combustion units to select their own methods of compliance. The program also sets NOx emission limitations (in pounds per million British thermal units or lb/mmBtu) for certain coal-fired electric utility boilers, representing about a 27% reduction from 1990 levels.