The Acid Rain Program was established under Title IV of the 1990 Clean Air Act Amendments (the Act). The program calls for major reductions of sulfur dioxide (SO2) and nitrogen oxides (NOX), the pollutants that cause acid rain. Using a new, market-based approach to environmental protection, the program sets a permanent cap on the total amount of SO2 that may be emitted by electric utilities nationwide at about one half of the amount emitted in 1980, and allows flexibility for individual utility combustion units to select their own methods of compliance. The program also sets NOX emission limitations (in lb/mmBtu) for certain electric utility boilers, representing about a 27 percent reduction from 1990 levels. The Acid Rain Program is being implemented in two phases: Phase 1 ran from 1995 through 1999 for SO2 and from 1996 through 1999 for NOX; Phase 2 for both pollutants began in 2000 and involves over 2,000 units. In 2000, there were 2,262 units affected by the SO2 provisions of the Acid Rain Program, meaning they operated, submitted emissions data for SO2, and were subject to annual reconciliation of allowable emissions with actual emissions.