An early example of an emission trading system has been the sulfur dioxide (SO2) trading system under the framework of the Acid Rain Program of the 1990 Clean Air Act in the U.S. Under the program, which is essentially a cap-and-trade emissions trading system, SO2 emissions were reduced by 50% from 1980 levels by 2007. Some experts argue that the cap-and-trade system of SO2 emissions reduction has reduced the cost of controlling acid rain by as much as 80% versus source-by-source reduction. The SO2 program was challenged in 2004, which set in motion a series of events that led to the 2011 Cross-State Air Pollution Rule (CSAPR). Under the CSAPR, the national SO2 trading program was replaced by four separate trading groups for SO2 and NOx.