Powder River Basin (PRB) coal is classified as "sub-bituminous" and contains an average of approximately 8,500 btu/lb, with low sulfur. Contrast this with eastern, Appalachian bituminous coal containing an average of 12,500 btu/lb and high sulfur. PRB coal was essentially worthless until air pollution emissions from power plants (primarily sulfur dioxide, or "SO2") became a concern. A coal-fired plant designed to burn Appalachian coal must be modified to remove SO2 at a cost estimated in 1999 to be around $322 per ton of SO2. If it switched to burning PRB coal, the cost dropped to $113 per ton of SO2 removed. Removal is accomplished by installing scrubbers.