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Cross-State Air Pollution Rule (CSAPR)


On July 6, 2011, the US Environmental Protection Agency (EPA) finalized the Cross-State Air Pollution Rule (CSAPR) to address air pollution from upwind states that crosses state lines and affects air quality in downwind states. Sulfur dioxide (SO2) and oxides of nitrogen (NOx) emissions react in the atmosphere and contribute to the formation of fine particle (soot) pollution. NOx also contributes to ground-level ozone (smog) formation. These emissions and the resulting soot and smog they form can affect air quality and public health locally, regionally, and in states hundreds of miles downwind. For more information on interstate transport of air pollution, see EPA’s interstate air transport page.


Brian Carley, Air Quality Division

CSAPR replaced EPA’s 2005 Clean Air Interstate Rule (CAIR), following the direction of a 2008 court decision that required EPA to issue a replacement regulation. CSAPR regulates annual emissions of NOx and SO2 as well as NOx emissions during the ozone season (May 1 through September 30) from subject electric generating units (EGUs). CSAPR implementation began on January 1, 2015.

CSAPR requires fossil fuel-fired EGUs at coal-, gas-, and oil-fired facilities in 27 states to reduce emissions to help downwind areas attain fine particle and/or ozone with the National Ambient Air Quality Standards (NAAQS). EPA sets a pollution limit (emission budget) for each of these states. Authorizations to emit pollution, known as allowances, are allocated by the EPA to affected sources based on these state emissions budgets. Sources can buy and sell allowances and bank (save) allowances for future use as long as each source holds enough allowances to account for its emissions by the end of the compliance period. CSAPR includes provisions to assure that each state will meet its pollution control obligations. These provisions ensure that each state eliminates the SO2 and NOx emissions that significantly contribute to downwind nonattainment or interfere with maintenance of the NAAQS.

On September 7, 2016, the EPA revised the CSAPR ozone season NOx program by finalizing an update to CSAPR for the 2008 ozone NAAQS, known as the CSAPR Update. The CSAPR Update ozone season NOx program will largely replace the original CSAPR ozone season NOx program starting on May 1, 2017. The CSAPR Update will further reduce summertime NOx emissions from power plants in the eastern U.S.

Who is subject?

The following are subject to NOx Annual, NOx Ozone Season, and SO2 Annual requirements.

  • Any stationary fossil fuel fired boiler or combustion turbine in operation since January 1, 2005, serving a generator with a nameplate capacity of more than 25 megawatts producing electricity for sale.
  • Any new unit start-up involving a combustion chamber that serves a generator with a nameplate capacity of more than 25 megawatts producing electricity for sale. The new unit will become subject to CSAPR on the first date on which it both combusts fossil fuel and serves such generator.

Non-EGUs that were subject to CAIR are not subject to CSAPR (see the CSAPR Fact Sheet for Non-EGUs). However, non-EGUs are still required to comply with the requirements of the NOx State Implementation Plan (SIP) Call.

CSAPR Interface with Renewable Operating Permits

CSAPR requirements will be incorporated as an appendix into each subject source’s ROP. The appendix will list all subject units and the monitoring methodology each one uses for SO2, NOx, and heat input. The appendix will also include the requirements for the CSAPR NOx Annual Trading Program (40 CFR Part 97, Subpart AAAAA), CSAPR NOx Ozone Season Trading Program (40 CFR Part 97, Subpart BBBBB), and CSAPR SO2 Group 1 Trading Program (40 CFR Part 97, Subpart CCCCC) that these units are subject to.