The Cross-State Air Pollution Rule (CSAPR) requires 28 states—predominantly in the eastern half of the United States—to reduce SO₂ and NO_x emissions to levels that will not impact downwind states' abilities to comply with National Ambient Air Quality Standards (NAAQS) for ozone and fine particle pollution. The rule is an extension of the "good neighbor" provision of the Clean Air Act, and established four interstate transport trading programs that were originally scheduled to go into effect in 2012 - two for annual SO₂, one for annual NO_x, and one for ozone-season NO_x.  A further reduction in SO₂ emissions was planned for 2014.

The US EPA published CSAPR in 2011 as a replacement for the Clean Air Interstate Rule (CAIR), which was ruled unlawful by a DC District Court in 2008. However, in 2011, just before CSAPR was scheduled to go into effect, it was vacated by the DC Circuit Court, and CAIR was permitted to remain in effect until a suitable replacement was implemented. The US EPA appealed this decision and in April of 2014 the United States Supreme Court ruled 6 to 2 that the EPA acted within its authority and reinstated CSAPR. In October 2015, the DC District Court granted an EPA motion to lift the stay, with Phase-1 limits effective January 2015.  Limits associated with Phase 2 of the program will now go into effect in 2017.

In January of 2015, the DC Circuit ruled that the emission budgets for several states were invalid. While the rule remains in effect, the US EPA must now reconsider these emission budgets.

AECOM Process Technologies offers several NO_x and acid gas control technology that can help ensure continued compliance with CSAPR.