EEI > Issues & Policy > Environment > Clean Air > Interstate Transport
Interstate Transport


The movement of ozone, particulate matter and other pollutants by wind currents, a phenomenon known as interstate transport, can cause pollution levels to be elevated in downwind areas. Multiple actions have been taken to address interstate transport:
  • In 1990, Congress created the ten-state Ozone Transport Commission (OTC), directing the states in the Northeastern U.S. to collaboratively find a solution to persistent summertime ozone. The OTC set out to accomplish this goal, in part, by implementing a regional trading program for NOx (an ozone precursor).
  • In 1998, the U.S. EPA finalized the NOx SIP Call, ultimately setting up the NOx Budget Trading Program (NBP), a new program covering 20 states and the District of Columbia. The states in the original OTC trading program joined the NBP.
  • In 2005, EPA promulgated the Clean Air Interstate Rule (CAIR) to continue to deal with interstate transport of fine particulate matter (caused by NOx and SO2 emissions) and ozone. Under CAIR, the number of states in the trading region was again extended to help states in their efforts. CAIR's emission reduction requirements began in 2009/2010, and a second round of reductions is required in 2015. CAIR was challenged in U.S. Court of Appeals for the District of Columbia, which ultimately sent the rule back to EPA to fix flaws identified by the court. However, the court allowed the CAIR emission reductions to take effect in order to protect the environment.
  • In 2011, EPA promulgated the Cross-State Air Pollution Rule (CSAPR) to replace the remanded CAIR rule. EPA again relied on a trading program to implement reductions, with the geographic region growing to include 28 states. Like CAIR, CASPR was challenged in the courts. On August 21, 2012, the District Court vacated CSAPR and told EPA to continue to administer CAIR while it dealt with the flaws the court identified in CSAPR. In April 2014, the U.S. Supreme Court reversed the vacatur, and EPA is now working with the Court and petitioners to resolve many outstanding issues with CSAPR.