On October 17, the U.S. Court of Appeals for the D.C. Circuit heard oral arguments in a case in which environmental groups are seeking more stringent rules for cutting fine particulate matter (“PM2.5”) emissions in non-attainment states. EPA issued the current PM2.5 rules under Title I, part D, subpart 1 of the Clean Air Act. The environmental groups argue that PM2.5 should be regulated under Title I, part D, subpart 4, which applies to coarse particulate matter (“PM10”). Subpart 1 allows more flexibility to states in adopting pollution controls. Environmentalists claim that this flexibility does not require adequately strict controls to reduce PM2.5 emissions in states that are out of attainment with national ambient air quality standards (“NAAQS”). 

A win for the environmental groups could require EPA to overhaul its existing rules for cutting PM2.5 in non-attainment states.