Two recent lawsuits decided by the U.S. Court of Appeals for the D.C. Circuit regarding PM2.5 NAAQS may result in stricter rules regarding PM2.5 NAAQS implementation. On January 4, the D.C. Circuit vacated EPA's PM2.5 NAAQS implementation rules, finding that the rules must comply with the strict requirements of Clean Air Act Title 1, Part D, Subpart 4, rather than the more lenient general requirements of Subpart 1. On January 22, the D.C. Circuit invalidated EPA's rules allowing exemptions from PM2.5 permit and monitoring requirements where a facility's PM2.5 emissions are below certain thresholds. 

The environmental community is arguing that these rulings indicate that EPA should tighten rules and limit exemptions for PM2.5, as well as rules for other NSR pollutants, across the board. Other stakeholders, however, argue that the appropriate response to recent lawsuits is to simplify air rules so that they best serve their intended purpose, guaranteeing good controls on new sources, and do not lead to litigation which halts development and construction.