As previously reported in the HGS Update, on January 22, 2013, the U.S. District Court of Appeals for the District of Columbia vacated a 2010 rule establishing significant impact levels (SILs) and significant monitoring concentrations (SMCs) in relation to prevention of significant deterioration (PSD) permitting review for the fine particulate matter (PM2.5) national ambient air quality standards (NAAQS). In response to the D.C. Circuit’s ruling, the U.S. Environmental Protection Agency (EPA) has now issued guidance providing recommendations for completing PSD air quality analysis for PM2.5 in a manner consistent with the court’s the decision and prior EPA statements. EPA has also released a “Question & Answer” document regarding the implications of the court’s decision. EPA is accepting comments on its guidance document until April 17, 2013, and will be conducting a series of related webinars. Following consideration of all public comments, EPA expects to issue a final version of its guidance by July 31, 2013.