On April 7, the U.S. Court of Appeals for the District of Columbia Circuit (D.C. Circuit) dismissed environmentalists’ challenge to the U.S. Environmental Protection Agency’s (EPA’s) guidance on significant impact levels (SILs) for ozone and particulate matter (i.e., PM2.5) because the guidance is not final agency action. The SILs guidance, published on April 17, 2018, explains the agency’s view that permitting authorities may exempt sources from in-depth Prevention of Significant Deterioration (PSD) permit review if anticipated impacts fall below nonbinding SIL values. In Sierra Club v. EPA, No. 18-1167, a three-judge panel of the D.C. Circuit unanimously held that the guidance does not legally bind as “it does not determine rights or obligations and does not effectuate direct or appreciable legal consequences as understood by the finality inquiry.” The D.C. Circuit did not reach the merits of the guidance.