As part of a suite of guidance on New Source Review (NSR) permitting expected this spring, EPA recently issued a guidance document and legal memorandum providing new guidance that will allow permit applicants to use significant impact levels (SILs) for ozone and final particulate matter (PM2.5) in the Prevention of Significant Deterioration (PSD) permitting program (SIL Guidance). Under the PSD/NSR program, regulated entities seeking to expand or build facilities in an area that is in attainment of the NAAQS must apply for preconstruction permits. A project that would cause a facility to exceed SILs must either offset emissions or conduct additional modeling to demonstrate that the project will not harm air quality. The SIL Guidance provides nonbinding SILs for ozone and PM2.5 for permitting authorities to use on a case-by-case basis in preliminary and cumulative impact analyses for ozone and PM2.5. EPA had sought to codify SILs in a 2010 rulemaking that the U.S. Court of Appeals for the District of Columbia Circuit (DC Circuit) subsequently vacated for failure to preserve the discretion of permitting authorities to require additional analysis. Sierra Club v. EPA, 705 F.3d 458, 463–66 (D.C. Cir. 2013). According to the SIL Guidance, EPA will use the information gathered through applying the new SILs to evaluate whether it will undertake a new rulemaking to codify the SILs. Meanwhile, the higher SILs will likely ease applicants’ ability to secure preconstruction permits under the PSD program.