In this episode, we discuss U.S. EPA’s recent release of a supplement to the 2019 Fine Particulate (PM2.5) National Ambient Air Quality Standard (NAAQS) Integrated Science Assessment (ISA). The supplement document is a continuation of U.S. EPA’s re-evaluation of the PM2.5 NAAQS that is anticipated to be further tightened next year. We discuss where U.S. EPA stands in the NAAQS revision process. We also explore the implications of a tightened NAAQS on air permitting projects, discuss the conservative layers associated with the air quality modeling tools that are available, and consider proactive steps that can be taken to prepare for a NAAQS revision.