On Thursday, November 14, 2024, Dr. Chris Frey gave an invited presentation to the Environmental Metrology and Public Policy Program at Georgetown University titled, “The Role of Scientific Evidence to Inform Decisions to Protect Human Health and the Environment: Examples Based on the National Ambient Air Quality Standards“. This presentation focuses on the various roles of scientific evidence based on measurements of air quality, exposure, and health effects that are the basis for hazard identification, dose-response assessment, exposure assessment, and risk characterization. These roles are illustrated based on the science that informs review, revision, or setting of new National Ambient Air Quality Standards, using the example of fine particulate matter. Furthermore, an overview is given of the weight of evidence framework for causal inference of hazard.