Under the Clean Air Act Amendments of 1977, EPA is required to review and if appropriate revise the air quality criteria for the primary (health-based) and secondary (welfare-based) national ambient air quality standards (NAAQS) every 5 years. On December 18, 2020, the EPA published a final decision retaining the NAAQS for particulate matter (PM), which was the subject of several petitions for reconsideration as well as petitions for judicial review. As directed in Executive Order 13990, "Protecting Public Health and the Environment and Restoring Science to Tackle the Climate Crisis,” signed by President Biden on January 20, 2021, EPA is undertaking a reconsideration of the December 2020 decision to retain the PM NAAQS because the available scientific evidence and technical information indicate that the current standards may not be adequate to protect public health and welfare, as required by the Clean Air Act. As part of this reconsideration, EPA developed a Supplement to the 2019 PM Integrated Science Assessment (ISA) and a Policy Assessment to take into account the most up-to-date science on public health impacts of PM, and engaged with the chartered Clean Air Scientific Advisory Committee (CASAC) and a newly-constituted expert CASAC PM panel.