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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 review of the decision to retain the PM NAAQS. Based on that review, EPA is undertaking a rulemaking to reconsider the December 18, 2020 decision 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 intends to develop an updated Integrated Science Assessment (ISA) and revised policy assessment to take into account the most up-to-date science on public health impacts of PM, and to engage with the Clean Air Scientific Advisory Committee (CASAC) and a newly constituted expert PM panel.