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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 undertook a reconsideration of the December 2020 decision to retain the PM NAAQS because the available scientific evidence and technical information indicated that the current standards may not be adequate to protect public health and welfare, as required by the Clean Air Act. As part of the 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. The notice of proposed rulemaking was signed on January 5, 2023, and a final rule was announced on February 7, 2024. EPA set the level of the primary annual PM2.5 standard to 9 µg/m3. EPA retained all other PM NAAQS, including the primary and secondary 24-hour PM2.5 standards, the primary and secondary 24-hour PM10 standards, and the secondary annual PM2.5 standard. EPA also finalized revisions to the Air Quality Index (AQI) and monitoring network requirements.