The United States Environmental Protection Agency (“EPA” or “Agency”) announced earlier this year that it would reconsider its decision in December 2020 to retain the existing suite of National Ambient Air Quality Standards (“NAAQS”) for particulate matter (“PM”).1 Those standards include an annual primary NAAQS for fine particles (measured as PM2.5) of 12 μg/m3, annual primary and secondary PM25 NAAQS of 15 μg/m3, 24-hour primary and secondary PM2,5 NAAQS of 35 μg/m3 and 65 μg/m3, and 24-hour primary and secondary NAAQS for coarse particles (measured as PM10) of 150 μg/m3.2 The Clean Air Act (“CAA” or “Act”) 3 requires that, in reconsidering the decision to retain these standards, the EPA Administrator must judge, based on the latest scientific knowledge, whether those standards are requisite to protect public health with an adequate margin of safety and protect public welfare.4 The Act does not require the Administrator set NAAQS at a zero-risk level.5 To inform the Administrator’s judgment, his professional staff has prepared and released for public comment drafts of both an update to the Integrated Science Assessment (“ISA”)6 – a Draft ISA Supplement7 – that summarizes the relevant scientific information, and a Policy Assessment (“PA”) – the Draft PA.8 – that provides the air staff’s advice to the Administrator on options for regulatory actions he should consider at the conclusion of this reconsideration.