Exercising one of its most important and far-reaching powers under the Clean Air Act, the Environmental Protection Agency (EPA) has lowered the primary annual National Ambient Air Quality Standard (NAAQS) for fine particulate matter (PM2.5) from 12 micrograms per cubic meter (ug/m3) down to 9 ug/m3, changing the game on air quality permitting for much of the U.S. EPA’s February 7, 2024 final rule, which will become effective 60 days following its publication in the Federal Register, represents a reversal of the Trump administration’s decision to retain the PM2.5 standard of 12 ug/m3 set under the Obama administration in 2012. The lower standard will set off a chain reaction of additional requirements for state air agencies, and ultimately industrial sources, in places designated as nonattainment with the new standard, but one impact of the new standard will be felt almost immediately: increased difficulty in obtaining air permits.