What is the regulatory standard for ambient (outdoor) PM₂.₅ levels?

In May 2024, the U.S. EPA adopted a stronger annual standard for PM₂.₅ from 12 micrograms per cubic meter (µg/m³) to 9 micrograms per cubic meter (µg/m³). The following diagram shows different levels of PM₂.₅ and who may be most affected at each level.