Abstract: "In July 1997, after evaluating hundreds of health studies and conducting an extensive peer-review process, EPA established PM standards that specifically addressed particles smaller than 2.5 microns (PM2.5). The annual standard was set at 15 micrograms per cubic meter (μg/m3), based on the 3-year average of annual mean PM2.5 concentrations. The 24-hour standard was set at 65 μg/m3 based on the 3-year average of the annual 98th percentile concentrations. The 1997 standards retained, but slightly revised, standards for PM10, which limited PM10 concentrations to 50 μg/m3 based on an annual average, and 150 μg/m3 based on a 24-hour average".
https://www3.epa.gov/region1/airquality/pm-aq-standards.html