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The majority of U.S. counties don’t have monitors — which means that many communities, especially rural ones, don’t have official monitored information on their air quality. Please note that the population figures in this statistic are only for those places that collect air pollution data, and do not reflect the entire population of these groups in the U.S. PM2.5 concentration information is for the three-year period for 2021-2023 for each monitoring site.

Grades are assigned based on the weighted averages as follows: A=0.0, B=0.3-0.9, C=1.0-2.0, D=2.1-3.2, F=3.3+. The concentration of a pollutant is based on the National Ambient Air Quality Standard for PM2.5, which is 9 µg/m3. Counties with design values of 9 or lower received a grade of “Pass” for Annual PM2.5. Counties with design values of 9.1 or higher received a grade of “Fail.”