In addition to the 25 worst cities for each pollutant list, the most polluted counties for ozone and particle pollution are ranked in the tables below.
Twenty counties, listed alphabetically by state below, received failing grades for all three measures of pollution:
| Arizona | Maricopa |
California | Fresno, Imperial, Kern, Kings, Los Angeles, Merced, Riverside, San Bernardino, Stanislaus, Tulare |
| Indiana | Lake, Marion |
| Michigan | Wayne |
| Ohio | Butler, Cuyahoga |
| Pennsylvania | Allegheny, Dauphin, Philadelphia |
| Texas | Bexar |
Page last updated: April 9, 2026