In addition to the 25 worst cities for each pollutant listed previously, the 25 most polluted counties for ozone and particle pollution are ranked in the tables below:
Eleven counties received failing grades for all 3 measures of pollution: Fresno, Kern, Kings, Los Angeles, Madera, Riverside, San Bernardino, Stanislaus, Sutter, and Tulare in California and Pinal in Arizona.
Page last updated: April 18, 2023