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:
Fourteen counties received failing grades for all three measures of pollution: Butte, Fresno, Imperial, Kern, Kings, Los Angeles, Madera, Merced, Riverside, San Bernardino, San Joaquin, Stanislaus, and Tulare in California and Pinal in Arizona.
Page last updated: April 17, 2024