Top 10 most polluted cities

Most ozone-polluted U.S. cities

1 Los Angeles-Long Beach-Riverside, CA

2 Visalia-Porterville-Hanford, CA

3 Bakersfield, CA

4 Fresno-Madera, CA

5 Sacramento-Roseville, CA

6 Houston-The Woodlands, TX

7 Modesto-Merced, CA

8 Dallas-Fort Worth, TX

9 Washington-Baltimore-Arlington, DC-MD-VA

10 Las Vegas-Henderson, NV

*Based on a three-year average of the number of days measurements exceeded federal health standards for ozone.

Most polluted U.S. cities by short-term particle pollution

1 Fresno-Madera, CA

2 Visalia-Porterville-Hanford, CA

3 Bakersfield, CA

4 Los Angeles-Long Beach-Riverside, CA

5 Modesto-Merced, CA

6 Pittsburgh-New Castle-Weirton, PA, OH, WV

7 Fairbanks, AK

8 Salt Lake City-Provo-Orem, UT

9 El Paso-Las Cruces, TX-NM

10 San Jose-San Francisco-Oakland, CA

* Based on a three-year average of the number of days measurements exceeded federal health standards for fine particle pollution.Source: American Lung Association