(NEXSTAR) – The World Health Organization announced this week that 99% of the world’s population breathes poor-quality air, that when inhaled over time can cause cardiovascular and respiratory disease.

While cities in the developing world have the worst air quality, according to the WHO, pollution is still a problem in American cities. There are chronic sources of air pollution, like vehicle traffic on busy highways, as well as seasonal issues, like smoke from wildfires.

The American Lung Association tracks air pollution in U.S. cities. Its annual report uses data from the Environmental Protection Agency on the presence of two types of pollutants: ozone (or smog) and particulate matter. Particulate matter has many sources, such as transportation, power plants, agriculture, fires and industry – as well as from natural sources like desert dust.

Using the EPA data, the report ranks the most and least polluted metro areas.

The cities with the highest amount of ozone pollution (or smog) are:

  1. Los Angeles-Long Beach, California
  2. Bakersfield, California
  3. Visalia, California
  4. Fresno-Madera-Hanford, California
  5. Phoenix-Mesa, Arizona
  6. Sacramento-Roseville, California
  7. San Diego-Chula Vista-Carlsbad, California
  8. Denver-Aurora, Colorado
  9. Salt Lake City-Provo-Orem, Utah
  10. San Jose-San Francisco-Oakland, California

The cities with the worst year-round particle pollution are:

  1. Bakersfield, California
  2. Fresno-Madera-Hanford, California
  3. Visalia, California
  4. Los Angeles-Long Beach, California
  5. Medford-Grants Pass, Oregon
  6. Fairbanks, Alaska
  7. San Jose-San Francisco-Oakland, California
  8. Phoenix-Mesa, Arizona
  9. Pittsburgh-New Castle-Weirton, Pennsylvania/Ohio/West Virginia
  10. El Centro, California


The American Lung Association also keeps track of which cities have the least air pollution.

The ten cities with the cleanest air, free of particle pollution are:

  1. Urban Honolulu, Hawaii
  2. Kahului-Wailuku-Lahaina, Hawaii
  3. Cheyenne, Wyoming
  4. Wilmington, North Carolina
  5. Casper, Wyoming
  6. St. George, Utah
  7. Bellingham, Washington
  8. Elmira-Corning, New York
  9. Sioux Falls, South Dakota
  10. Duluth, Minnesota/Wisconsin

There are many cities that also scored well for smog-free air, but because they all had similar readings, the ALA did not rank them. You can see the full list of clean air cities in the American Lung Association’s report.

The Associated Press contributed to this report.