10 most polluted cities in the U.S.

Pollution is down in the U.S., with air quality showing improvement in all cities listed in the American Lung Association's annual report on air quality. Time to celebrate? Not quite. The report indicates that 60 million Americans - roughly one in five - live in areas with unhealthy spikes in pollution levels, and 18 million live with unhealthy levels year-round. And high levels of air pollution are linked to asthma, stroke, heart attack, cancer, and other often-deadly health problems.

Just where are these air pollution hotspots? Keep clicking to see the association's list of the 10 most polluted metropolitan areas in the nation. Each is ranked according to the average concentration of microscopic particles in the air (measured in micrograms per cubic meter) from diesel exhaust, coal-fired power plants, etc.

10. Modesto, California (tie)

Does Modesto need a new slogan? It tied for 10th on the list of cities with bad air, with a score of 14.7.

10. Louisville-Jefferson County-Elizabethtown-Scottsburg, Kentucky - Indiana (tie)

The Louisville area had a score of 14.7, putting it in a tie with Modesto, Calif. as the tenth most polluted city in the U.S.

9. Cincinnati-Middletown-Wilmington, Ohio - Kentucky - Indiana

The Cincinnati metropolitan area came in ninth on the list, with a pollution score of 15.0.

8. Birmingham-Hoover-Cullman, Alabama

With a score of 15.1, Birmingham finished eighth on the list of the nation's most polluted cities.

7. Pittsburgh-New Castle, Pennsylvania

Pittsburgh got a pollution score of 17.0, putting it at #7 on the list of polluted cities.

6. Fresno-Madera, California

With a score of 17.1, Fresno-Madera came in sixth on the list.

5. Hanford-Corcoran, California

One of six cities in California on the list of polluted cities, Hanford-Corcoran came in eighth. Its pollution score was 17.3.

2. Visalia-Porterville, California (tie)

A score of 18.8 put Visalia-Porterville in a three-way tie for second place the list of the most polluted cities in the U.S.

2. Phoenix-Mesa-Glendale, Arizona (tie)

People who flock to Arizona for health reasons may be surprised to find Phoenix on the list of cities with bad air. But it is. Its score of 18.8 put it in a three-way tie for second place.

2. Los Angeles-Long Beach-Riverside, California (tie)

Having oodles of commuters didn't help Los Angles air much. The metro area tied for second place on the list, with a score of 18.8.

1. Bakersfield-Delano, California

Looking for the worst air in America? You'll find it in Bakersfield-Delano. Its score was a whopping 22.6.