Seventeen of the 25 most polluted cities, including L.A., Pittsburgh, and Cincinnati, had their lowest particle pollution levels in the history of the annual report.

For the first time in over a decade, Birmingham, Ala., Detroit, and York, Pa., were no longer among the 25 most polluted cities.

Santa Fe, N.M., topped the list of the nation's cleanest cities, followed by Cheyenne, Wyo., and Prescott and Tucson, Ariz.

4 in 10 Americans Live With Dangerous Air

Despite the improvements, nearly 50 million Americans live in counties with unhealthy spikes in particle pollution levels:

Roughly 4 in 10 people in the U.S. are exposed to unhealthy levels of smog or particle pollution year-round or at various times during the year.

Nearly 2.5 million children and 7.4 million adults with asthma live in parts of the United States with very high ozone levels, and more than 3 million adults and close to 1 million children with asthma live in areas with high levels of short-term particle pollution.

More than 11 million adults with heart disease and 3 million with diabetes live in areas with high levels of short-term particle pollution.

The Rest of the Best -- and Worst

Cleanest Cities in the U.S. - Lowest Year-Round Particle Pollution

Santa Fe, N.M.

Cheyenne, Wyo.

Prescott, Ariz.

Tucson, Ariz.

Albuquerque, N.M.

Redding, Calif.

Colorado Springs, Colo.

Flagstaff, Ariz.

Anchorage, Alaska

Boise City, Idaho

10 Areas With the Most Year-Round Particle Pollution

Bakersfield-Delano, Calif.

Hanford-Corcoran, Calif.

Los Angeles-Long Beach-Riverside, Calif.

Visalia-Porterville, Calif.

Fresno-Madera, Calif.

Pittsburgh-New Castle, Pa.

Phoenix-Mesa-Glendale, Ariz.

Cincinnati-Middletown-Wilmington, Ohio-Ky.-Ind.

Louisville-Jefferson County-Elizabethtown-Scottsburg, Ky.-Ind.

Philadelphia-Camden-Vineland, Pa.-N.J.-Del.-Md.

10 Most Ozone-Polluted Areas

Los Angeles-Long Beach-Riverside, Calif.

Visalia-Porterville, Calif.

Bakersfield-Delano, Calif.

Fresno-Madera, Calif.

Hanford-Corcoran, Calif.

Sacramento-Arden-Arcade-Yuba City, Calif.-Nev.

San Diego-Carlsbad-San Marcos, Calif.

Houston-Baytown-Huntsville, Texas.

San Luis Obispo-Paso Robles, Calif.

Merced, Calif.

To see where your community ranks on the list, check out www.stateoftheair.org.