11 U.S. Cities With the Worst Air Pollution

The good news: Air has gotten cleaner in recent years. Now the bad: More than half of people in the U.S. still breathe air dirty enough to cause health problems, according to the American Lung Association.

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Bakersfield, Calif.

Population: 807,407

Bakersfield and Delano, Calif., residents breathe some of the country’s most polluted air, based on several measures. The area ranks highest in annual and 24-hour peak averages in fine particulates.

Agriculture in this region of the San Joaquin Valley "whips up" a lot of dust, pesticides, and fertilizers, says Dr. Kaufman.

North of Los Angeles, the area also has the second-highest level of ozone pollution in the U.S. Mountains surround the valley on three sides, creating inversion layers that trap pollution, with little wind to carry it away.

Next: Visalia, Calif.

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