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The 10 most polluted cities in the U.S.

Birds fly across the sky at daybreak over downtown Los Angeles in 2011. (Frederic J. Brown/AFP/Getty Images)
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California has had a rough go this year. The state’s historic drought is impacting the lives of Californians, and now the drought is exacerbating California’s air pollution, according a new report by the American Lung Association.

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But Californians are not alone. More than 4 in 10 Americans live in counties where the air is unhealthy to breathe due to ozone or particle pollution. That’s almost 138.5 million people, or 44 percent of the U.S. population, according the association’s 16th annual “State of the Air” report, which studied air quality data collected from 2011 through 2013.  The report also ranks U.S. cities by air quality based on measurements of particle pollution and ozone pollution.

Particle pollution is quite dangerous and made of fine particulates of chemicals, metals, acids, soil and dust that are small enough to infect the bloodstream and trigger asthmatic attacks.

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According to CBS, the number of Americans living in unhealthy areas has slightly decreased from last year’s report, but overall the report is mixed. Although some cities saw improvements, others had more “episodes of unhealthy air,” according to the association. The “best progress,” the association said, came in the eastern half of the country, where cleaner diesel fleets and cleaner power plants led to a continued reduction of year-round particle pollution.

The West did not fare as well. “Many cities, especially in the West, had record numbers of days with high short-term particle pollution,” Janice Nolen, the association’s vice president for national policy and advocacy, told the Palm Springs Desert Sun. California, in particular, scored low on air quality due in part to the drought, which causes warmer weather that results in increased levels of ozone or smog.

[California’s drought: What losing 63 trillion gallons of water looks like]

“Heat is one of the ingredients that is key to making ozone,” Nolen told CBS.

“As we are seeing temperatures increase across the nation, it means that we have a harder time cleaning up ozone,” she added.

“Overall, we have made great improvements but we do know we are still facing challenges, especially challenges created by climate change and some of the impacts warmer climates have on creating more ozone and particle pollution.”  Nolen said.

Here’s a look at the nation’s most polluted cites, according to the American Lung Association’s report.

Top 10 U.S. cities most polluted by short-term particle pollution

  1. Fresno-Madera, Calif.
  2. Bakersfield, Calif.
  3. Visalia-Porterville-Hanford, Calif.
  4. Modesto-Merced, Calif.
  5. Los Angeles-Long Beach, Calif.
  6. San Jose-San Francisco-Oakland, Calif.
  7. Salt Lake City-Provo-Orem, Utah
  8. Logan, Utah-Idaho Metropolitan Statistical Area
  9. Fairbanks, Alaska
  10. Pittsburgh-New Castle-Weirton, Pa.-Ohio-W.Va.

Top 10 U.S. cities most polluted by year-round particle pollution

  1. Fresno-Madera, Calif.
  2. Bakersfield, Calif.
  3. Visalia-Porterville-Hanford, Calif.
  4. Modesto-Merced, Calif.
  5. Los Angeles-Long Beach, Calif.
  6. El Centro, Calif.
  7. San Jose-San Francisco-Oakland, Calif.
  8. Cincinnati-Wilmington-Maysville, Ohio-Ky.-Ind. Metropolitan Statistical Area
  9. Pittsburgh-New Castle-Weirton, Pa.-Ohio-W.Va. Metropolitan Statistical Area
  10. Cleveland-Akron-Canton, Ohio

Top 10 most ozone-polluted cities

  1. Los Angeles-Long Beach, Calif.
  2. Visalia-Porterville-Hanford, Calif.
  3. Bakersfield, Calif.
  4. Fresno-Madera, Calif.
  5. Sacramento-Roseville, Calif.
  6. Houston-The Woodlands, Tex.
  7. Dallas-Fort Worth, Tex.-Okla. Metropolitan Statistical Area
  8. Modesto-Merced, Calif.
  9. Las Vegas-Henderson, Nev.-Ariz. Metropolitan Statistical Area
  10. Phoenix-Mesa-Scottsdale, Ariz.