Take a deep breath — or maybe not.

More than four in 10 people live in counties with unhealthy levels of either ozone or particle pollution. That's according to the American Lung Association's annual "State of the Air 2015" report released today. For the past 16 years, the report has examined pollution levels across the U.S.

This year's report found mixed progress in air quality improvement across the country. While most of the nation experienced much cleaner air quality than just a decade ago, some cities had their worst bouts of unhealthy air since the report started. In fact, nearly 138.5 million Americans — almost 44 percent of the nation — live with pollution levels that are often too dangerous to breathe, according to the report.

The "State of the Air 2015" reports looks at monitoring data from 2011 through 2013 for the two most common and dangerous types of pollution: ozone (smog) and particle (soot) pollution. The report card compiles information on how much of each type of pollution is in various areas throughout the country. Some highlights (and lowlights) from the findings:

  • The best overall progress was in the continued reduction of year-round particle pollution in the eastern half of the U.S., courtesy of cleaner diesel fleets and cleaner fuels used in power plants.
  • Many cities, particularly in the West, had a record number of days with high levels of short-term particle pollution. In the West, drought and heat may have contributed to dust, grass fires and wildfires, while in smaller cities burning wood as a heat source may be an issue.
  • Climate change is a factor for air pollution, especially in the West, where high temperatures and drought create the perfect conditions for high-particle days.
Six cities had no days when the air quality reached the unhealthy level for ozone or short-term particle pollution and they were on the list of the cleanest cities for year-round particle pollution. (Note that areas on all lists that include more than one state are near state lines.)
  • Bismarck, ND
  • Cape Coral-Fort Myers-Naples, FLA
  • Elmira-Corning, NY
  • Fargo-Wahpeton, ND-MN
  • Rapid City-Spearfish, SD
  • Salinas, CA
But not all the cities fared so well, as the next lists will show.

Nation's most polluted cities

By ozone                            

 1. Los Angeles-Long Beach, CA

 2. Visalia-Porterville-Hanford, CA

 3. Bakersfield, CA

 4. Fresno-Madera, CA

 5. Sacramento-Roseville, CA

 6. Houston-The Woodlands, TX

 7. Dallas-Fort Worth, TX-OK

 8. Modesto-Merced, CA

 9. Las Vegas-Henderson, NV-AZ

 10. Phoenix-Mesa-Scottsdale, AZ

By year-round particle pollution                              

 1. Fresno-Madera, CA

 2. Bakersfield, CA

 3. Visalia-Porterville-Hanford, CA

 4. Modesto-Merced, CA

 5. Los Angeles-Long Beach, CA

 6. El Centro, CA

 7. San Jose-San Francisco-Oakland, CA

 8. Cincinnati-Wilmington-Maysville, OH-KY-IN

 9. Pittsburgh-New Castle-Weirton, PA-OH-WV

 10. Cleveland-Akron-Canton, OH

By short-term particle pollution

 1. Fresno-Madera, CA

 2. Bakersfield, CA

 3. Visalia-Porterville-Hanford, CA

 4. Modesto-Merced, CA

 5. Los Angeles-Long Beach, CA

 6. San Jose-San Francisco-Oakland, CA

 7. Salt Lake City-Provo-Orem, UT

 8. Logan, UT-ID

 9. Fairbanks, AK

 10. Pittsburgh-New Castle-Weirton, PA-OH-WV

Want to see how your city did? Check out the American Lung Association's "Compare Your Air" rankings.

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Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.