You may want to think twice about your next breath of “fresh air.” More than 4 in 10 Americans live in counties with pollution levels that make the air unhealthy to breathe.

That’s according to the American Lung Association’s recently released State of the Air report, which examines air pollution levels across the United States. The report looks at ozone (smog) and particle pollution (soot) and gives each county a report card based on that data.

“[I]mprovement in the nation’s air quality was mixed, with many cities experiencing strong improvements, while others suffered increased episodes of unhealthy air, and a few even marked their worst number of unhealthy days,” the 16th annual report found.

Although the ALA found that cleaner power plants and cleaner diesel fleets helped the air quality in the eastern half of the country, climate change is having a significant impact on air quality in the Western United States. The ALA said:

Climate change creates the ideal conditions for both particle and ozone pollution. Warmer weather increases the risk of ozone pollution and makes cleaning it up even more challenging. Rising temperatures also increase droughts, wildfires and other sources of particle pollution.

Based on the report, the 10 cities with the worst air quality (based on year-round particle pollution) are:

1. Fresno-Madera, California.

2. Bakersfield, California.

3. Visalia, California.

4. Modesto, California.

5. Los Angeles.

6. El Centro, California.

7. San Francisco Bay.

8. Cincinnati metro area

9. Pittsburgh metro area

10. Cleveland metro area

These cities have the cleanest air in the U.S. (so breathe deeply if you live there):

1. Prescott, Arizona.

2. Farmington, New Mexico.

3. Cheyenne, Wyoming.

4. Casper, Wyoming.

5. Flagstaff, Arizona.

6. Duluth, Minnesota.

7. Salinas, California.

8. Palm Bay (metro area), Florida.

9. Kahului-Wailuku-Lahaina, Hawaii

10. Rapid City (metro area), South Dakota.

Unfortunately, fewer than 1,000 of the 3,068 counties in the U.S. have air quality monitors. My county’s air pollution, like many across the United States, is listed as DNC, or Data Not Collected, because there are no monitors to track pollutant information.

Other counties are listed as INC, or Incomplete, because they don’t have three years of pollution data.

The ALA said that if you want to breathe healthy air, you can take steps now to improve the air quality in your area:

Drive less. Use less electricity. Don’t burn wood or trash. Support measures in your community that can cut air pollution. Tell your local and state officials to take steps to clean up air pollution.

What grade did your county earn based on its air pollution? Share your comments below or on our Facebook page.