More than 40% of Americans live in a place with unhealthy air quality, according to a new report from the American Lung Association. But, when it comes to how bad that air pollution can get, there’s a real range.

Los Angeles, which has some of the most polluted air in the country, experiences unhealthy levels of particle pollution for the equivalent of nearly a month out of each year and unhealthy ozone pollution for the equivalent of more than two months annually. Six cities, all much smaller in size, went an entire year without an unhealthy day of either.

Particle pollution refers to the toxic exhaust emitted by processes like smoking or driving a car, and ozone pollution refers to an invisible substance present in smog. Exposure to either pollutant can exacerbate breathing problems and increase residents’ chance of developing cancer.

People most harmed by air pollution include those with cardiovascular diseases, diabetes and asthma, as well as children and the elderly. The report also notes that great disparities in risk exist even within cities. People who live close to highways or busy roads, for instance, are at increased risk, and overall, low socioeconomic status is associated with exposure to poorer air quality.

The report offers a few guidelines for individuals to manage pollution, like avoiding high-traffic areas, but the report also suggests a more top-level approach to public health: strengthening clean-air regulation.

Here are the U.S. cities with the most ozone pollution in 2015:

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

The six cities without any days of unhealthy ozone or particle pollution are Bismarck, N.D.,Cape Coral-Fort Myers-Naples, Fla., Elmira-Corning, N.Y., Fargo-Wahpeton, N.D.-Minn., Rapid City-Spearfish, S.D. and Salinas, Calif.

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