Four in 10 Americans live in regions with poor air quality, according to a new American Lung Association ranking of the nation’s 10 most polluted cities.

Los Angeles takes the No. 1 spot in the nation, experiencing dangerous levels of particle pollution for the equivalent of a month out of each year and unhealthy ozone pollution for two months annually, Time.com reports.

Ozone (smog) and particle pollution are contained in toxic emissions that come primarily from automobiles and some industrial processes. Both can exacerbate breathing problems, increase the odds of developing cancer, and pose particular risks to people — especially seniors and children — with heart disease, diabetes, and asthma.

The ALA suggests individuals can reduce their risks by avoiding high-traffic areas in polluted cities, but also calls for greater global efforts to curb pollution.

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

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, TX.
7. Dallas-Fort Worth, TX.
8. Modesto-Merced, Calif.
9. Las Vegas-Henderson, NV.
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.