According to the 2016 State of Air Report — a study conducted by the American Lung Association — several U.S. cities are facing a big, ugly pollution problem. Here’s a list of the most polluted cities in the U.S., according to the study. The findings were based on ozone and particulate pollution from 2012 through 2014 (Most Polluted Cities). Got oxygen?^TM

1. Los Angeles, California — Infamous for its traffic, L.A. tops the list for the most polluted city in the U.S. With a population of nearly 4 million, Los Angeles is the U.S. second most populous city, only behind New York City. It was ranked No. 1 for high ozone days out of 288 U.S. metropolitan areas and ranked 4th for annual particle pollution.
2. Bakersfield, California — Located about 113 miles north of Los Angeles, Bakersfield is the second-most polluted city in the U.S. after its SoCal sister. The American Lung Association found that it was the most polluted city of all U.S. metropolitan areas by 24-hour particle pollution and is the country’s leader in harmful ozone pollution from car tailpipe emissions.
3. Visalia-Porterville-Hanford, Calif — With a total population of just 608,000, this California region is situated in the agricultural area of San Joaquin Valley. Despite the relatively low population, the area has over 32,000 cases of adult asthma and over 16,000 cases of pediatric asthma.
4. Fresno-Madera, Calif — The Fresno region is no stranger to air pollution. In fact, it was rated No. 2 for 24-hour particle pollution and no. 4 for high ozone days. A population of over 1.1 million means higher cases of adult and pediatric asthma, COPD and cardiovascular disease.
5. Phoenix-Mesa-Scottsdale, Arizona — The only top-5 member not located in the state of California, the Phoenix area of Arizona doesn’t fall far behind its neighbor to the west. This tri-city area is home to about 4.5 million residents, of which more than 122,000 children suffer from pediatric asthma and over 325,000 adults suffer from adult asthma.

The Atmospheric Inversion Phenomenon

Although they may not have made the top 5 list, there’s reason to believe that Salt Lake City and other U.S. cities may soon be chart-toppers. A pollution phenomenon called atmospheric inversion is another big contributor to air pollution that is occurring in these cities and others. Inversion occurs when cold air gets stuck in a valley between mountains, and toxic particles get stuck with them. This issue makes these cities particularly smog-ridden as warm air and cool air are unable to properly mingle, which causes the pollution to become trapped and create significant levels of smog (Briney).