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These are the nation’s most polluted cities

Hollywood Hills with Smog and FogHollywood Hills with Smog and Fog
Hollywood Hills with Smog and Fog, Los Angeles, California, USA Photograph by UniversalImagesGroup via Getty Images

The American Lung Association (ALA) released its annual “State of the Air” report, which measures both ozone pollution and particle pollution (long term and short term) in cities across the U.S. The 2015 report shows that the Los Angeles-Long Beach, Calif. area takes the top spot in ozone pollution and Fresno-Madera, Calif. leads in both long term and short term particle pollution.

Although air pollution has generally decreased in recent years, over 138 million people, about 44% of the U.S. population, live in areas with dangerously high levels of pollution, 24/7 Wall St. reports, and 6 cities set local records of air pollution in the 2015 report.

The top 5 polluted cities in each category are located in or near the Central Valley in California, which is surrounded by mountains that effectively trap pollution in the valley. The state has also been suffering from wildfires and drought conditions that create unhealthy particles, contributing to abnormally high levels of pollution.