Los Angeles still has some of the worst air quality in the U.S.

Los Angeles has the worst ozone levels in the U.S. with four of the five most-polluted cities found in California, according to a report by the American Lung Association.

Following LA, the 2024 "State of the Air" report lists Visalia, Bakersfield and Fresno as the metro areas with the worst levels of ozone, a harmful pollutant that's formed when sunlight reacts chemically with nitrogen oxides and volatile organic compounds that come from the emissions of cars, power plants and refineries, according to the Environmental Protection Agency. Traffic is considered a leading contributor to so-called "bad ozone," which can damage living cells in the lungs and cause breathing issues.

Bad ozone is distinct from "good ozone," which occurs naturally in the upper atmosphere and shields people from the sun's UV rays. The EPA states bad ozone is particularly harmful on "hot sunny days."

Meanwhile, the report also lists cities with the worst short-term fine particle pollution and those with the worst yearly average of fine particle pollution. Bakersfield topped both lists. 

Across all three lists measuring air quality, California cities make up the majority of the top 10 cities.

LA's air has seen some improvements. The yearly average of fine particle pollution declined from last year's report, reaching its lowest level yet. City residents were exposed to unhealthy ozone levels an average of 55 days fewer a year now compared to the year 2000.  

Still, the American Lung Association also indicates some concerning changes in the newest report.

Since last year, there has been a notable increase in the number of people living in places with poor air quality nationwide. An estimated 131.2 million people -- about 39% of people living in the U.S. -- live in places with failing grades for ozone levels and fine particle pollution, according to the report.

"This is 11.7 million more people breathing unhealthy air compared to last year's report," the American Lung Association states.

Fine particle pollution has been linked to health conditions such as asthma and heart disease and comes from sources like fires, construction sites and smokestacks, according to the the EPA . 

All 25 of the cities with the worst fine particle pollution are located in the western U.S. 

Wildfires in the western U.S. and Canada are the  "major contributing factor" to the rising number of days and places where fine particle pollution is being reported, the report states. 

Other weather conditions have also been linked to California having some of the worst air quality in the country.

In particular, the Southern California region has long been especially susceptible to bad ozone levels given the combination of warm weather and harmful pollutants from fuel emissions. The report indicates "higher temperatures, dry, sunny skies" are contributing to the number of unhealthy ozone days being reported.

"Simply, climate change is undercutting the progress we would have made," the report states.