California leads the nation in the number of cities with the most polluted air, according to the American Lung Association’s 2026 “State of the Air” report, which evaluates U.S. metropolitan areas based on ozone levels, short-term particle pollution, and annual particle pollution.

The report found that multiple California cities appear at or near the top of national rankings for poor air quality, with eight metropolitan areas in the state listed among the 25 most polluted in the country.

Bakersfield and the surrounding Delano region ranked as the most polluted metro area in the nation for year-round particle pollution, a category measured across 211 U.S. metropolitan areas. Other California regions also placed high in the rankings, including Fresno-Hanford-Corcoran at No. 4, San Diego-Chula Vista-Carlsbad and Visalia tied at No. 5, and Los Angeles-Long Beach at No. 7.

The Bay Area metros of San Francisco, Oakland, and San Jose were grouped in a tie at No. 13, while El Centro ranked No. 21 for annual particle pollution.

In short-term particle pollution, Bakersfield improved slightly from the previous year, moving from first to third place, with Fairbanks, Alaska, taking the top spot in the latest report.

Los Angeles again ranked first nationally for ozone pollution, also known as smog, marking the 26th time in 27 years that the metro area has held that designation.

The American Lung Association report defines ground-level ozone as a harmful pollutant formed through chemical reactions involving sunlight and emissions from vehicles, industry, and other sources. The organization notes that both ozone and particle pollution have been linked to serious health risks, including respiratory and cardiovascular disease, adverse pregnancy outcomes, and increased mortality risk.

According to the report, 44% of Americans, about 152.3 million people, live in areas that received failing grades for either ozone or particle pollution. It also found that roughly 46% of U.S. children are exposed to at least one category of unhealthy air quality, while about 7.3 million people live in areas that failed all three pollution measures evaluated in the study.