In its 2019 "State of the Air" report, the American Lung Association listed the 25 US cities with the worst air pollution. Nine are located in California.
Five of those California cities failed to meet national air-quality standards.
The report identified Los Angeles as the US city with the worst ozone pollution, which can cause breathing problems and increase one's risk of heart attack. Ozone pollution results from a chemical reaction between sunlight and pollutants from cars, power plants, industrial boilers, and refineries.
The Fresno metropolitan area in the Central Valley, meanwhile, had the worst year-round particle pollution, which comes from dust and other tiny particles (usually carbon dioxide and methane). The city of Bakersfield had the sharpest spike in particle pollution in 2019.
Particle pollution has been linked to lung and throat cancers, as well as heart attacks. A recent study found that it may also increase one's risk of brain tumors.