Six of the ten U.S. cities with the worst air pollution are in Southern California, report says

A new report shows that some of the nation's most polluted cities are in Southern California, four of which call the Inland Empire home. 

The study, conducted by IQAir, analyzed the fine particulate matter concentration over cities across the United States, revealing that Ontario was ranked as the worst, followed closely by Bloomington, Huntington Park, San Bernardino and Fontana, which round out the top five. Glendora was ranked No. 8 on the list, while two other central California cities — Visalia and Hanford — were also in the top ten. 

Fine particulate matter, which is also known as PM2.5, can cause serious health risks when inhaled, including lung and heart disease and reproductive issues. According to the Environmental Protection Agency, PM2.5 can come from a variety of sources, including cars, construction sites, unpaved roads, smokestacks or fires. 

It's likely a combination of these factors that helped create such a high level of pollution over much of Southern California, especially when one considers the daily bumper-to-bumper traffic all over freeways. 

"We have so many vehicles, trucks, cars and buses," said Billl Magavern, a Policy Director at the Coalition for Clean Air who says that much of Los Angeles' pollution lands in Riverside and San Bernardino Counties because of the wind blowing from the west. According to IQAir's report, Los Angeles has the highest fine particulate matter annual average of the 25 most populated U.S. cities.

That traffic includes the shipping and logistics industry. Cargo containers arrive in Southern California ports from ships burning polluting fuels, before they're loaded onto diesel trucks and shipped to warehouses in the Inland Empire.

"Those warehouses are essentially magnets for diesel pollution," Magavern said. 

He added that the pollution levels are likely even worse in the wake of the devastating wildfires that raged across Los Angeles County in early-2024, with thousands of buildings and materials getting torched in the process, some of which include toxic metals and products that aren't measured by fine particulate matter testing. 

The report nearly mirrors another study released last year, which ranked eight of the nations' 50 "dirtiest cities" In Southern California. That report took factors like living conditions, city infrastructure, pollution and consumer satisfaction into account