The answer depends on which pollutant you’re measuring. For year-round fine particle pollution (PM2.5), Bakersfield, California, and other San Joaquin Valley cities consistently rank among the worst in the country. For ozone, or smog, the Los Angeles-Long Beach metropolitan area holds the number one spot out of 226 metro areas tracked by the American Lung Association. In 2023, IQAir’s World Air Quality Report named Beloit, Wisconsin, as the most polluted U.S. city overall and Columbus, Ohio, as the most polluted major city, largely driven by wildfire smoke and industrial emissions during that particular year.
Why Rankings Change Year to Year
Air quality rankings shift depending on which pollutant is measured, how the data is collected, and what happened that year. A city downwind of a major wildfire season can leap to the top of the list one year and drop off the next. That’s why Beloit, Wisconsin, and Columbus, Ohio, showed up in the 2023 IQAir report despite not being traditional pollution hotspots. Wildfire smoke from Canadian fires blanketed much of the Midwest and Northeast that summer, temporarily spiking fine particle levels far above normal.
The American Lung Association’s annual “State of the Air” report takes a longer view, averaging data across multiple years to identify cities with chronic pollution problems. That approach tends to keep the same California cities near the top of the list year after year, because their air quality problems are driven by geography, climate, and population density rather than one-off events.
California Dominates the Chronic Pollution Lists
California cities occupy most of the top spots for both ozone and particle pollution in the American Lung Association’s rankings. The Los Angeles-Long Beach-Riverside metro area ranks first in the nation for high ozone days and receives an “F” grade. Bakersfield, Fresno, and Visalia in the San Joaquin Valley consistently rank among the worst for year-round particle pollution.
The San Joaquin Valley is essentially a massive bowl. The Sierra Nevada mountains to the east and the Coast Ranges to the west trap air pollutants in the valley, preventing them from dispersing. The EPA has flagged this region specifically, noting that the valley “has some of the nation’s worst air quality, failing to meet federal health standards for both ozone and particulate pollution.” Temperature inversions, where a layer of warm air sits on top of cooler air near the ground, are common in winter and act like a lid, holding pollution close to the surface for days or weeks at a time.
Los Angeles faces a similar but distinct problem. Emissions from millions of vehicles and industrial sources get trapped against the San Gabriel Mountains by onshore breezes. Intense sunlight then bakes those emissions into ozone, the primary ingredient in smog. The combination of traffic volume, sunshine, and mountain-backed geography makes the LA basin uniquely prone to ozone buildup.
Ozone vs. Particle Pollution
These are the two major types of outdoor air pollution that affect health, and they behave differently. Ozone forms when vehicle exhaust and industrial emissions react with sunlight. It’s worst on hot, sunny afternoons and peaks in summer. You can’t see it directly, but it irritates the lungs and worsens asthma. Los Angeles leads the nation in this category because of its combination of heat, traffic, and geography.
Particle pollution, or PM2.5, refers to tiny particles less than 2.5 micrometers across, small enough to pass deep into the lungs and even enter the bloodstream. Sources include vehicle exhaust, power plants, industrial operations, agriculture, and wildfire smoke. In February 2024, the EPA tightened the annual PM2.5 standard from 12.0 micrograms per cubic meter down to 9.0 micrograms per cubic meter, meaning more cities now fall short of federal health standards. San Joaquin Valley cities regularly exceed this threshold, and wildfire seasons are pushing previously clean cities above it as well.
Cities Most Affected by Wildfire Smoke
Wildfire smoke has reshaped the air quality map in the past decade. Cities in Oregon, Washington, and Northern California now experience sharp spikes in PM2.5 during fire season that can push daily readings into the “hazardous” range, levels comparable to the most polluted cities in South Asia. Portland, Seattle, and Sacramento have all recorded multi-day stretches where residents were advised to stay indoors.
The 2023 Canadian wildfire season demonstrated that this isn’t just a West Coast issue. Smoke drifted across the Great Lakes and into the Northeast, giving cities like New York, Detroit, and Columbus some of their worst air quality days on record. These events are becoming more frequent. A city that historically had clean air can now experience dangerous pollution levels for days at a time during fire season.
What This Means for Where You Live
If you’re evaluating air quality for health reasons or a potential move, the single-worst-city ranking matters less than the type of pollution and how often it occurs. A city with chronically elevated PM2.5, like Bakersfield, poses different risks than a city that spikes during wildfire events but has clean air the rest of the year. Chronic exposure to fine particle pollution is linked to heart disease, lung cancer, and shortened life expectancy. Short-term spikes are more likely to trigger asthma attacks and respiratory infections.
You can check real-time air quality for any U.S. location on AirNow.gov, which reports both ozone and PM2.5 levels on a color-coded scale. For a longer-term picture, the American Lung Association’s “State of the Air” report lets you look up individual metro areas and counties, including letter grades for each pollutant category. If your area receives an “F” for either ozone or particle pollution, that means unhealthy air days are a regular occurrence rather than a rare event.