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Deadly short-term particle pollution continues to impact communities in many parts of the country. The found that 77.2 million people lived in counties that experienced unhealthy spikes in particle pollution. This is the highest number in the last 16 years of the report.

Fine particulate matter air pollution—also known as PM2.5, particle pollution, or soot—comes from wildfires, wood-burning stoves, coal-fired power plants, diesel engines, and other sources. The ALA report has two grades for particle pollution: one for “short-term” particle pollution, or daily spikes, and one for the annual average “year-round” level that represents the concentration of particles in each location.

The number and severity of unhealthy spikes in particle pollution improved slightly in the western states but worsened in the Midwest and Northeast. In the three years covered by this report, individuals in the U.S. experienced the highest number of days when particle pollution reached  levels in the 26 years of reporting the “State of the Air.” This year’s report includes data from the summer of 2023, when smoke from wildfires in Canada significantly impacted midwestern and eastern states, resulting in worse particle pollution.

Top 10 cities most polluted by short-term particle pollution:

1. Bakersfield-Delano, California
2. Fairbanks-College, Alaska
3. Eugene-Springfield, Oregon (tied for 3rd)
4. Visalia, California (tied for 3rd)
5. Fresno-Hanford-Corcoran, California
6. Reno-Carson City-Gardnerville Ranchos, Nevada-California
7. Los Angeles-Long Beach, California
8. Yakima, Washington
9. Seattle-Tacoma, Washington
10. Sacramento-Roseville, California

Year-round pollution

This year’s report reveals that 85 million people lived in a county that received a failing grade based on the nation’s standard for year-round levels of particle pollution. This is the second largest number in the report’s history—after the 90.7 million posted in last year’s report.

The top 10 cities most polluted by year-round particle pollution include:

1. Bakersfield-Delano, California
2. Visalia, California
3. Fresno-Hanford-Corcoran, California
4. Eugene-Springfield, Oregon
5. Los Angeles-Long Beach, California
6. Detroit-Warren-Ann Arbor, Michigan (tied for 6th)
7. San Jose-San Francisco-Oakland, California (tied for 6th)
8. Houston-Pasadena, Texas
9. Cleveland-Akron-Canton, Ohio
10. Fairbanks-College, Alaska

Ozone pollution
Ground-level ozone pollution, also known as smog, is a respiratory irritant with effects likened to a sunburn of the lungs. Inhaling ozone can cause shortness of breath, trigger coughing and asthma attacks, and may shorten life. Warmer temperatures driven by climate change make ozone more likely to form and harder to clean up.

After years of progress on cleaning up ozone, some communities are seeing the worst ozone levels in years. More than 125 million people (37% of the nation’s population) lived in an area with unhealthy ozone pollution, which is 24.6 million more than last year’s report. Extreme heat and wildfires contributed to the increase in ozone levels for many parts of the country, most notably in central states from Minnesota to Texas.

Top 10 cities most polluted by ozone pollution:

1. Los Angeles-Long Beach, California
2. Visalia, California
3. Bakersfield-Delano, California
4. Phoenix-Mesa, Arizona
5. Fresno-Hanford-Corcoran, California
6. Denver-Aurora-Greeley, Colorado
7. Houston-Pasadena, Texas
8. San Diego-Chula Vista-Carlsbad, California
9. Salt Lake City-Provo-Orem, Utah-Idaho
10. Dallas-Fort Worth, Texas-Oklahoma

Cleanest cities
The report also recognizes the nation’s cleanest cities. To make the cleanest list for all three measures, a city must experience no high ozone or particle pollution days and rank among the 25 cities with the lowest year-round particle pollution levels. This year, only two cities made the cut: Bangor, Maine, and San Juan, Puerto Rico. This reflects an overall worsening of air quality across the country. Last year, five cities made the list.

The State of the Air report relies on data from air quality monitors managed by state, local, and tribal air pollution control authorities in counties across the U.S. Regrettably, out of 3,221 counties in the U.S., only 922 counties can monitor for at least one pollutant. More than 72.8 million people live in counties where neither their ozone nor their particle pollution levels are being monitored.