Short-Term Particle Pollution Trends

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In the years 2021, 2022 and 2023, there were 77.2 million people living in counties across the U.S. that earned an F grade for unhealthy spikes in particle pollution. This represents an increase of 12.1 million more people than in last year’s report, the seventh straight year of increasing health threat from this deadly pollutant.

cirlce with 77 MM

77 million people live in counties with F grades for daily particle pollution

Even compared with the past several years of “State of the Air” reports—in which many cities and counties experienced their highest weighted average number of days ever reported for fine particle pollution—results this year are again worse. In “State of the Air” 2025, 154 counties in 27 states get failing grades for short-term particle pollution. This is 44 more counties and 8 more states, plus Washington DC, than in last year’s report. Although 27 counties in the West, including 17 counties in California, improved enough to get passing grades this year, those improvements are more than offset by the 68 counties that have been added, many of them in the Midwest and East. Connecticut, DC, Georgia, Illinois, New York, Ohio, South Dakota, Virginia and Wisconsin are all represented on the F list for the first time in years.

Wildfire has clearly emerged as a major driving factor in determining where in the country people are being exposed to unhealthy spikes in particle pollution. As states and counties experience shifting conditions of heat and precipitation – “good fire years” and “bad fire years” – their levels of air pollution can vary significantly. For example, compared to the disastrous 2020 fire year in California, the three years included in “State of the Air” 2025 were relatively better in the state, allowing counties like Santa Barbara and Marin to go from an F to an A grade in this year’s report. In contrast, smoke from extensive wildfires in Canada in 2023 resulted in extremely high levels of fine particle pollution on many days throughout parts of the northeastern and north central U.S. that have not historically been thought of as “fire country.”

Wildfires are also continuing to increase the severity of pollution, with smoke in eastern states resulting in this report’s highest ever number of red and purple days for particle pollution (1,280 and 231 days, respectively). These are levels on the Air Quality Index that carry strong health warnings. On red Unhealthy days, not only are members of sensitive groups likelier to “experience more serious health effects,” but also “some members of the general public may experience health effects.” On purple Very Unhealthy days, “the risk of health effects is increased for everyone.”

There were also 27 maroon Hazardous days, the highest category, days on which a health warning of emergency conditions is issued, saying, “Everyone is more likely to be affected.” Although this is fewer maroon days than in “State of the Air” 2023 and 2024, it is a sharp change from the zero maroon days reported from 2004 to 2016.

This year’s report finds that the health of 56.3 million people across 140 counties in 25 states was put at risk on severely polluted Very Unhealthy (purple) and Hazardous (maroon) days for fine particle pollution. This is 24 million more people than in last year’s report. This is drastically worse than the findings in last year’s “State of the Air” and a shocking demonstration of a trend that not only is continuing but worsening as a consequence of climate change.  

In better news, comparing cities ranked the worst 25 in last year’s report with those in this year’s, the average number of days per year that residents were exposed to high levels of fine particle pollution decreased by about three days. (However, it was to a still seriously poor weighted average of 16.5 days.) All but one of the ten worst cities on the list improved in this year’s report, including Bakersfield, California, which experienced a weighted average of 17.5 fewer bad air days in 2021-2023 for spikes in particle pollution. The exception was Visalia, California, which recorded its highest level of particle pollution spikes in the history of the report – for the third year in a row.

As a result of the geographic shifts in high levels of particle pollution, eight of last year’s Worst 25 cities have been replaced in this year’s report. Medford, Oregon and Lancaster and Pittsburgh, Pennsylvania rejoined the list after a one-year hiatus. Worsened air quality in Indianapolis, Indiana; Detroit, Michigan; and Bismarck, North Dakota led to them being added to the list. Helena, Montana and Minot, North Dakota, both newly designated Metropolitan Statistical Areas in 2024, join the list for the first time, though Helena’s air quality would have put it among the worst 25 in last year’s report had it been classified as a metro region.

Improved enough to leave the Worst 25 list this year are the western cities of Phoenix, Arizona; Chico, Salinas, and San Diego, California; Denver, Colorado; Boise City, Idaho; Las Vegas, Nevada and Portland, Oregon.

Did You Know?

  1. Nearly half of the people in the U.S. live where the air they breathe earned an F in “State of the Air” 2025.
  2. More than 156 million people live in counties that received an F for either ozone or particle pollution in “State of the Air” 2025.
  3. More than 42 million people live in counties that got an F for all three air pollution measures in “State of the Air” 2025.
  4. Breathing ozone irritates the lungs, resulting in inflammation—as if your lungs had a bad sunburn.
  5. Breathing in particle pollution can increase the risk of lung cancer.
  6. Particle pollution can cause early death and heart attacks, strokes, and emergency room visits.
  7. Particles in air pollution can be smaller than 1/30th the diameter of a human hair. When you inhale them, they are small enough to get past the body's natural defenses.
  8. Ozone and particle pollution are both linked to increased risk of premature birth and lower birth weight in newborns.
  9. If you live or work near a busy highway, traffic pollution may put you at greater risk of health harm.
  10. People who work or exercise outside face increased risk from the effects of air pollution.
  11. Millions of people are especially vulnerable to the effects of air pollution, including children, older adults, and people with lung diseases such as asthma and COPD.
  12. People of color and people with lower incomes are disproportionately affected by air pollution that puts them at higher risk for illness.
  13. Air pollution is a serious health threat. It can trigger asthma attacks, harm lung development in children, and even be deadly.
  14. You can protect yourself by checking the air quality forecast in your community and avoiding exercising or working outdoors when unhealthy air is expected.
  15. Climate change enhances conditions for ozone pollution to form and makes it harder to clean up communities where ozone levels are high.
  16. Climate change increases the risk of wildfires whose smoke spreads dangerous particle pollution.
  17. Policymakers at every level of government must take steps to clean the air their constituents breathe.
  18. The nation has the Clean Air Act to thank for decades of improvements in air quality. This landmark law has successfully driven pollution reduction for over 50 years.
  19. The U.S. Environmental Protection Agency is critical for cleaning up air pollution. EPA’s staff ensure that air pollution is monitored, write sound rules to clean it up, and make sure those rules are enforced.
  20. The U.S. Environmental Protection Agency is under threat. Despite EPA’s lifesaving role in protecting people’s health from air pollution, big staffing and funding cuts are endangering their work.
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Page last updated: March 19, 2025