Short-Term Particle Pollution Trends

In the years 2019, 2020 and 2021, some 63.7 million people lived in the 111 counties that earned an F for unhealthy spikes in particulate matter air pollution. This represents close to half a million more people than in last year’s “State of the Air” report, and more people in total than in any of the last eight reports—since the current version of the Air Quality Index was adopted.  

U.S. map with 64 MM

More than 64 million Americans live in counties with F grades for spikes in daily particle pollution

Even compared with the past three 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 were again worse throughout much of the western U.S. Among those cities ranked the worst 25, the average number of days residents were exposed to high levels of fine particle pollution increased by almost 2 days, to a weighted average of 18.3 days, up from 16.5 days in last year’s report.  

Wildfires in the western U.S. are a major contributing factor to the increasing number of days and places with unhealthy levels of particle pollution. They are also increasing the severity of pollution, resulting in a sharp rise in the number of days designated as either purple or maroon. These are the levels on the Air Quality Index that carry the strongest health warnings. On purple very unhealthy days, “the risk of health effects is increased for everyone.” On maroon hazardous days, a health warning of emergency conditions is issued, saying, “Everyone is more likely to be affected.”  

In the years 2019, 2020 and 2021, the health of nearly 32 million people across 56 counties in ten states was put at risk on “purple” or “maroon” days for fine particle pollution. This is very similar to the findings in last year’s “State of the Air” and a worrisome sign of a trend that is likely to continue as climate change worsens. 

Seven of the 25 most polluted cities for this measure posted their highest-ever weighted average number of days with unhealthy levels of particle pollution. Two of those, Denver, Colorado and Fargo, North Dakota, are new on the list.  The remaining five are Visalia, California; Reno, Nevada; Yakima and Spokane, Washington; and Boise, Idaho.  

U.S. map with light blue lines along west coast

All but two of the 25 worst cities for short-term particle pollution are in the western U.S.

Twenty-one of last year’s worst 25 cities remained listed among the worst 25 in this year’s report, though their relative ranks often shifted by several places. Missoula, Montana and Lancaster, Pennsylvania both rejoined the ranks of the worst 25 cities after a short hiatus in 2022. San Luis Obispo, California; Portland, Oregon; and Seattle and Bellingham, Washington all moved off the list of worst 25 cities.  

In “State of the Air” 2023, only two cities among the 25 worst for short-term particle pollution were not in the western U.S. Both of the eastern cities, Pittsburgh and Lancaster, Pennsylvania, posted more days high in fine particle pollution in this year’s report, and remained the two worst metro areas in the country east of the Mississippi River for this pollutant measure. 

Did You Know?

  1. More than one in three Americans live where the air they breathe earned an F in “State of the Air” 2023.
  2. Nearly 120 million people live in counties that received an F for either ozone or particle pollution in “State of the Air” 2023.
  3. More than 18 million people live in counties that got an F for all three air pollution measures in “State of the Air” 2023.
  4. Breathing ozone irritates the lungs, resulting in inflammation—as if there were a bad sunburn within the lungs.
  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 lower birth weight in newborns.
  9. If you live or work near a busy highway, traffic pollution may put you at greater risk of 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.
  12. People of color and those 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 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 residents breathe.
  18. The nation has the Clean Air Act to thank for decades of improvements in air quality. This landmark law has driven pollution reduction for over 50 years.
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Page last updated: April 10, 2023