Short-Term Particle Pollution Trends

In the years 2017, 2018 and 2019, close to 54.4 million people lived in the 88 counties that experienced unhealthy spikes in particulate matter air pollution. This represents a million more people than in last year’s “State of the Air”, and higher numbers than in any of the last five reports.

U.S. map with 54 MM

Over 54 million Americans live in counties with F grades for spikes in daily particle pollution

Many cities reached their highest number of days with unhealthy levels of particle pollution ever reported. Of the 25 most-polluted cities, 5 of them, including worst-ranked Fairbanks, posted their highest-ever average number of days with spikes in particle levels – for the second year in a row.

The list of the 25 worst cities for short-term particle pollution is very similar to last year’s report, with minor shifts in rank-order. The exceptions are Salt Lake City, which improved dramatically from 7th worst to 17th worst; Salinas, California, which improved enough to be removed from the list; and Lancaster, Pennsylvania which returned to the list for the first time since the 2018 report (see Figure 2).

Thirteen of the 25 most-polluted cities improved and had fewer unhealthy air days than in the 2020 report. However, as a general rule, improvements were modest, and all of these areas remained seriously polluted. For example, Bakersfield, though the only city among the worst 25 that improved to its best ever, nevertheless ranked third worst in the nation.

25 cities most polluted by 24-hr PM

In “State of the Air” 2021, all but two of the 25 worst cities for short-term particle pollution are in the western U.S., with 10 in California, 8 in the Pacific Northwest and 5 in the Southwest. Only two cities are in the eastern U.S. This continues a shifting geographic trend being driven in large part by the increasing number and size of wildfires resulting from climate change-induced heat and drought.

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.

Because of significant wildfires in 2019, Fairbanks, AK recorded three days when levels spiked to hazardous, the highest “maroon” level in the Air Quality Index. In California, the Los Angeles-Long Beach and Chico metro areas each had two maroon days, and three other cities in California, Oregon and Washington posted one. In the three years covered by “State of the Air” 2021, 60 very unhealthy “purple” air quality days were recorded in 33 counties in 8 states, affecting 16.3 million people. This is nearly twice as many very unhealthy days as occurred in the period from 2015 to 2017, and six times as many as from 2014 to 2016.

Wildfires, however, are not the only source of high particle pollution days. Other sources including wood stove use, older diesel vehicles and equipment, and industrial sources contribute to significant particle pollution. Changes in weather patterns can create atmospheric inversions that trap particles in place, leading to days with spikes.


Did You Know?

  1. More than 4 out of 10 people live where the air they breathe earned an F in State of the Air 2020.
  2. 150 million people live in counties that received an F for either ozone or particle pollution in State of the Air 2020.
  3. More than 20.8 million people live in counties that got an F for all three air pollution measures in State of the Air 2020.
  4. Breathing ozone irritates the lungs, resulting in something like a bad sunburn within the lungs.
  5. Breathing in particle pollution can increase the risk of lung cancer, according to the World Health Organization.
  6. Particle pollution can also cause early death and heart attacks, strokes and emergency room visits for people with asthma and cardiovascular disease.
  7. Particles are 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. Do you live near, or work on or near a busy highway? Pollution from the traffic 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 infants, older adults and people with lung diseases like asthma.
  12. People of color and those earning lower incomes are often disproportionately affected by air pollution that put them at higher risk for illnesses.
  13. Air pollution is a serious health threat. It can trigger asthma attacks, harm lung development in children, and can even be deadly.
  14. You can protect your family by checking the air quality forecasts in your community and avoiding exercising or working outdoors when the unhealthy air is expected.
  15. Climate change enhances conditions for ozone to form and makes it harder to keep ozone from forming.
  16. Climate change increases the risk of wildfires that spread particle pollution and ozone in the smoke.
  17. This Administration is trying to roll back or create loopholes in core healthy air protections under the Clean Air Act. The Lung Association opposes these actions that will add pollution to the air we breathe.
  18. Cutting air pollution through the Clean Air Act will prevent at least 230,000 deaths and save $2 trillion annually by 2020.
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Page last updated: June 3, 2021