Download the 2020 State of the Air full report


What Are "Short-Term Levels" of Particle Pollution?

Particle pollution can be harmful even if it is inhaled over just a few hours or days, even if the year-round averages are low. "Short-term levels" refers to just such spikes. These represent levels averaged over a 24-hour period. Those days or weeks of high levels can be dangerous, even deadly.

More cities experienced more days of spikes in particle pollution, compared to the 2019 report. Twenty-two of the 25 most polluted cities had more days on average in the 2020 report. Many cities reached their highest number of such days ever reported.  

More people experienced unhealthy spikes in particle pollution than in the last three reports. More than 53.3 million people suffered those episodes of unhealthy spikes in 86 counties where they live. In the 2019 report, the total was approximately 49.6 million people who experienced too many unhealthy days; in the 2018 report, approximately 35.1 million people; and in the 2017 report, approximately 43 million people.

Why? Wildfires in 2017 and 2018, especially in California, were a main reason for many of these spikes. In the western U.S., climate change has made more likely the conditions of heat and drought that promote wildfire hazards. In some communities, wood smoke from home heating, especially when worsened by stagnant air masses known as inversions, has also contributed to high levels of particle pollution.

Nine of the ten most polluted cities had more days when particle pollution reached unhealthy levels; four of those reached their worst exposure ever recorded. Of the 25 most polluted cities, 22 had more days on average in this year’s report, with nine cities reaching their highest number of days on average ever recorded.

Fresno-Madera-Hanford, CA, returns to rank as the #1 most polluted city for short-term particle levels.  This marks the third time Fresno-Madera-Hanford has ranked at the top in this category; the last period was from 2011-2013 covered in the 2015 report. Bakersfield, which had been ranked in that spot for eight of the last ten reports, shifted to the 2nd most polluted city.

Nine cities had their highest-ever weighted average number of days with spikes in particle levels: Fairbanks, AK; Yakima, WA; Redding-Red Bluff, CA; Phoenix, AZ; Spokane-Spokane Valley-Coeur d’Alene, WA-ID; Chico, CA; Salinas, CA; Santa Maria-Santa Barbara, CA; and Las Vegas, NV.

Showing the impact of wildfires, this year’s report marks the second year that Santa Maria-Santa Barbara, CA, showed up on the list of the most polluted for short-term particle pollution. Prior to the 2019 report, this city had been on the list of cleanest cities in the nation for the previous six years for the same pollutant.

Just as people move around, so too does harmful pollution. Wildfire smoke is just one example of pollution threatening health far from the source.

Twelve other cities on the most-polluted list also suffered from more days with unhealthy levels of particle pollution. These include Bakersfield; San Jose-San Francisco; Los Angeles; Salt Lake City, UT; Sacramento; Visalia, CA; Logan, UT; Medford-Grants Pass, OR; El Centro, CA; Eugene, OR; Reno, NV; and Portland, OR.

Only 3 of the 25 most polluted cities improved and had fewer unhealthy air days on average than in the 2019 report. Though it improved from its worst performance in last year’s report, Missoula, MT, remained among the nation’s 10 most polluted cities. Two other cities on the list had fewer unhealthy days on average: Seattle and Pittsburgh.  

In California, Montana, Oregon and Washington, extended wildfires increased the days when PM levels spiked during 2016-2018. The Los Angeles metro area had two days when levels spiked to “hazardous,” the highest, “maroon” level in the Air Quality Index. The Chico, CA, metro area also recorded two hazardous days in Butte County, reaching its highest ever short-term weighted average. Eugene, OR, and rural counties Mendocino County, CA, Okanagan County, WA and Gallatin County, MT, each reached one hazardous day.

Wildfires are not the only source of high particle pollution days. Other contributing sources include wood stove use (especially in Fairbanks, AK), older diesel vehicles and equipment, and industrial sources (as in Pittsburgh, PA). Changes in weather patterns can create atmospheric inversions that trap particles in place, leading to days with spikes.

Pittsburgh is the only city in the 25 most polluted that is east of the Mississippi River.

Did You Know?

  1. Nearly 5 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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