Exposure to unhealthy levels of ozone air pollution continues to make breathing difficult for millions of Americans all across the country. In the years 2017, 2018 and 2019, more than 123.2 million people lived in the 163 counties that earned an F for ozone. That is fewer than in the 2020, 2019 and the 2018 reports, but more than in the 2017 report. There are still a lot of vulnerable people, including 28.1 million children and 18.2 million people age 65 or older, exposed to ozone air pollution and increased risk of harm.

3 out of 8 Americans live in counties with F grades for ozone

3 out of every 8 Americans live in counties with F grades for ozone.

The list of 25 cities with the worst ozone pollution in “State of the Air” 2021 remains the same as in last year’s report, although a few moved up or down a bit in the rankings (see Figure 1). Cities in the West and the Southwest continue to dominate the most-ozone-polluted list. California retains its historic distinction of having the most cities on the list, with 10 of the 25 most-polluted cities. The Southwest fills most of the remaining slots, with eight of the 25 cities on the list. Only seven of the worst 25 cities are east of the Mississippi River. Once again this year, no metropolitan areas in the Southeast rank among the 25 worst cities most polluted by ozone.

Overall, the 25 most ozone-polluted cities in the U.S. experienced fewer bad air days on average from 2017 to 2019 than did those in last year’s report covering 2016 to 2018. Four California cities on the list recorded their fewest bad ozone days on average in the report’s 22-year history, although they are still among the ten most ozone-polluted cities in the nation.

The geographical distribution of cities with the worst ozone problems confirms a trend seen over the past five reports: fewer Eastern cities and more Western cities. The increased oil and gas extraction in the Southwest and the cleanup of power plants in the East have shifted the cities that experienced the greatest number of unhealthy ozone days. However, there are still problems in the East with transported pollution when ozone and ozone precursors enter from upwind sources in the Midwest. For example, Fairfield County, Connecticut is the county with the highest ozone in the eastern half of the nation, in part because of pollution transported from other states.

Thermometer with arrow pointing up

The three years covered by State of the Air 2021 ranked among the six hottest years on record globally.

Heat also makes a difference. High temperatures again played a major role in the number of unhealthy ozone days. The previous three years’ reports included data from 2016, which remains the hottest year on record globally. Although the years 2017, 2018 and 2019 included in “State of the Air” 2021 were somewhat cooler, they still rank among the six hottest years on record globally, showing the strong impact of warmer temperatures on air quality. Climate change is driving warmer temperatures, which make ozone pollution more likely to form and harder to clean up.


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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  1. NOAA National Centers for Environmental Information, Global Climate Report--Annual 2017, published online January 2018. Accessed at https://www.ncdc.noaa.gov/sotc/global/201713