Exposure to unhealthy levels of ozone air pollution continues to make breathing difficult for more Americans across the country than any other single pollutant. In the years 2020, 2021, and 2022, some 100.6 million people lived in the 125 counties in 26 states that earned an “F” grade for ozone. This means that three of every ten people, including 22.5 million infants and children, 15.5 million people age 65 or older, and tens of millions in other groups at highest risk of health harm, are exposed to high levels of ozone on enough days to earn the air they breathe a failing grade.

more than 100 million

More than 100 million people in America live in counties with F grades for ozone smog.

Although ozone air pollution remains a serious threat to public health, one trend in this year’s “State of the Air” report is continuing in a positive direction. For the fourth consecutive report, the number of people living in counties with a failing grade for ozone declined, this year by 2.4 million people. 

Ambient ozone levels are influenced by a complex interaction of natural and human-driven factors that can vary from year to year, with some years being better overall than others. However, the long-term trend of improvement in ozone levels can be attributed to the fact that the Clean Air Act has been working. Controls placed on emissions have increasingly resulted in the replacement of more polluting engines, fuels, and industrial processes nationwide. The transition of the economy away from coal-fired power plants, the dirtiest fossil fuel, and towards clean renewable sources of energy, has unquestionably had an impact, especially in parts of the eastern United States.

Snapshot from the 2000 "State of the Air" report

When the first “State of the Air” report was released in 2000, with ozone data from 1996, 1997 and 1998, 332 counties in 38 states earned an F grade, putting the health of 132.5 million people at risk. The air quality standard was weaker than it is now, meaning that in fact many more people were breathing unhealthy air. If the current standard had been in effect at that time, 463 counties in 43 states would have gotten failing marks. Of the counties where data was being collected, only 9 would have received an A.

In many parts of the country, the weighted average number of days of unhealthy ozone has declined substantially over the years. It is worth noting that the severity of ozone pollution has also dropped. In 2001, the “State of the Air” report recorded a 25-year high of 1,563 very unhealthy Purple days for ozone around the country. In 2024, that number has dropped to a 25-year low of 71 days. That is still 71 days too many, but an important indicator of progress. 

In spite of the promising trend, some trouble signs appear in this year’s report, and time will tell whether they are the result of expected fluctuations or more systemic threats. Nationwide, nearly twice as many counties worsened as improved. The weighted average number of unhealthy ozone days of all counties taken together increased by about 6%. And although the number of counties earning “F” grades remained nearly unchanged from the 2023 report, those earning “A” grades dropped by more than 10%, from 302 to 270. 

The list of 25 cities with the worst ozone pollution in “State of the Air” 2024 and their order of ranking remained remarkably stable compared with last year’s report. The largest change in rank was only 5 places as Dallas-Fort Worth worsened from 18th to 13th worst. This year, Hartford Connecticut was the only city that improved enough to move off the worst 25 list. It was replaced by Grand Rapids, Michigan, which earned a spot on the list for the first time in 8 years. None of the cities on the list reported a worst-ever average number of days of ozone smog. Two cities, Fresno and El Centro, California recorded their fewest-ever number of unhealthy days for ozone, though they still earned “F” grades.

The geographical distribution of cities with the worst ozone problems confirms a pattern seen over nearly a decade of reporting: the great majority are western cities. Cities in the West and the Southwest continue to dominate the list of the most ozone-polluted cities. California retains its position of being the state with the most metro areas on the list, with 10 of the 25 most-polluted cities, while the six states of Arizona, Colorado, Nevada, New Mexico, Texas, and Utah accounted for 12 others. They are joined this year by only three more easterly cities, New York, Chicago, and Grand Rapids.

Although cleanup of ozone precursor pollutants has been working to reduce ozone concentrations, the impact of climate change in the West has meant higher temperatures, dry, sunny skies and more frequent stagnation events that are contributing to the number of unhealthy ozone days being higher than it would otherwise be. Simply, climate change is undercutting the progress we would have made.

Mike Nelson, chief meteorologist at Denver7, goes beyond the seven-day forecast to educate viewers on climate issues. Alongside the daily high temperature and precipitation, his weather graphics include the Denver area’s air quality rating and the planet’s current amount of carbon dioxide, the heat-trapping gas that’s a major contributor to global warming. 

He says on-air meteorologists have a special responsibility to talk about climate change.  

“The television meteorologist is as close to a scientist as most Americans are going to get. People invite us into their homes every single day to explain something complicated – the weather,” Nelson says. “Why would we shy away from talking about the most important thing we face globally?” 

Nelson has been forecasting weather in Colorado for more than 30 years. During that time, the Clean Air Act and other regulations have helped improve air quality, he says. But he notes that the increase in large wildfires across the West in recent years is exacerbating particulate matter pollution. A hotter, drier climate is helping to fuel these more intense wildfires and days with heavy smoke, Nelson says. 

Although related, climate and weather aren’t the same. Climate is the long-term average of conditions, whereas weather refers to short-term changes and is often fast-changing. Nelson uses a football analogy: “Climate is the history of the National Football League; weather is one play in a game.” 

Nelson says the birth of his first grandchild 11 years ago pushed him to speak more about climate implications. Today’s children and future generations are most at risk if we don’t reduce carbon emissions, he notes. 

“We can still fix it,” Nelson says, “but the clock is ticking, and we should get going on it at a faster pace.” 

Mike Nelson
Chief Meteorologist at Denver7 

Did You Know?

  1. Nearly four in ten people in the U.S. live where the air they breathe earned an F in “State of the Air” 2024.
  2. More than 131 million people live in counties that received an F for either ozone or particle pollution in “State of the Air” 2024.
  3. Nearly 44 million people live in counties that got an F for all three air pollution measures in “State of the Air” 2024.
  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 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. Particle pollution exposure from wildfire smoke harms health in ways that range from mild irritation to serious illness and premature death.
  20. Recent updates to the Air Quality Index give the public more accurate information about the health risk from air pollution, and when to take measures to protect themselves on bad air days.
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