This year’s “State of the Air” Report found that ground-level ozone pollution is getting worse – strikingly worse. This pollutant, also known as smog, makes it harder to breathe and is dangerous for your health. Between 2021 and 2023, more than 1 in 3 people living in the U.S. were exposed to unhealthy levels of ozone pollution. The areas hit hardest include some Midwest states and Plains down to Texas.
You may already know that hot temperatures lead to ozone pollution. That’s because ozone doesn’t come straight from cars, factories or any other polluting source. Instead, it forms when pollutants mix in the atmosphere and “cook” in the presence of sunlight and heat.
Extreme heat (including a deadly heat wave in Texas in 2023) was definitely a factor in dramatic rise in harmful levels of ozone pollution seen in this year’s report. But there’s also a second, surprising reason that helps explain this uptick in ozone pollution: wildfires.
We often hear how wildfire smoke causes particle pollution, but it impacts ozone pollution too. In 2023, wildfires in Canada blanketed the Upper Midwest and East Coast in smoke. This raised ozone and particle pollution levels in dozens of states higher than they have been in many years. The Upper Midwest experienced record-breaking high ozone levels between May and August. Scientists discovered that the smoke helped create extreme levels of ozone pollution even hundreds of miles downwind.
How Wildfires Affect Ozone
Wildfires produce large amounts of fine particles (PM2.5), as well as hundreds of reactive gases, including nitrogen dioxide (NO2), volatile organic compounds (VOCs) and carbon monoxide. These gases are “ingredients” that help form ozone, especially on hot and sunny days. The ingredients needed to make ozone can come from many different sources, including emissions from cars and trucks, power plants, agriculture and wildfires.
As wildfire smoke travels away from the fire, these gases react in sunlight and form ozone. In general, ozone formation increases as the smoke moves downwind. Research shows that even when smoke is days old and more than 1,000 miles away, it can still contain the necessary ingredients for ozone to form. If smoke drifts over reservoirs of NO2 pollution, such as cities, highways, railroads or ports, it is more likely to produce high ozone levels.
Why It Matters
Ozone pollution can cause serious harm to the lungs, both right away and over time. When you breathe it in, it reacts with the lining of your airways, causing inflammation and other damage. Breathing in too much ozone can be harmful – and in some cases, even deadly.
Exposure to ozone can cause immediate breathing problems like shortness of breath, wheezing and coughing, and an increased risk of respiratory infections. For people with lung diseases, like asthma or COPD, severe symptoms may require an emergency room visit. Breathing ozone over long periods (that is, for periods longer than eight hours) is linked to a range of health harms, including:
- increased respiratory illnesses,
- metabolic disorders,
- nervous system issues,
- reproductive issues, and
- increased respiratory- and cardiovascular-related mortality.
Anyone who spends time outdoors when ozone levels are high can be affected. But some people are more at risk to the effects of breathing ozone, including people who are pregnant, children and teens, anyone 65 and older, people with pre-existing medical conditions (such as asthma), people of lower socioeconomic status, and people who work or exercise outdoors.
The bottom line is - wildfires are making ozone pollution (not just particle pollution) worse. For years, ozone levels were going down across much of the country because of successful efforts to cleanup emissions from tailpipes, smokestacks, factories and other sources of pollution. But climate change is threatening to reverse that progress. Wildfires are becoming more common and more intense, putting more people at risk of unhealthy levels of pollution and undoing years of progress.
To learn more, visit Lung.org/SOTA for the full “State of the Air” report or Lung.org/wildfire for tips for protecting your health during a wildfire.
Blog last updated: May 5, 2025
