Wildfire Smoke Has Erased Years of U.S. Ozone Gains, NASA-Supported Study Finds

Wildfire smoke has substantially worsened ground-level ozone pollution across much of the contiguous United States over the past decade, wiping out years of air-quality gains even in places far from active fires, according to a NASA-supported study published Thursday in Science.

The finding shifts attention to a less visible health threat from wildfires. Smoke is often discussed for its fine particle pollution, but this study focused on surface ozone, a federally regulated pollutant linked to respiratory and cardiovascular harm. Unlike soot, ozone is not emitted directly. It forms in sunlight from precursor gases such as nitrogen oxides, volatile organic compounds and carbon monoxide, including those produced by combustion from wildfires. Because smoke can travel long distances, the ozone effects can reach communities well beyond fire zones.

NASA said researchers created what it described as a first-of-its-kind daily surface ozone dataset for the contiguous U.S. at roughly 1-kilometer resolution spanning 2003 through 2024. The team used deep learning to combine data from about 1,000 ground monitors with atmospheric models, weather information, wildfire pollution data and satellite products including VIIRS and MODIS. Corresponding author Jun Wang is at the University of Iowa, according to NASA. In separate coverage Thursday, The Associated Press identified Weizhi Deng of the University of Iowa as the study’s lead author and reported that average U.S. ground-level ozone has increased by about 4% since 2015.

Nationally, NASA said, wildfire impacts “offset nearly four years’ worth of ozone-control gains.” The setbacks were larger in the West and Midwest. In the Midwest, the agency said, ozone likely would have continued declining without wildfire contributions, but fires instead “erased about 5.3 years’ worth of ozone-control progress since 2015.”

The study also tied those pollution changes to measurable health consequences. NASA said the researchers estimated that premature deaths associated with long-term wildfire-related ozone exposure in the U.S. increased by 318 deaths per year after 2013. The post-2013 average was 46% higher than in the previous decade, according to the agency. From 2022 through 2024, NASA said, wildfires exposed an additional 43 million people in the U.S. to air that did not meet current federal ozone standards.

The results add to evidence that wildfire pollution is reshaping air quality far from the flames. For roughly two decades, U.S. ozone levels had generally declined because of tighter controls on emissions from vehicles, power plants and other sources. This study says that since around 2015, wildfire smoke has increasingly eroded that progress.

That matters because ozone is a regional pollutant as well as a local one. The gases that help create it can be carried across state lines in wildfire plumes, then react in sunlight days later and hundreds of miles away.

“People in the Midwest may think fires burning far away will not affect them,” Wang said in NASA’s news release. “But once wildfire pollution is in the air, it can move across regions. Pollution from one place can affect air quality in another.”

NASA said its Earth-observing tools can help public health and air-quality officials track that risk as smoke affects more communities. “NASA Earth observations, along with ground monitoring networks, help reveal air quality risks from wildfires that can cross state lines, giving air quality managers better decision-making information as wildfire smoke affects more communities,” said John Haynes, manager of NASA Earth Action’s Health and Air Quality program.