Ozone pollution has worsened in much of the continental United States over the past decade, fueled by wildfires and the long-distance transport of unhealthy air, according to a new study led by University of Iowa researchers and published in the journal Science.
Exposure to surface ozone pollution stemming from wildfires has contributed to more than 300 additional premature deaths each year in the U.S. since 2013, the researchers conclude, with significant spikes in premature deaths in 2020, 2021, and 2023.
Study authors calculated the concentration of surface ozone, also known as smog, on a kilometer-by-kilometer (0.6 miles) grid for the continental U.S between 2003 and 2024. The researchers report that air quality due to ozone pollutants worsened from 2015 to 2024 across much of the Midwest and large swaths of the Western U.S.
The researchers also found that the increase in wildfire-driven ozone pollution over the past decade has undercut air-quality improvements achieved before 2015, largely through reductions in automobile emissions, which are a main contributor to surface ozone.
The study comes as wildfire activity in the U.S. has reached historic levels this spring, with blazes charring millions of acres from California to Georgia.
Surface ozone is caused by a chemical reaction involving carbon monoxide, an odorless, colorless gas emitted when wildfires don't fully combust organic matter, such as trees. Surface ozone can form near fires when nitrogen oxide combines with carbon monoxide and sunlight.
But smog also can form far away from wildfires when carbon monoxide rises into Earth's upper atmosphere and travels long distances before falling to the surface and undergoing chemical reactions involving sunlight and sources such as tailpipe emissions.
"The bottom line is the air is getting worse in these regions, and the reason is pollutants are being transported long distances from wildfires in the western U.S. and Canada," says Jun Wang , Lichtenberg Family Chair in the Department of Chemical and Biochemical Engineering and the study's corresponding author. "We show in high spatial resolution how a large part of the continental U.S. has been affected by worsened air quality through surface ozone pollution."
Surface ozone levels in the U.S. have increased by 0.13 parts per billion (ppb) annually between 2015 and 2024, the researchers calculated. That reverses annual decreases in surface ozone levels since 2003, due in part to tightened federal regulations around tailpipe emissions. The study authors report that surface ozone levels would have continued to decrease during the study period "if fire impacts were removed, suggesting that fires are the major driver of the national trend reversal."
"While U.S. air quality regulations have reduced surface ozone, a pollutant linked to respiratory and cardiovascular diseases, this progress has reversed since around 2015," says Weizhi Deng , a graduate research assistant in Wang's group who led the modeling linking wildfires and surface ozone pollution and is the study's first author. "Wildfire smoke has become a major driver of increasing ozone pollution, especially in the western and midwestern United States."
The Canadian wildfires in 2023 were especially noteworthy for their effects on surface ozone pollution. During that fire season alone, surface ozone levels exceeded federal air quality standards for 148 million Americans — 44% of the continental U.S. population, the authors write. In the Midwest, surface ozone exceeded safe levels for more than one week, and the pollution extended as far as New York, Texas, and Georgia.
The pollutants had discernible effects on human health: The researchers calculated 7,974 smog-induced premature deaths in the U.S. in 2023.
"These results underscore the escalating public health burden of wildfire-driven ozone pollution," the study authors write.
While the Environmental Protection Agency sets the national standard for surface ozone , it falls largely to states and cities to address unhealthy air in their communities. The problem with that, Wang notes, is those entities have little control over foul air that originates from wildfires far away.
"When air quality is poor — even when the pollution is from elsewhere — the responsibility is on the local or state authority to collect the evidence and then file an 'exceptional event claim' to the EPA," says Wang, who is associate director of the Iowa Technology Institute. "That could be fine, but those exceptional events are not becoming exceptional anymore."
The researchers derived surface ozone concentrations and premature deaths estimates from satellite data and approximately 1,000 ground-based stations that monitor air quality. The data from surface stations can be extensive, but it does not provide complete spatial coverage and can be lacking in rural areas. So, the researchers employed "deep learning," which enables computer systems to cluster data and produce accurate predictions, to calculate the surface ozone concentrations. They calculated premature deaths through a formula that incorporated average lifespan, ozone exposure, and population density.
The results in part mirror a 2023 study , led by Wang and published in the journal The Lancet Planetary Health, that tabulated wildfires' effect on air quality and human health in the continental U.S. through the rise in black carbon, a fine-particle air pollutant that has been linked to respiratory and heart disease.
"While there are regional differences, in general the impact of surface ozone is always bigger than black carbon," Wang says.
The study, "Fires reverse progress toward ozone air quality standards in the USA," was published online June 4.
The study's co-corresponding author is Meng Zhou, from NASA Goddard Space Flight Center. Other authors are Xi Chen, Huanxin Zhang and Xiaodong Wu from Iowa; Jason Cohen, based in New Brunswick, New Jersey; Jing Wei, from the University of Maryland- College Park; Arlindo da Silva, from NASA Goddard Space Flight Center; Guy Brasseur, from the U.S. National Center for Atmospheric Research and the Max Planck Institute for Meteorology, in Germany; Claire Granier, from the Université de Toulouse, in France and the University of Colorado-Boulder and the NOAA Chemical Sciences Laboratory; and Laurence Rouil, from the European Centre for Medium-Range Weather Forecasts, in Bonn, Germany.
NASA funded the research.
