A decade of improvements in American air quality has been undermined by increasingly severe wildfires, which are driving a resurgence in harmful ozone pollution across much of the country, according to a new study.
Researchers led by the University of Iowa found that surface ozone pollution has worsened across large portions of the Midwest and Western United States since 2015, reversing years of gains achieved through stricter vehicle emissions standards and other air quality regulations.
Jun Wang, corresponding author of the study said: ‘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 US and Canada. We show in high spatial resolution how a large part of the continental US has been affected by worsened air quality through surface ozone pollution.’
The study analysed ozone concentrations across the continental United States from 2003 to 2024 using satellite observations, data from around 1,000 air-quality monitoring stations and advanced deep-learning models. It was found that surface ozone levels increased by an average of 0.13 parts per billion annually between 2015 and 2024, reversing the downward trend seen during the previous decade.
According to the study, wildfire smoke has become the primary driver of that reversal. Carbon monoxide released by burning vegetation can travel thousands of miles through the atmosphere before contributing to ozone formation far from the original fire source.
Lead author Weizhi Deng, a graduate research assistant in Wang’s research group said: ‘While US air quality regulations have reduced surface ozone, a pollutant linked to respiratory and cardiovascular diseases, this progress has reversed since around 2015. Wildfire smoke has become a major driver of increasing ozone pollution, especially in the western and midwestern United States.’
The researchers estimate that exposure to wildfire-driven ozone pollution has contributed to more than 300 additional premature deaths annually in the United States since 2013. The health impacts were particularly severe during the record-breaking Canadian wildfire season of 2023, when ozone pollution exceeded federal air-quality standards for an estimated 148 million Americans, around 44% of the continental population.
The full research, which was funded by NASA, can be read here.
Graphic: Jun Wang lab, University of Iowa
Photo: Matt Palmer
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