Wildfire smoke has undone more than a decade of regulatory progress on ozone pollution, according to a study published Thursday in the journal Science. Ground-level ozone — a lung-damaging gas formed when pollutants from vehicles, refineries, and industrial sources react with sunlight — had been declining steadily as the US cut emissions of its chemical precursors. Since 2015, that trend has reversed.
“We found that despite regulated reductions in anthropogenic emissions of O3 precursors, observation stations indicate that policy-relevant surface O3 levels have plateaued,” wrote authors led by Weizhi Deng. The plateau, they said, is driven by wildfire smoke that emits carbon monoxide and other gases that contribute to ozone formation hundreds of miles from the fire itself.
The researchers developed a novel dataset by combining satellite observations, EPA air quality monitor readings, and meteorological data with deep learning models. Ground-based monitors in the continental US, they noted, cover only 2% of land area, leaving vast regions invisible to conventional measurements.
Ozone concentrations fell by an average of 0.65 parts per billion per year before 2015. After that year, they began rising by 0.13 ppb annually — a net reversal that the study attributes to wildfire emissions. The rise is correlated with approximately 318 excess premature deaths each year since 2013, the authors estimated.
The findings add to a growing body of evidence linking wildfire smoke to serious public health consequences. Wildfire smoke contains PM2.5 — microscopic particles that lodge deep in the lungs and enter the bloodstream — which has been tied to heart and lung disease, cancer, and premature death. A 2024 study found that more than 50,000 people in California died prematurely over a decade due to wildfire smoke PM2.5 exposure. Research published last fall projected that by the end of the century, wildfire smoke could kill up to 1.4 million people annually worldwide unless emissions are curbed. Another study estimated that at the current rate of warming, wildfire smoke would cause more than 70,000 premature deaths in the US each year by 2050.
Wildfires themselves have grown increasingly destructive. In California, 2018 was the state’s deadliest fire season with 100 fatalities; 2020 burned the most acreage, scorching 4.3 million acres. In January 2025, wildfires in Los Angeles County killed 31 people and destroyed more than 16,000 structures.
“Mitigating climate change and implementing fire prevention measures can lead to improved standards in air quality and potentially bring large benefits to public health,” the study concludes.