Wildfire smoke drives population exposure to ozone pollution and unhealthy air quality days across the United States

Minghao Qiu , Yangmingkai Li, Marissa Childs, Makoto Kelp, Xiaomeng Jin, Guanyu Huang, Mahdieh Danesh Yazdi, Yaguang Wei, Yuan Wang, Kai Chen

Wildfire smoke is an increasing threat to air quality and public health in the United States, affecting population exposure to multiple air pollutants, including ground-level ozone (O₃). Most prior studies have focused on smoke fine particulate matter (PM_2.5), but how smoke affects surface O₃ concentrations at a population scale is less well characterized. Much less is known about how smoke O₃ impacts the Air Quality Index (AQI) compared with smoke PM_2.5, or what this implies for regulatory attainment and population exposure. Here we quantify daily O₃ concentrations attributable to wildfire smoke (i.e., smoke O₃) across the contiguous U.S. from 2006 to 2023, using ensemble machine learning models that isolate the smoke contribution from meteorological variability. Smoke O₃ enhancement is widespread across all states, with the Midwest experiencing the largest increases. Wildfire smoke placed an additional 29 million people each year in areas exceeding the federal O₃ standard, and lengthened extreme O₃ episodes by 2.6 days. Smoke was responsible for 62.5% of person-days with unhealthy O₃ levels (defined as O₃ sub-index > 100) during 2021–2023. When smoke pushed the AQI into a worse category above 100, O₃ rather than PM_2.5 was the main pollutant 71.3% of the time. Smoke O₃ and smoke PM_2.5 are only weakly correlated and exhibit an inverted U-shaped relationship with substantial regional heterogeneity. Our results show that wildfire smoke is an increasingly important source of O₃ exposure and unhealthy air quality days, and that assessments based on PM_2.5 alone understate its air quality and health burden.