Growing impacts of fire smoke on ozone pollution and associated mortality burden in the United States.

Fire smoke pollution is an emerging health risk factor, yet prior studies have focused mostly on fine particulate matter (PM2.5) and neglected the health impacts from other pollutants, most notably ozone (O3). Here, we quantify changes in ground-level O3 concentrations during smoke episodes across the contiguous US using surface air quality measurements, satellite data, and machine learning models. After correcting for meteorological variability, smoke contribution to daily ground-level O3 can reach up to 6.9 parts per billion (16% increase relative to nonsmoke days) at certain US monitoring locations, averaged over all smoke days from 2006 to 2023. Smoke O3 patterns are poorly correlated with smoke PM2.5 patterns, demonstrating that their health impacts do not necessarily coincide in space and time. We estimate that smoke O3 can lead to 2045 annual excess deaths (95% confidence interval: 1325 to 2755) averaged across 2006 to 2023 using established exposure-response functions of all-source O3 exposure. The increasing mortality due to smoke O3 partially offset the observed declining trend in nonsmoke O3 mortality. Over the study period, estimated deaths from smoke O3 were equivalent to 15.8% of smoke PM2.5 mortality, and this proportion reached 61.5% in 2023.