For decades, the United States made steady progress in reducing surface ozone pollution, the main ingredient in smog. But that progress – made as vehicles, industries and power sources became cleaner – is increasingly being overshadowed by a different and growing source of ozone pollution: wildfires.

Our team of atmospheric and wildfire scientists analyzed wildfires’ contribution to surface ozone levels from 2003 to 2024 across the United States.

We found that the gases in wildfire smoke have reversed the national ozone trend, forcing a shift from declining ozone levels prior to 2015 to increasing ozone levels after 2015. We also found that the number of ozone-related premature deaths due to wildfires has been increasing by about 300 deaths per year since then.

Most people know ozone as the protective layer of the atmosphere high above the Earth that shields the planet from harmful ultraviolet radiation. But ozone has two very different faces.

High in the atmosphere, ozone is beneficial. Near the ground, it is a harmful air pollutant that can irritate the lungs and worsen respiratory diseases.

Los Angeles made ozone visible to the nation in the 1940s and 1950s, as thick, eye-stinging smog often blanketed the city. It turned an invisible chemistry........

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