Wildfires reversed U.S. ozone air quality gains since 2015, study finds

A study published Thursday in the journal Science found that wildfire smoke has reversed prior U.S. progress on lowering ground-level ozone since 2015. The research attributed the change to emissions of carbon monoxide and other gases from fires that form ozone when exposed to sunlight and can travel hundreds of miles.

Ground-level ozone forms when pollutants from vehicles, refineries, and industrial sources react with sunlight. The Environmental Protection Agency has tracked reductions in these human-made precursors, yet monitoring stations showed surface ozone levels plateauing rather than continuing to fall.

Researchers combined satellite observations, EPA ground measurements, and meteorological records using deep-learning models. Coverage from EPA stations is limited to about 2 percent of the continental U.S. land area, so the satellite-augmented dataset allowed broader estimates of ozone changes.

The analysis showed ozone concentrations decreasing 0.65 parts per billion per year before 2015 and increasing 0.13 parts per billion per year afterward. The study linked the post-2015 rise to an estimated 318 additional premature deaths annually since 2013.

Wildfire smoke also contains fine particulate matter known as PM2.5 that can reach deep into the lungs and bloodstream. Separate studies have associated such exposure with thousands of premature deaths in California over the past decade and projected higher future mortality if current warming trends continue.

The authors stated that mitigating climate change and implementing fire-prevention measures could improve air-quality standards and yield public-health benefits. The findings add to existing research on how wildfire frequency and intensity affect both emissions and population exposure.