The highly destructive wildfires that have battered the US and North America in recent years have significantly increased emissions and been linked to tens of thousands of premature deaths, but their impact on air quality is greater than previously known, according to new research.

A study published in Science on Thursday found that, since 2015, wildfires have reversed US progress toward ozone air quality standards, as the worsening pollution caused by wildfire smoke has undone years of efforts to reduce emissions. Ground-level ozone (O3) is created when pollutants from cars, refineries and industrial sources react with sunlight, according to the Environmental Protection Agency.

Wildfires also create their own massive ozone problem as they emit carbon monoxide and other gases that can contribute to the formation of ozone, and smoke, which can spread far beyond the source of a fire, can lead to increasing ozone hundreds of miles away.

“Despite regulated reductions in anthropogenic emissions of O3 precursors, observation stations indicate that policy-relevant surface O3 levels have plateaued,” Weizhi Deng and other authors wrote. In other words, despite emissions of the chemicals that create ozone pollution going down, the amount of ground-level ozone has stopped declining. The study links this plateau to wildfire emissions and finds that the rise in O3 emissions is correlated with premature deaths, some 318 each year since 2013.

There were limited ground measurements available to researchers, as the US Environmental Protection Agency’s monitoring stations cover only 2% of land in the continental US, the study noted.

But researchers developed a dataset by evaluating information from satellites and the EPA as well as meteorological data with deep learning models. They ultimately found that O3 went from decreasing by 0.65 parts per bn (ppb) per year before 2015 to rising 0.13ppb annually after that year.

Wildfires are worsening around the world with global heating leading to extreme blazes and causing billions of dollars in damage and catastrophic loss of life. Canada and the American west have seen particularly devastating fires in recent years.

In California, 2018 brought the state’s deadliest fire season with 100 people killed while 2020 was the most severe in terms of land burned with 4.3m acres scorched. In 2025, fires in Los Angeles killed 31 people and destroyed more than 16,000 structures.

Addressing the climate crisis, which has created conditions for more frequent and destructive wildfires, is an important step to reducing pollution.

“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 notes.

The findings add to a growing body of research that aims to shed light on the extent to which wildfires have affected the environment and people. Wildfire smoke contains PM2.5, microscopic particles that can embed themselves deep in the lungs and enter the bloodstream. The pollutant has been linked to numerous health conditions and premature death.

A 2024 study found that more than 50,000 people in California died prematurely over a decade due to exposure to toxic particles in wildfire smoke.

Research published last fall found that by the end of the century, smoke from wildfires was expected to kill as many as 1.4 million people around the world annually unless emissions were curbed. Another study found that wildfire smoke would lead to the deaths of more than 70,000 people in the US each year by 2050 at the current rate of heating.