Smog linked to wildfires is getting worse across much of the U.S., playing a role in more than 300 additional premature deaths every year since 2013, researchers say.

The main ingredient in smog is ozone, which is a gas molecule made of three oxygen atoms that, depending on where it forms, can be either helpful or harmful. In the layer of the upper atmosphere called the stratosphere, the gas creates a protective layer that shields Earthlings from harmful ultraviolet rays. But ozone is dangerous when inhaled. So on the ground, the gas can lead to respiratory issues, particularly in people with asthma or other breathing conditions.

This ground-level ozone can form when carbon monoxide gas emitted by wildfires chemically reacts with nitrogen oxides in the presence of sunlight. Wildfires have been on the rise because of climate change, according to NASA. To get a full picture of the effect of this recent increase in wildfire frequency and intensity, a team of researchers fed surface ozone levels and premature death data into a deep-learning model.

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The new analysis, funded by NASA, showed that, between 2003 and 2015, ground-level ozone decreased, likely resulting from tightened controls on ozone-forming chemicals released by human activity, such as car tailpipe emissions, the scientists said. Those gains were reversed between 2015 and 2024, when, the model showed, ozone levels increased, particularly in the Midwest and parts of the western U.S.

By looking at different scenarios in their model, the researchers found that, without the addition of wildfires, surface ozone in the Midwest would have continued its downturn after 2015.

The effects of wildfire smoke can spread far and wide, as anyone living in the northeastern or midwestern U.S. in the summer of 2023 knows. Wildfires blazing in Canada that summer clogged the air with smoke and turned the skies an orange hue in both U.S. regions. That fire season, the study showed, exposed 148 million Americans to surface ozone levels above the standards considered safe by the U.S. Environmental Protection Agency. Midwesterners experienced the unsafe levels for more than a week.

“People in the Midwest may think fires burning far away will not affect them,” said study co-author Jun Wang, an atmospheric scientist at the University of Iowa, in a NASA statement. “But once wildfire pollution is in the air, it can move across regions.”

The resulting smog was linked to 7,974 premature deaths in the U.S. in 2023 alone, the researchers calculated.

“These results underscore the escalating public health burden of wildfire-driven [ozone] pollution,” the researchers wrote in their study, which was published online on June 4 in the journal Science.