WASHINGTON -- For more than a decade, the United States dramatically reduced its national smog levels, but since 2015 smoke from increasingly larger wildfires is reversing that clean-up trend and making the air dirtier and deadlier, a new study finds.

Scientists say climate change deserves much, but not all, of the blame.

The national smog level dropped by 11% from 2003 to 2015 as strict federal regulations on power plants, cars and diesel engines kicked in. But since then, as wildfires have grown, the nation's average ground level ozone — which is smog — increased by 4%. That means if smoke increases at the current rate, smog will go back up to 2003 levels in 20 years, said study lead author Weizhi Deng, an atmospheric scientist at the University of Iowa.

Thursday's study in the journal Science also estimated an increase in deaths from ozone attacking lungs, using previously established epidemiology studies that compared death rates in clean and dirty air. They calculated an increase of 318 American deaths per year since 2013.

“For the last 20 years, by regulations, we keep decreasing the emissions" for human-caused smog-inducing chemicals, said study co-author Meng Zhou, a University of Iowa wildfire researcher. “However, because of wildfires, that is actually from natural hazards, all those kinds of effort were wiped out.”

The study was novel in the way it estimated the national smog level, compensating for how the U.S. Environmental Protection Agency has a limited number of smog monitors. Those cover only 2% of the nation, mostly in urban areas. So Deng and his colleagues used those observations — along with satellite, pollution and weather data and models — then used artificial intelligence to create a nationwide data set of ozone levels that showed smog count at a resolution slightly higher than half a mile (1 kilometer).

EPA figures show the national ozone level since 2015 has vacillated around the same mark, going up and down a few percentage points, but Deng said, “by considering everywhere in the U.S., we actually found an increase in ozone starting from 2015.”

The method using artificial intelligence is solid because it starts with “massive and reliable datasets,” then uses computer models to fill in the gaps in a sensible way to make an “exceptional” high-resolution picture, said University of Delaware environment professor Cristina Archer, who wasn’t part of the study.

Megafire Action policy director Teresa Feo said “experts have long called for expanding the air pollution monitoring network to improve research on wildfire smoke exposure and provide the data needed to better protect public health.”

For decades, the U.S. tracked six traditional air pollutants, including smog and soot, which are tiny particles. This new study looked only at ozone, while a 2023 study by many of the same team looked at small particle pollution. They found the downward trend in soot levels had similarly reversed. Wildfire smoke increased particle pollution deaths by about 670 per year, the 2023 study found.

Fires don't produce ozone itself, but they release precursor chemicals that become smog when they interact with sunlight, scientists said.

“Higher daily ozone concentrations can increase asthma attacks, hospital admissions, and mortality,” said University of Washington public health and climate scientist Kristie Ebi. It's not quite as deadly as tiny particles, she said, but it is “still a very important pollutant, which is why it's regulated.”

During the heavy wildfire smoke seasons of 2022, 2023 and 2024, much of the fires were in Canada, but the smoke came south. In the U.S., 43 million people were exposed to smog levels that exceeded the current EPA safety standard, the study found.

And that standard isn't stringent enough, said Dr. Lynn Goldman, former dean of the George Washington University School of Public Health and a former EPA assistant administrator. In 2023, the Biden administration delayed plans to tighten those standards and then the Trump administration changed regulations that consider deaths and health impacts in smog and soot rules.

The biggest increase in ozone levels were in the Northern Rockies, which were near many of the fires, and in the Midwest, where the smoke travelled next, Deng said.

The average amount of U.S. land that wildfires burn each year is now 9% higher than it was from 2003 to 2014, according to the National Interagency Fire Center. But the wildfires in Canada have been particularly bad since 2022, scientists said. They pointed to 2023 when the skies were orange and people in the East were wearing face masks because of the Canadian smoke.

The amount of land burned in 2023 in Canada was not only a record but two times higher than the old record, said atmospheric scientist Brendan Rogers of the Woodwell Climate Research Center. Smoke from that year's Canadian fires killed 82,100 people globally — 33,000 in the United States — because of the particle pollution, a study in 2025 calculated.

Climate change, from the burning of coal, oil and gas, increased the intensity of Canada's 2023 fire season by at least 50% and doubled the chances of the drier, hotter weather conditions that were needed for the fire, a 2023 study found.

“Human-caused climate change is an important contributor, because it increases hot, dry fire-weather conditions in many regions,” said Lixu Jin, an atmospheric scientist at Rutgers who wasn't part of the study. “But wildfire emissions also depend on fuels, land management, ignitions, suppression, and year-to-year meteorology.”

Former EPA Administrator Gina McCarthy, who served in the Obama administration, said it was discouraging to see smog improvements being eroded.

Wildfires cause death and destruction, but the greatest danger may come from smoke and extreme heat increasing the ozone that harms people’s health, she argued,

“So the big question is," she said, “when are we going to stop the nonsense from this administration to burn more and more ‘beautiful’ fossil fuels?”

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