A new Science study found that wildfire smoke has driven a 4% rise in average U.S. ground-level ozone since 2015, offsetting years of cleaner-air progress achieved through federal pollution rules.
By yourNEWS Media Newsroom
Wildfire smoke is eroding more than a decade of U.S. progress on cleaner air, with a new study finding that increasingly large fires have pushed national smog levels higher since 2015 and contributed to hundreds of additional deaths each year.
The study, published Thursday in the journal Science, found that the United States reduced national ground-level ozone by 11% from 2003 to 2015 as federal regulations on power plants, cars and diesel engines lowered emissions of smog-forming pollutants. Since 2015, however, average ground-level ozone has risen by 4% as wildfire smoke has become a larger factor in air quality.
Ground-level ozone is commonly known as smog. It forms when precursor chemicals react in sunlight. Wildfires do not directly emit ozone, but they release chemicals that help produce it.
Study lead author Weizhi Deng, an atmospheric scientist at the University of Iowa, said the current trend could erase two decades of air-quality gains. If wildfire smoke continues increasing at the same pace, national smog levels could return to 2003 levels within 20 years, Deng said.
The researchers also estimated that higher ozone exposure has contributed to 318 additional U.S. deaths per year since 2013. The estimate was based on established epidemiological research comparing death rates in cleaner and more polluted air.
“For the last 20 years, by regulations, we keep decreasing the emissions” for human-caused smog-forming 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 used a new method to measure smog across the country. The Environmental Protection Agency’s ozone monitors cover only about 2% of U.S. land area and are concentrated mostly in cities. To create a broader national picture, Deng and his colleagues combined monitor readings with satellite data, pollution information, weather data and models. They then used artificial intelligence to produce a nationwide ozone dataset at a resolution slightly finer than half a mile.
EPA data since 2015 has shown national ozone levels moving up and down within a narrow range. Deng said the new analysis found a clearer trend because it included areas beyond the limited monitoring network.
“By considering everywhere in the U.S., we actually found an increase in ozone starting from 2015,” Deng said.
Cristina Archer, an environment professor at the University of Delaware who was not part of the study, said the artificial intelligence method was reliable because it began with “massive and reliable datasets” and used models to fill gaps in a way that produced an “exceptional” high-resolution view.
Teresa Feo, research director and senior policy adviser for Megafire Action, said wildfire smoke exposure remains difficult to track without broader monitoring.
“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,” Feo said.
The federal government has long tracked six major air pollutants, including smog and soot, or fine particle pollution. The new study focused on ozone. A 2023 study by many of the same researchers found that wildfire smoke had also reversed progress on fine particle pollution, increasing particle-related deaths by about 670 per year.
Public health experts said ozone remains a serious concern, even though fine particles are generally considered more deadly.
“Higher daily ozone concentrations can increase asthma attacks, hospital admissions, and mortality,” said University of Washington public health and climate scientist Kristie Ebi. She said ozone is “still a very important pollutant, which is why it’s regulated.”
The study found that during the heavy wildfire smoke seasons of 2022, 2023 and 2024, many of the fires were in Canada, but smoke moved south into the United States. Researchers calculated that 43 million people in the U.S. were exposed to smog levels above the current EPA safety standard.
Some public health experts said that standard is not strict enough. Dr. Lynn Goldman, former dean of the George Washington University School of Public Health and a former EPA assistant administrator, said the existing limit does not adequately protect public health. In 2023, the Biden administration delayed plans to tighten ozone standards, and the Trump administration later changed regulations involving how deaths and health impacts are considered in smog and soot rules.
The largest ozone increases were found in the Northern Rockies, which were close to many fires, and in the Midwest, where smoke traveled afterward, Deng said.
National fire data shows the average amount of U.S. land burned by wildfires each year is now 9% higher than it was from 2003 to 2014, according to the National Interagency Fire Center. Scientists also pointed to unusually severe Canadian fire seasons since 2022, including 2023, when smoke turned skies orange in parts of the eastern United States and prompted many residents to wear face masks.
The amount of land burned in Canada in 2023 broke records and was twice as high as the previous record, said atmospheric scientist Brendan Rogers of the Woodwell Climate Research Center. A 2025 study estimated that smoke from the 2023 Canadian fires caused 82,100 deaths globally from particle pollution, including 33,000 in the United States.
Scientists said climate change is a major factor in worsening fire conditions, though not the only one. A 2023 study found that climate change caused by the burning of coal, oil and gas increased the intensity of Canada’s 2023 fire season by at least 50% and doubled the likelihood of the hotter, drier weather conditions needed for the fires.
“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 was not 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 under President Barack Obama, said the findings show the health danger from fires extends beyond flames and property loss.
Wildfires cause death and destruction, but smoke and extreme heat can also increase ozone levels that harm human health, McCarthy said.
“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?”
The study’s findings show that U.S. air-quality gains from pollution controls are increasingly vulnerable to wildfire smoke. Researchers said the trend raises new questions for regulators and public health officials as larger fires push smog into communities far from the flames.
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