Wildfires are making the US smoggy again, reversing progress on cleaner
air, study finds
[June 05, 2026]
By SETH BORENSTEIN
WASHINGTON (AP) — 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.”
Limited smog monitor coverage
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 showing 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's research director and senior policy advisor 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.
How fires trigger health problems
Fires don't produce ozone itself, but they release precursor chemicals
that become smog when they interact with sunlight, scientists said.
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Firefighters are silhouetted amid an operation to control the Sandy
Fire, May 19, 2026, in Simi Valley, Calif. (AP Photo/Caroline
Brehman, File)
 “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's “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 was in the Northern Rockies,
which were near many of the fires, and in the Midwest, where the
smoke traveled next, Deng said.
More fires, more smoke
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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