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%, reports the AP. 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." 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 US, we actually found an increase in ozone starting from 2015."
For decades, the US 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.
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 US, 43 million people were exposed to smog levels that exceeded the current EPA safety standard, the study found. 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 fires, 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."
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