A decade ago, wildfire smoke was barely on many Americans’ radar.
But spurred by climate change, fires now rage on the West Coast every summer. Smoke spews down from Canadian forests. The Northwest, Midwest, Northeast, Southeast, and Southwest have all been affected. And we’re learning more about what that means for our health.
Scientists have found that people are being exposed to more toxins than previously known, and beyond damaging lungs, wildfire smoke may affect hearts, brains, and other organs. One study in 2025 estimated that smoke could kill up to 1.9 million people in the United States over the next three decades.
This area of science is rapidly developing, and many conclusions aren’t definitive — partly because the studies generally can’t prove causation. (It would be difficult and likely unethical to design a randomized study that deliberately exposed people to wildfire smoke.)
“It’s a bit of a young field,” said Dan Westervelt, an associate research professor at Columbia University. But, he added, recent research is compelling.
Here is some of what scientists have discovered.
Wildfire smoke can be unusually toxic.
Wildfire pollution is often measured by the concentration of airborne particles known as PM2.5, which can penetrate deep into the lungs.
But that can understate risk, said Kari Nadeau, a physician and dean of the UCLA Fielding School of Public Health, who is studying the consequences of the 2025 Los Angeles fires. That’s because PM2.5, a primary pollutant measured by air quality indexes, doesn’t reflect other substances that wildfire smoke can contain, including lead, mercury, and formaldehyde.
Some research suggests preliminarily that wildfire smoke may be more harmful than air pollution from other sources, like car exhaust. Fires in populated areas may be especially toxic, Nadeau’s team found, because they consume things like buildings, vehicles, and electronics.
Scientists are also starting to appreciate how greatly two fires can differ depending on what they burn and how far the smoke travels, said Rima Habre, director of the CLIMA Center at the University of Southern California, which studies health effects of extreme weather.
It may harm many organs.
Studies have found associations between wildfire smoke and cardiovascular hospitalizations, diabetes complications, preterm birth, cancer, impaired cognition, and other problems, in addition to respiratory effects, including asthma attacks.
A 2024 study linked wildfire smoke to deaths from cardiovascular, digestive, endocrine, and kidney diseases. And on days with severe smoke, asthma attacks and heart attacks can increase within an hour, said Michael Brauer, a professor at the University of British Columbia who also does research at the University of Washington.
Research into cognitive effects has expanded significantly, said Stephanie Cleland, an assistant professor at Simon Fraser University. In 2022, Cleland was the lead author of a study that found smoke exposure was associated with lower scores on a “brain training” game. Research also suggests a link between cumulative exposure over a school year and lower standardized test scores; other studies show a possible association with dementia.
These neurological associations are not definitive, but several experts cited them as an area where science is advancing rapidly and evidence is beginning to accumulate.
There are also indications that the risk of preterm birth, and of ear and respiratory infections in infancy, may increase as exposure levels during pregnancy increase.
Total exposure matters.
The most intense smoke events — the ones that blot out the sun and make the air smell like a campfire — get the most attention.
But lower-level exposures over time, even in the “moderate” category on air quality indexes, add up, said Kai Chen, an associate professor at the Yale School of Public Health. There is no amount of smoke exposure that scientists can say confidently isn’t harmful. As exposure increases, so does risk.
Chen suggested using masks and air filters more frequently.
“It’s not just a few days in these extreme events that you have orange skies that you need to protect yourself,” he said. “It is also the days when the sky looks OK,” but air quality indexes or weather reports indicate smoke is present.
A 2025 study linked average wildfire exposure over three years to the risk of death from all causes: The more exposure, the greater the risk, whether exposure was low but persistent or came in severe but isolated events.
Another 2025 study looked at nine years of data and found that as smoke exposure increased, Americans older than 65 had a higher risk of dying from chronic obstructive pulmonary disease. And in a 2026 study of people age 65 and older, incremental increases in exposure over as little as two years were associated with a higher risk of stroke; the longer the exposure, the greater the risk.
Many questions remain, and research is proceeding rapidly, despite difficulties caused by federal funding cuts.
The goal is to guide public health actions. Habre said it was urgent, for instance, to develop better monitoring tools for substances besides PM2.5.
But Nadeau and Chen said the best way to mitigate risks would be to prevent the catastrophic fires we’re seeing.
That means addressing the root cause: above all else, climate change.
This article originally appeared in The New York Times.