Wildfire Smoke: A Systemic Threat to Human Health, from Acute Inflammation to Cancer and Dementia

Wildfire smoke—an increasingly pervasive consequence of climate change—assaults nearly every system in the human body, killing tens of thousands of people annually and leaving survivors with lasting health consequences, according to a growing body of medical research. Scientists have identified at least 1,000 toxins in wildfire smoke, and the evidence now spans from immediate respiratory distress to long-term risks including cancer, dementia, and adverse pregnancy outcomes.

"Wildfire smoke is the toxic product of combustion of whatever burned," said Dr. Courtney Howard, an emergency room physician, chair of the Global Climate and Health Alliance and president-elect of the Canadian Medical Association. "So really it's a big giant toxic soup of particles and gases."

The Composition and Reach of Wildfire Smoke

Unlike typical air pollution, wildfire smoke can linger in the atmosphere for weeks and travel thousands of miles. It burns not only vegetation but also vehicles, buildings, and their contents, carrying traces of chemicals, metals, plastics, and other synthetic materials. Colorado State University environmental toxicologist Luke Montrose noted that the smoke contains compounds "oftentimes associated with the burning of diesel fuel or cigarette smoke, things like formaldehyde or volatile organic compounds."

The smallest particles—approximately one micron in size—travel the farthest and do the most damage, according to Colleen Reid, a University of Colorado geographic health professor. These particles bypass the body's defenses, entering the lungs and then the bloodstream, where they trigger an aggressive immune response.

Montrose explained the mechanism: the particles, coated in chemicals with large surface areas, prompt the body's defense system to "send signals to other cells that say, 'We have a problem. We need to mount an immune response to this.' And that's where you get your acute effect or your effect within minutes, hours or even that day."

Acute and Chronic Health Impacts

The immediate toll is stark. Dr. Howard described findings from Yellowknife, Canada: "We found a full doubling of emergency department visits for asthma and about 50% increase in pneumonia" following two-and-a-half months of intermittent smoke exposure.

Studies have linked wildfire smoke with higher rates of heart attacks, strokes, cardiac arrests, weakened immune defenses, and poorer survival rates after surgery. A 2023 study in Maryland identified a spike in heart and lung illnesses associated with wildfire smoke originating up to 2,100 miles away.

On average, 24,100 people died each year in the Lower 48 states between 2006 and 2020 due to long-term exposure to tiny particles from wildfire smoke, according to a study published in Science Advances. Globally, a 2021 study combining observational data and computer models calculated that wildfire smoke particles cause 677,745 deaths annually, with almost 39% of those being children under age 5.

The effects can persist for years. After Australia's 2014 Hazelwood Coal Mine fire, heart disease rates remained elevated for two-and-a-half years and respiratory illnesses for five years.

Cancer, Dementia, and Pregnancy Risks

A 2026 U.S. study linked wildfire smoke exposure with elevated risks for lung, colorectal, breast, bladder, and blood cancer, with risks increasing alongside pollution levels from the fires.

Cognitive effects are also emerging as a major concern. Exposure to the 2018 Camp Fire in California was linked to changes in cognition and brain activity six to 12 months later. Several studies have now connected longer-term wildfire smoke exposure to higher risks for dementia. Dr. Mary Johnson, a Harvard School of Public Health environmental health scientist, noted that people over 60 become more prone to stroke in wildfire smoke.

Pregnant women face particular vulnerability. Wildfire exposure in pregnancy has been associated with miscarriage, low birth weight, and preterm delivery. A California study found a link between wildfire exposure and cellular damage in first- and second-trimester placentas. A separate study found that wildfire smoke exposure during the third trimester may raise the risk of an autism diagnosis.

No Safe Level of Exposure

"The bad news is, there's no safe level" of inhaled particles from wildfire smoke, said Doug Brugge, chair of the Department of Public Health Sciences at the University of Connecticut School of Medicine. "The higher the exposure, the worse the risks, but even at levels below the national safety standards, these particles can make people sick."

Even in healthy adults, effects such as sore throats, excessive phlegm, coughing, headaches, and brain fog can appear immediately and persist long after the smoke plume is gone, according to Keith Bein of the Air Quality Research Center at UC Davis.

Mitigation Strategies

Experts recommend wearing N95 masks when wildfire smoke is present and limiting outdoor activities, especially strenuous exercise. Indoor air purifiers that trap particles smaller than 2.5 microns are also advised. "Air purifiers reduce exposure and in our studies reduced blood pressure and cognitive impacts of air pollution," Brugge said.

However, particulates from wildfire smoke enter most buildings in high concentrations. On average, indoor pollutant levels reach about half of outdoor concentrations, and up to 70% in poorly sealed buildings. Bein compared indoor wildfire smoke exposure to second-hand tobacco smoke, noting that particles "end up everywhere—on your clothes, on your walls, on surfaces, and they're still outgassing after the smoke plume is gone."

"Staying away from the smoke is No. 1 if you can," Johnson said.

The Climate Connection

So far this year, more than 5,740 square miles of the United States has burned from wildfires—31% more than the 10-year average for this date. The amount of U.S. land burned annually in the 2020s is now more than twice what it was 30 years ago. A Stanford study projects that U.S. wildfire smoke deaths will increase with climate change, reaching an annual economic cost of $244 billion by midcentury.

Researchers continue to investigate the long-term effects of smoke particles in water supplies, on crops, and whether wildfire smoke amplifies the adverse effects of extremely hot weather. As Bein noted, "It is hard to make predictions because it is hard to say how many fires people will be exposed to, how long the fires will burn, or what the smoke will contain."