Researchers are increasingly documenting the long-term harms of wildfire smoke exposure. “Your whole body is affected,” said Yang Liu, Professor of Environmental Health at Emory University in Atlanta. “Wildfire smoke causes oxidative stress to your system and exacerbates or accelerates the development of diseases.”
A study published in January 2026 found that repeated exposure to even moderate amounts of wildfire smoke raised stroke risk among older adults. Another study, published a month later, estimated that long-term exposure to wildfire smoke contributed to an average of 24,100 deaths per year in the United States between 2006 and 2020.
Wildfire smoke is a complex mixture of hazardous pollutants. One of its most harmful components is fine particulate matter known as PM2.5. These tiny particles—composed of soot, dust and other materials—can bind toxic metals and organic compounds. When inhaled, PM2.5 can penetrate the lungs and enter the bloodstream, triggering inflammation and damaging organs throughout the body.
Long-term PM2.5 exposure has been linked to respiratory diseases such as asthma, as well as heart disease, diabetes, dementia and cancer. Pregnant women are especially vulnerable: wildfire smoke exposure can increase the risk of premature birth and low birth weight.
What makes wildfire PM2.5 particularly toxic is the mixture of materials burned at different sites. Smoke composition varies by location and by what is burning; it can even differ from block to block. Mary Johnson, a principal research scientist at the Harvard T.H. Chan School of Public Health, led an international study of toxic substances in wildfire smoke from recently exposed firefighters and civilians to see how these toxins disrupt immune function and raise susceptibility to infection and chronic disease.
Her team detected heavy metals such as mercury and cadmium, carcinogenic compounds, and PFAS—so-called “forever chemicals” used in many products and building materials. PFAS have been linked to cancers and metabolic disorders. The researchers suggested follow-up work should focus on treatments or protections for vulnerable groups, including children, the elderly and people with chronic conditions.
Wildfire smoke also becomes more toxic as it ages. Chemical reactions in the atmosphere can increase its cell-damaging potential over days. Researchers in Greece found smoke could become up to four times more toxic in the days following a fire. Smoke can travel long distances—during the 2023 Canadian wildfires, PM2.5 pollution spread across North America and reached Europe and Asia. In one analysis, the 2023 fires were associated with an estimated 5,400 acute deaths in North America and about 64,300 chronic deaths in North America and Europe.
Still, other everyday sources of air pollution—traffic exhaust, industrial emissions and indoor fuel burning—cause more overall harm because of continuous exposure. “When we look at what really drives mortality in the population as a whole, it’s still all these other sources [of air pollution] that are really responsible for so many deaths,” said Cathryn Tonne, an environmental epidemiologist at the Barcelona Institute for Global Health.
How to protect yourself against wildfire smoke
Wildfire smoke is uniquely hazardous, especially for people living near fire-prone areas. The best protections reduce exposure: stay indoors during and immediately after fires, use filters in heating and air conditioning systems, have N95-grade masks available, and ensure vulnerable people have emergency medication and inhalers on hand. Ahead of fire season, clear properties of flammable materials and organize neighborhood monitoring to detect and contain small fires before they become catastrophic.
Edited by: Zulfikar Abbany