Researchers are beginning to understand the long-term impact of wildfire smoke exposure on human health. And it’s not good. “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.”
In a study published in January 2026, Yang Liu and other researchers found that repeated exposure to even moderate amounts of smoke increases the risk of stroke among older adults.
A month later, another study found that long-term exposure to wildfire smoke had contributed to an average of 24,100 deaths a year in the United States between 2006 and 2020.
Wildfire smoke contains a mixture of hazardous air pollutants. But one of the most aggressive components appears to be tiny specks of particulate matter, known as PM2.5.
PM2.5 is made up of soot, dust, and other substances that act like sponges, binding toxic metals and organic compounds to them.
If inhaled, PM2.5 particles can get into the lungs and bloodstream, from where they cause inflammation and damage to various other organs in the body.
Long-term exposure to PM2.5 has been linked to lung diseases like asthma, as well as heart disease, diabetes, dementia, and cancer.
Wildfire smoke is particularly dangerous for pregnant women. It can increase the risk of premature births and low birth weight.
What makes PM2.5 in wildfire smoke so toxic
Wildfire PM2.5 is particularly toxic due to complex mixtures of burning materials found at any site, which can differ between rural fires and those that spread to urban areas.
“It really does depend on the location of the fire. It might even vary from block to block — the smoke may consist of different components,” said Mary Johnson, a principal research scientist in the Department of Environmental Health at the Harvard T.H. Chan School of Public Health.
Johnson led an international team that studied toxic substances in wildfire smoke in a group of healthy firefighters and civilians who had recently been exposed to wildfire smoke. They wanted to understand how these toxins disrupt immune cell function and increase the body’s susceptibility to infections and chronic diseases.
The researchers found mercury and cadmium — both heavy metals found naturally in plants and soils, as well as in man-made materials. They also found carcinogenic compounds and so-called forever chemicals, known as PFAS.
PFAS are synthetic chemicals used in many everyday products and building materials. They have been linked to various cancers and metabolic conditions.
Their findings led them to suggest future research should focus on developing treatments for vulnerable groups like children and elderly people and those with chronic health conditions.
Wildfire smoke becomes more toxic over time and distance
Wildfire smoke becomes more dangerous as it ages, undergoing chemical reactions that increase its ability to damage cells.
Researchers in Greece found that smoke can get up to four times more toxic in the days following a fire. It can also travel long distances quickly and spread to areas thousands of miles away from the original source of the fire.
“If there is no rain to wash out these particles, they can stay in the atmosphere for ten days or sometimes more. They can go around the whole hemisphere and come back to the departure point within this time,” said Nikolaos Mihalopoulos, director of the Institute for Environmental Research and Sustainable Development at the National Observatory of Athens.
During the Canadian wildfires of 2023, for example, 6,000 fires burned 15 million hectares of land, causing “long-range PM2.5 pollution” to spread across North America and even reach Europe and Asia.
In a paper published in September 2025, researchers estimated that about 5,400 acute deaths in North America and about 64,300 chronic deaths in North America and Europe could be linked to PM2.5 exposure from the 2023 Canadian wildfires.
However, it is worth noting that other sources of air pollution — traffic exhaust fumes, industrial emissions, and burning fuel for cooking — still cause more health problems overall because people are exposed to them daily.
“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 toxic, especially for people living near areas prone to fires.
“The more you can do to minimize exposure, the better,” said Johnson, who recommends staying indoors during and immediately after fires, using filters on heating and air conditioning systems, keeping N95-grade masks available, and ensuring vulnerable people have emergency supplies of medication and inhalers readily accessible.
Ahead of fire seasons, people should clear their properties of flammable materials and set up neighborhood monitoring groups that look out for early fire signs and help stop small fires from becoming catastrophic ones.
Edited by: Zulfikar Abbany
