Wildfire smoke — increasingly common across the Northern Hemisphere as the planet heats up — affects nearly every organ in the human body and is responsible for tens of thousands of deaths each year, according to a growing body of medical research.

Its impact can be swift. Within hours, asthma attacks rise and ambulance calls increase. Within a day or two, emergency departments fill with patients suffering from heart attacks, breathing difficulties, cardiovascular complications and even mental health crises, physicians and researchers report.

Smoke exposure also poses dangers during pregnancy, raising the likelihood of premature delivery and low birth weight, which can lead to lifelong respiratory problems. Long-term exposure has been associated with elevated risks of certain cancers and neurodegenerative conditions such as dementia.

A child looks out from the observation deck of the CN Tower as smoke from wildfires in Northwestern Ontario blankets the city of Toronto, Thursday, July 16, 2026.

A child looks out from the observation deck of the CN Tower as smoke from wildfires in Northwestern Ontario blankets the city of Toronto, Thursday, July 16, 2026.

After the massive wildfire seasons of 2018 and 2019, scientists intensified their focus on the health consequences of smoke exposure. Since then, a steady stream of studies has revealed a wide range of effects that were not fully recognized before, said Dr. Mary Johnson, an environmental health researcher at the Harvard School of Public Health.

Wildfire smoke provokes inflammation by pushing the immune system into overdrive as it attempts to fend off invading particles. Researchers have linked exposure to damage in the brain, irritation of the skin and even reduced sperm quality, with few bodily systems untouched. Adults over 60 appear to face a heightened risk of stroke during smoky conditions, Johnson added.

“Wildfire smoke is the byproduct of whatever materials have burned,” said Dr. Courtney Howard, an emergency physician and leader in global climate and health advocacy.

That means it can contain residues from homes, vehicles and industrial materials — forming what she describes as a complex and hazardous mixture of microscopic particles and gases.

Researchers have identified at least 1,000 different toxic substances in wildfire smoke, according to Colorado State University environmental toxicologist Luke Montrose.

Many of these chemicals are familiar pollutants also found in diesel exhaust or cigarette smoke, including formaldehyde and volatile organic compounds. Even without additional contaminants, Montrose said, smoke itself poses significant health risks.

Rising global temperatures from climate change means more fires

A plane lands at Billy Bishop Toronto City Airport as wildfire smoke fills Toronto, on Wednesday, July 15, 2026.

A plane lands at Billy Bishop Toronto City Airport as wildfire smoke fills Toronto, on Wednesday, July 15, 2026.

More than 5,740 square miles of land in the United States had burned by this point in the year — about 31% above the 10-year average for the same date, according to federal fire data. Over the past decade, the annual average area scorched in the U.S. has more than doubled compared with 30 years ago.

Europe recorded unprecedented burned acreage in 2025. Canada has experienced multiple record or near-record fire seasons in the 2020s, and parts of the Arctic have seen extraordinary levels of burning.

“As climate change drives hotter and drier conditions, wildfires are becoming both more frequent and more intense,” said Colleen Reid, a geographic health expert at the University of Colorado. “And where there is fire, there is smoke.”

Larger particles in wildfire smoke tend to settle near the flames, but the smallest particles — those considered most harmful — can travel hundreds or even thousands of miles. These fine particles are often about one micron in size, small enough to penetrate deep into the lungs, Reid explained.

Inside the body, particles attack

A person wearing a mask walks in Times Square as smoke from wildfires blankets the sky, Thursday, July 16, 2026, in New York.

A person wearing a mask walks in Times Square as smoke from wildfires blankets the sky, Thursday, July 16, 2026, in New York.

To cause harm, the particles must first bypass the body’s natural defenses, including nose hairs and mucus. Once inhaled, they lodge in the lungs and can pass into the bloodstream.

Because these particles often carry a variety of chemicals and have large reactive surfaces, they activate the immune system. Cells send out distress signals, triggering inflammation that can produce symptoms within minutes or hours. The heart and lungs are especially vulnerable, Montrose said.

For many people, the consequences are fatal.

Between 2006 and 2020, an estimated 24,100 deaths per year in the contiguous United States were linked to long-term exposure to fine particles from wildfire smoke, according to research published this year. Projections suggest that as the climate continues to warm, the economic and human toll will climb sharply by midcentury.

Globally, wildfire smoke is estimated to contribute to 677,745 deaths annually. Nearly 39% of those deaths occur in children under 5, based on a comprehensive analysis combining environmental measurements, medical studies and computer modeling.

Nonfatal health effects are most evident in the respiratory system, particularly among people with asthma.

After prolonged smoke exposure in Canada’s subarctic region, Howard and colleagues observed a doubling of asthma-related emergency visits and roughly a 50% increase in pneumonia cases.

Even individuals without preexisting respiratory disease can experience coughing, chest tightness, sore throats and headaches when smoke levels are high, Johnson said.

There are ways to minimize the risks

National Guardsmen patrol the Lincoln Memorial as the sun, obscured by wildfire smoke, rises above the Washington Monument and the Reflecting Pool, Friday, July 17, 2026, on the National Mall in Washington.

National Guardsmen patrol the Lincoln Memorial as the sun, obscured by wildfire smoke, rises above the Washington Monument and the Reflecting Pool, Friday, July 17, 2026, on the National Mall in Washington.

Research also suggests that smoke exposure may impair concentration and decision-making. Some patients seek emergency care for worsening depression during heavy smoke events, Howard noted. Finding indoor spaces with clean, filtered air — such as public cooling or clean-air shelters — can offer temporary relief and allow for light physical activity away from pollution.

Health experts recommend wearing well-fitted, high-quality masks outdoors during smoky periods, acknowledging that no mask offers complete protection. Indoors, sealing windows and doors, maintaining ventilation systems and replacing air filters can reduce exposure.

“If possible, limiting your time in smoky air is the most important step,” Johnson said.