Wildfire smoke seeps into homes. Global protection would cost billions.

When wildfires burn, residents as far as hundreds of miles away may be advised by health officials to stay inside, to protect themselves from the harmful smoke that can travel across entire continents.

But sheltering indoors might not be as safe as we think, according to research published Wednesday in the journal Science Advances, because microscopic pollution from these blazes can seep into homes and other buildings. More than a billion people worldwide - around 1 in 8 on Earth - were exposed to at least one day of unhealthy air from wildfires annually from 2003 to 2022 while staying inside, researchers found.

When it comes to protecting yourself from wildfire smoke, the indoors are safer than the outdoors, but exposure to dangerous particles from smoke is still a possibility.

Air purifiers and proper insulation can help stave off these hazards. But they are costly, creating disparities in which poorer parts of the world bear the brunt of the harm, the researchers said. According to their models, the countries that experienced the most indoor wildfire smoke exposure were predominantly low-income and lower-income countries in central Africa.

“The unequal distribution of resources, resulting from socioeconomic disparities, has the potential to exacerbate global injustice,” the authors wrote, “by exposing those who cannot afford the cost of air purifiers to higher health risks.”

Wildfires - and their smoke - are becoming more and more of a reality for much of the world. The season has kicked off in North America with fierce blazes and plummeting air quality in many parts of the country, including the Southeast, Midwest and Mid-Atlantic. The American Lung Association’s 2025 report on air quality found that 25 million more people in the United States breathed unhealthy air last year than the year before, due in part to extreme heat, drought and wildfires.

Past research has mostly focused on outdoor exposure to wildfire particulate matter, but people spend more than 80% of their time indoors, the authors said. And “during wildfire smoke events, people are often advised and inclined to stay indoors to seek refuge from both the smoke and heat.”

Smoke contains fine particles, referred to as PM2.5, that are about a 30th the width of a human hair and can travel into our hearts and lungs - as well as creeping through the minuscule cracks in our windows and doors. Exposure to these particles has been linked to respiratory illnesses, dementia, Alzheimer’s disease and increased overall mortality.

Air purifiers can mitigate these hazards, but they are expensive. The researchers looked at three hypothetical scenarios of global air-purifier use and estimated the cost per country. Ensuring every house experiences the World Health Organization-recommended annual limit of PM2.5 concentrations under 5 micrograms per cubic meter could cost more than $4 trillion, according to the researchers’ models. But pollution and even certain kinds of cooking can all produce these fine particles. Just mitigating wildfire PM2.5 would cost more than $68 billion.

The western coast of North America and northern Asia - where large Siberian wildfires regularly rage - would see the highest total price tag, but Africa would incur the highest cost per person.

Wednesday’s study is one of the few to look at wildfire smoke indoors across the world.

“Looking at the issue from the global perspective is very difficult to do,” said Yifang Zhu, a professor in the Department of Environmental Health Sciences at the UCLA Fielding School of Public Health. Data across countries on wildfire-related indoor air quality is limited and uneven. “There is also a general scarcity of prior research on this topic in many parts of the world,” she added. Zhu and her team have been collecting samples and data since the Los Angeles fires in January to understand more about their impacts on public health.

The authors’ global ambition comes with caveats, noted Zhu, who was an anonymous peer reviewer of the paper. Given the lack of localized data from every country, the scientists had to make assumptions. But because the authors combined their data from many sources and developed a range of scenarios, the main findings of cost and vulnerability are clear, she said - including the financial burden many regions face to improve indoor air quality during fires.

“Air purifiers are simple and easy to implement, but they have a certain economic burden in low- and middle-income countries and require policy design,” said Bin Zhao, one of the paper’s authors and an associate professor at Tsinghua University’s School of Environment.

For Zhu, such blueprints would entail more subsidized assistance.

“The cost in my mind, especially for those low-income countries, should not be borne by individuals at a personal level. This is more a government responsibility,” she said, adding that people shouldn’t have to decide between whether to eat or whether to buy an air purifier.

“These people have the right to breathe clean air inside their homes,” she said.