More than 1 billion people each year experienced heat waves and fire pollution simultaneously from 2010 through 2019, according to research published Wednesday in Science Advances. That marked a sharp increase from the previous decade and exposed a worsening public health crisis driven by global warming.

The research represents the largest global assessment to date of simultaneous exposure to heat waves and fires, particularly wildfires. Researchers combined detailed information about hourly temperatures, fire emissions, air quality, and populations to identify when and where people encountered both threats.

The number of people exposed to at least one day of heat and fire pollution climbed from 761 million annually between 2000 and 2009 to more than 1 billion annually between 2010 and 2019. Some of the greatest exposure occurred in countries with fewer resources for confronting the dangers.

“There are two reasons why the total number of days people spend exposed to both hazards can increase: more people are exposed, or people experience these conditions more often,” lead author Yao Wu, a postdoctoral research fellow at Monash University in Australia, said in a written statement to Inside Climate News.

“Our findings show that the second reason plays a much bigger role,” Wu added, noting that “the problem is growing mainly because people are experiencing these two hazards together more often, not simply because there are more people.”

Researchers from universities in Australia and China called the trend “alarming.” They said public health plans that address only one climate related hazard at a time are inadequate as disasters increasingly overlap and expose communities to multiple threats.

The average annual number of days that each person experienced both heat and fire pollution rose from one day during 2000 through 2009 to 1.7 days during 2010 through 2019. Almost every global region recorded increases in annual average dual exposure days.

Zambia, the Democratic Republic of Congo, Malawi, Guatemala, and Burundi recorded the greatest exposure per person. Zambians experienced nearly a month of exposure per person each year, while smaller countries with frequent exposure tended to rank highly under that measurement.

A separate measurement counted total exposure days across each country’s entire population, favoring more populous nations where many residents were affected for fewer days. The Democratic Republic of Congo, Indonesia, Mexico, Brazil, and Russia together accounted for more than half of global exposure days between 2010 and 2019.

Countries with lower living standards, poorer health, and less education experienced the highest exposure per person and the largest increases, researchers said. Lower income countries often have less capacity to prepare for climate risks, while their air pollution connected to fires is four times worse than in wealthy countries.

Both dangers can be deadly. Extreme heat strains the heart, drains fluids through sweating, and can cause heatstroke that damages internal organs, including the brain, while wildfire smoke carries toxic fine particles associated with numerous health problems.

The combined health consequences require more research, but several studies have found that overlapping heat and smoke events send people to emergency rooms with worse heart and lung problems than either threat alone. Tarik Benmarhnia, a professor of climate epidemiology who was not involved in the study, said major research gaps remain.

“This study captures how climate change is actually showing up in people’s lives today, and how it will increasingly affect communities in the future: not one hazard at a time, but several at once,” said Benmarhnia, who is at the Scripps Institution of Oceanography at the University of California San Diego.

Human caused climate change is shifting global fire patterns, with rising temperatures now playing a larger role than in the past. Hotter temperatures, drier air, and parched soil make fires easier to ignite and spread, while fires can intensify heat waves by releasing heat and pollution.

“These processes create a positive feedback loop in which heatwaves and fires amplify one another,” the researchers said. Global warming also increases the likelihood that heat waves and major fires will strike at the same time.

The researchers cited record setting fires in Western Canada and the United States in 2021 that burned 3.2 million hectares. A heat dome contributed to between 21% and 34% of the burned area, while the study also noted fires across Portugal, Spain, and France in 2022.

It also cited Brazil’s 2020 Pantanal fire season, which was worsened by heat and drought and destroyed about one third of the world’s largest tropical freshwater wetland. Central and South America, Central and Eastern Africa, Southeast Asia, and parts of North America, Australia, and Russia were particularly affected.

Researchers recommended accessible emergency shelters, cooling centers, and buildings designed to protect occupants from extreme heat and smoke through ventilation that filters toxic particles and gases. Benmarhnia also suggested adding air quality warnings to heat alerts as governments prepare for combined climate dangers.