The smoke from fires that burn near populated areas has far greater health impacts than smoke from wildfires in remote areas, new research finds.

The new study, published last week in Science Advances, estimates that emissions from fires at the wildland-urban interface (WUI) are about three times more likely to lead to annual premature deaths than emissions from wildfires in general. This is because the fires, and their associated emissions, are far closer to populated areas.

The wildland-urban interface is the geographic area where wildland vegetation and developed land come together or intermingle. WUI areas have been expanding on all populated continents and now constitute about 5 percent of the world’s land area, excluding Antarctica.

With this expansion have come devastating fires. Some of the deadliest WUI blazes in recent years include the 2009 Black Saturday bushfires in Australia that directly killed 173 people, the 2018 Attica fires in Greece that killed 104, and the 2023 Lahaina Fire in Hawaii that killed 100. At the beginning of this year, a disastrous outbreak of fires in Southern California burned an estimated 16,000 homes, businesses, and other structures, with estimates of financial losses ranging up to $250 billion or higher.

“Even though the emissions of WUI fires are relatively small globally, the health impacts are proportionately large because they’re closer to human populations,” said Wenfu Tang, an atmospheric scientist at the National Science Foundation National Center for Atmospheric Research (NSF NCAR) and lead author of the new study. “Pollutants emitted by WUI fires such as particulate matter and the precursors to ozone are more harmful because they’re not dispersing across hundreds or thousands of miles.”

“It’s important we start modeling these types of fires and their impacts, and this is a really important step forward,” said Christine Wiedinmyer, CIRES Associate Director for Science and co-author of the new study. “The results show fires near urban landscapes can have devastating effects, and the study emphasizes that we should be studying them more.”

One of Tang’s previous studies used satellite observations and machine learning techniques to show the fraction of global fires that occur in WUI areas has increased significantly this century.

Building on that work, Tang and her colleagues wanted to estimate the health effects of the fire emissions beyond the immediate deaths. Certain pollutants associated with smoke, such as fine particulate matter and ground-level ozone, are especially harmful to cardiovascular and respiratory systems.

In the new study, the researchers turned to an advanced NSF NCAR-based computer model to simulate pollutants from fires. Their modeling included carbon monoxide chemical tracers, which enabled them to estimate the sources of emissions and differentiate between wildland and WUI fires.

They also used a dataset of WUI fires in the recent two decades worldwide, which Tang and her colleagues developed last year.

The results showed that WUI fire emissions constituted 3.1 percent of all fire emissions across the six populated continents in 2020. However, the fractional contribution of WUI fire emissions to premature deaths was 8.8 percent of all fire emissions because of how many people were affected by smoke from WUI fires.

A critical factor that Tang wants to examine next is the difference in emissions from wildland fires that consume trees and other vegetation as opposed to WUI fires that burn down structures that often contain additional toxic substances. The smoke from different burned materials may have widely varying impacts on human health.

“It is very important to have an emission inventory that explicitly accounts for the burning of structures,” Tang said. “We need to know what is being burned in order to determine what is going up in smoke.”