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Increased wildfires on wildland-urban interface create unique risks

by Wyoming Livestock Roundup

Increasing wildfire activity in the U.S. has been recorded, spurred by hotter, drier conditions in North America in recent decades. 

During a Sept. 20 webinar on their recent report, “The Chemistry of Fires at the Wildland-Urban Interface,” the National Academies of Sciences, Engineering and Medicine (NASEM) noted, “Many more fires are occurring at the wildland-urban interface (WUI) – the place where housing and other structures intermix or interface with wildland.”  

Nearly 70,000 communities and 43 million homes risk the impact of fire in the WUI, and the number continues to climb, with 41 percent of new housing units built since 1990 existing in the WUI. 

Because of growing risk, NASEM convened a cross-disciplinary team from across the U.S. to evaluate existing research and identify knowledge gaps related to fires on the WUI. The result was a peer-reviewed white paper defining opportunities and challenges.

Dave Allen, committee chair from the University of Texas-Austin, explains the study was guided by a statement of task, which convened the committee to “describe chemistry information which would improve mitigation of acute and long-term health effects of residential burning during wildfires at the WUI.” 

“Recognizing there may be knowledge gaps, our statement of task also directed us to describe opportunities for research to fill key decisions gaps and critical information gaps to aid decision makers who are charged with addressing the impacts of WUI fires,” Allen adds.

Risk and extent
of WUI fires

From 1970 to 2000, the WUI has grown 52 percent, and another 10 percent expansion is predicted by 2030, according to the National Academy of Sciences (NAS). Communities on the WUI see higher risk from the loss of structures since firefighting infrastructure and water sources are often limited in these areas. 

NAS cites several fires in particular demonstrating the severity of fires on the WUI, including the Marshall Fire of 2021. Named the most destructive fire in Colorado history, high winds launched a grass fire across Superior and Louisville, Colo., resulting in the damage or destruction of over 1,000 structures and 6,000 acres in Boulder County, Colo.

“Certainly, one of the things I learned in this study is the WUI is not strictly a Western phenomenon,” Allen states. “We hear a lot of coverage about Western fires, which tend to be larger than fires in the East, but we have interfaces between vegetation and urban structures throughout the U.S.” 

Not the average fire

“Another main message of the report is WUI fires are different than wildland fires and different than urban fires,” Allen explains. “There are differences in the composition of the fuels. A mix of vegetation and urban structures gives us a unique composition of fuels which might burn when the urban interface burns.”

He also cites differences in fuel loading, combustion conditions, atmospheric transformations, exposure and health effects. 

“Structures burn differently in an urban fire compared to a WUI fire. In an urban fire, the ignition source is typically within the structure, and the structure will burn from the inside out,” he explains. “In a WUI fire, the ignition site is on the outside, so the structure will burn from the outside in. Therefore, the combustion components are very different.” 

Difference in combustion conditions lead to differences in emissions, which result in different chemistries in the fire plume, which may be distributed thousands of miles. The resulting exposures and health effects can be different as well. 

“WUI fire can have substantial negative impacts on human health, visibility and quality of life, not only in the vicinity of the fire, but also hundreds of kilometers downwind,” the report continues. “This can affect millions of people outside the fire zone.” 

“For example,” the report continues, “about seven million people in the Bay Area of northern California were affected by elevated particulate matter from the 2018 Camp Fire, which was more than 240 kilometers away.”

Further, chemicals in soils and water are of additional concern, as they “find their way into nearby buildings, soils and water streams from runoff,” according to the report. “The chemical composition of the runoff may include soot, ash and other suspended solids, combustion products from the buildings and materials that burned and firefighting foam often used to extinguish fires. 

Need for research

While it’s known WUI fires result in different health effects, impacts and chemistries, Allen also acknowledged data on these topics are extrapolated and inferred from known data, rather than from direct measurement.

“Direct data available on fires at the WUI are quite sparse,” he comments. “Not only is the data scarce, it is hard to get because we need to be at those interfaces.” 

The NAS committee identified a need for further research to adequately understand the chemistry of fires at the WUI. Topics needing to be further defined include: fuels and emissions; transports and transfer of the affluents; and exposure and health.  

“As part of these research activities, deliverables should address the needs of decision makers,” Allen says. “We can make a lot of progress in research, but we also have to get information to decision makers who are addressing fires on a day-to-day basis.”

He continues, “We also suggested it [research activities] be made a part of these research programs to develop these actionable items aimed at decision makers.” 

Reducing risks

With high-risk situations across the West, actions can be taken to both minimize exposure and inhibit the spread of wildfires, both in wildland and urban areas, according to researchers. 

“WUI fire risks can be reduced through building codes and actions to protect homes,” researchers agreed, noting landowners on the WUI should consider using ignition resistant, hardened and non-combustible building materials. 

“In wildlands, we should use land management approaches to reduce fuel loading or otherwise modify fuel characteristics,” researchers explain, citing data from the U.S. Forest Service. “Approaches include grazing, prescribed fire, thinning and other biomass reduction programs.” 

For communities, it’s important to maintain protective buffers around communities and provide setbacks from areas more at risk when it comes to fast-moving wildfires. 

“Maintain a defensible space around structures by removing dead or dying vegetation, replacing vegetation with noncombustible materials and maintaining space in between trees and homes,” researchers note. 

NASEM cites more research is necessary to “enhance exposure mitigation specific to WUI fires.” 

For wildland firefighters, NAS recommends showering and wearing clean clothes, use of respiratory protection and use of exposure monitoring equipment for firefights. Additionally, use of job rotation to reduce exposure risk is important to minimize impacts to firefighters. 

“Risk communication is critical for minimizing harm to residents affected by any disaster. However, risk communication can help only to some extent,” the report emphasizes. “Health care providers also play a critical role in advising patients on actions they can take to reduce exposure related to fires.”

The study was sponsored by the Centers for Disease Control and Prevention, National Institute of Environmental Health Sciences, National Institute of Standards and Technology and the President’s Committee Funds of the National Academies. 

Saige Zespy is a corresponding writer for the Wyoming Livestock Roundup. Send comments on this article to

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