The line between human settlement and wilderness is becoming the front line of a new chemical crisis.
In 2003, atmospheric chemist Ryan Sullivan moved to San Diego, eager to begin his PhD studies. Within weeks, the Cedar Fire—driven by powerful Santa Ana winds—exploded from wildlands into the city's outskirts. The air filled with acrid smoke that would linger for days, destroying thousands of homes, claiming lives, and forcing residents to breathe the poorly understood pollution7 . Two decades later, this scenario has become alarmingly familiar to millions worldwide.
As climate change creates hotter, drier conditions and communities expand into natural areas, wildfires are increasingly invading developed landscapes. These aren't ordinary wildland fires anymore. When flames consume not just trees and brush but homes, vehicles, and entire communities, they undergo a dangerous chemical transformation that scientists are just beginning to understand.
The Wildland-Urban Interface (WUI) represents zones where human development meets undeveloped wildland. Since 1990, approximately 41% of new housing units in the United States have been built in these interface areas, placing an estimated 43 million U.S. homes at risk from WUI fires1 2 .
41% of new U.S. housing since 1990 built in WUI zones
43 million U.S. homes at risk from WUI fires
What truly distinguishes WUI fires is their fuel. Unlike traditional wildfires that consume primarily biomass, WUI fires feast on a complex mixture of natural materials and synthetic substances from our homes and possessions.
"The chemistry and ultimate health impacts of WUI fires are still poorly understood. They have unique chemistry due to the combination of natural and human-made fuels that are burned, which may lead to the formation or release of toxic emissions not found in purely wildland fires"1 .
The average home contains a surprising variety of potential fuels, each with distinct chemical compositions that transform into dangerous emissions when burned:
Wood, asphalt shingles, plastics in wiring, vinyl siding, and various insulation materials7
Plastics, synthetic fabrics, electronics, and household chemicals7
Plastics, rubber tires, flammable fluids, and batteries2
When these materials burn alongside natural vegetation, they create a complex chemical cocktail with serious implications for human health.
| Material | Most Commonly Released Fire Emissions |
|---|---|
| Polyurethane foam insulation | Hydrogen cyanide (HCN), carbon monoxide (CO), isocyanates, volatile organic compounds (VOCs), polycyclic aromatic hydrocarbons (PAHs) |
| Vinyl siding/PVC windows | Hydrogen chloride (HCl), dioxins (PCDDs), furans (PCDFs) |
| Oriented strand board | Formaldehyde, acrolein, VOCs, isocyanates |
| Electrical wiring insulation | HCl, dioxins, furans |
| Acrylic clothing | HCN, CO, ammonia (NH₃), VOCs |
Source: Adapted from The Chemistry of Fires at the Wildland-Urban Interface2
Cancer, reproductive and developmental effects, immune suppression, endocrine disruption
Cancer, reproductive and developmental toxicity, kidney and liver damage
Cancer, respiratory irritation, neurotoxicity
Cardiopulmonary effects, cancer, immunosuppression
Source: Adapted from The Chemistry of Fires at the Wildland-Urban Interface2
To understand this complex chemistry, researchers are conducting controlled burns of fully furnished structures—literally burning buildings in the name of science.
At the Insurance Institute for Business and Home Safety Research Center in South Carolina, researchers designed a crucial experiment to mimic real WUI fire conditions. They constructed detached dwelling units and furnished them realistically—complete with wiring, plumbing, and everyday items purchased from big-box stores7 .
"I have a house, I've got three kids," said EPA researcher Amara Holder, explaining how she decided what to include in the experimental rooms. "I know what a house looks like." This attention to detail even extended to including popular stuffed toys missing from traditional scientific literature7 .
An 18-meter-wide fume hood was positioned above the structure to capture all smoke emissions7
The smoke traveled through ducts to a sophisticated suite of analytical instruments including liquid and gas chromatographs and mass spectrometers7
Drone-mounted filters and sorbent tubes collected samples of particulate matter, organic compounds, metals, and gases directly from the smoke plume7
Researchers used multidimensional chromatography and mass spectrometry to identify hundreds to thousands of different chemicals in the emissions7
Preliminary results from these experiments reveal how specific materials directly impact emissions. Holder noted, "When we had drywall in there, we had really high sulfur dioxide emissions. If we didn't have drywall, we didn't have those emissions"7 .
This precise understanding of emission sources is crucial for forecasting air quality during fire events and developing safer building practices. The UC Berkeley team involved in these experiments, including atmospheric chemist Allen Goldstein and fire scientist Michael Gollner, has now conducted multiple burns to build a comprehensive dataset of WUI fire emissions7 .
Understanding WUI fires requires specialized approaches and instruments. Here are key tools and methods researchers use to study these complex events:
Replicate real-world fire conditions at full scale to study combustion and emissions
Identify and quantify chemical species in smoke samples
Collect samples directly from fire plumes for analysis
Inventory and characterize combustible materials in WUI communities
Predict how toxicants move through air, water, and soil
Determine mass of pollutants produced per kilogram of fuel burned
The health impacts of WUI fires extend far beyond the evacuation zones, creating significant equity concerns. The National Academies committee emphasized that vulnerability to WUI fire effects falls disproportionately on certain groups1 :
Face increased exposure due to their jobs, with physically demanding work increasing respiratory exchange with polluted air1
Take about 40 breaths per minute compared to 12-20 for adults, leading to higher relative exposure2
May lack proper ventilation systems or the financial means to relocate during smoke events1
A 2017 report estimated that wildfires in the United States, including WUI fires, cost between $71 billion and $347 billion per year—including preventive measures, disaster response, injuries, mental health impacts, and loss of life1 .
In 2022, the National Academies released a consensus study report calling for an integrated, multidisciplinary research agenda to address critical knowledge gaps about WUI fires5 7 . Priority research areas include:
Mapping WUI communities and their material compositions5
Characterizing combustion conditions specific to WUI environments5
Identifying secondary toxicants formed as emissions travel downwind5
Linking specific exposures to health outcomes across diverse populations1
This research could inform practical solutions such as improved building codes, better-informed evacuation guidance, and targeted air quality monitoring. As climate change intensifies and more people live in interface areas, understanding the unique chemistry of WUI fires becomes increasingly crucial for protecting communities.
What remains clear is that the problem of WUI fires represents not just a fire management challenge, but a complex chemical and public health crisis—one that demands our immediate attention and a deeper understanding of what happens when our suburbs burn.