Summary¹

Wildland fire activity in the United States is increasing as climate change drives more frequent extreme weather events, including heat waves and droughts. Growth in wildland fire size and intensity has also been driven by historical land management practices that emphasized fire exclusion. Simultaneously, urban development has been expanding into wilderness areas, and fires at the interface between urban and wildland areas are also increasing.

The wildland-urban interface, or WUI, is conceptually defined as the area where structures and other human development meet undeveloped wildland or vegetation fuels. Many slight variations on this broad definition exist, and this report uses a specific definition based on the densities of structures and how those structures either intermix or interface with wildland. In these WUI areas, the combination of natural and human-made materials that burn in fires leads to additional atmospheric emissions, residues, and effluents not typically found in wildland fires. The reactive emissions from the combustion of urban structures will alter the chemistry of WUI fires from that of wildland fires. Wildland fires will also alter the combustion of urban structures, with structures at the WUI generally burning from the exterior inward, rather than the interior outward, leading to different emissions than those encountered in urban structure fires. WUI fires may also lead to higher human exposures than wildland fires because of their proximity to communities, and because individuals whose occupations bring them in proximity to wildland fires are increasingly being exposed to WUI-type fires. While WUI fires lead to emissions that are different than for wildland and urban fires, the precise nature and extent of those differences is not well understood.

As efforts are made to understand fire dynamics; fire progression; emissions, effluents, and residues from fires; and human exposure to toxicants at the WUI, the interdisciplinary quality of this problem has led to the involvement of multiple governmental agencies. To enhance their efforts, the National Institute of Standards and Technology, the Centers for Disease Control and Prevention, and the National Institute of Environmental Health Sciences asked the National Academies of Sciences, Engineering, and Medicine (the National Academies) to evaluate how information about chemistry can be used to inform the mitigation of acute and long-term health effects of WUI fires and to recommend chemistry research that could help to inform decision-makers charged with mitigating wildfire impacts. To address this request, the National Academies created the Committee on the Chemistry of Urban Wildfires, which prepared this report.

Three overarching themes of the committee’s work were the growing importance of WUI fires, the complexity of the topic, and the lack of data specifically relevant to the chemistry of fires at the WUI. Tens of millions of homes

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¹ This Summary does not include references. Citations for the findings presented in this Summary appear in subsequent chapters of the report.

are currently found in WUI communities; this number of structures is growing, and the risk to these communities from fires is growing. Health risks associated with toxicants from WUI fires can persist well beyond the active burning of the fire, due to contamination of the ecosystem (water and soil) and built environments. Health impacts can also extend well beyond the WUI communities, as smoke can be transported for hundreds to thousands of kilometers. While toxicants from WUI fires are a recognized problem, their variability and complexity present significant challenges. For example, the emissions from a WUI fire can vary depending on whether the fire burns homes, cars, or commercial areas; even a subset of these fuel types can vary, such as homes of different ages made of different materials. Emissions vary depending on fuel composition, fire characteristics, and the heating dynamics that the fuels experience. Human exposure can vary greatly, depending on weather patterns during and after the fire, personal activities, and the living and working circumstances of the people exposed. While few data are available on emissions and exposures from fires at the WUI, knowledge of the chemistry, emissions, exposures, and health impacts of wildland fires and urban fires can be combined to hypothesize characteristics of WUI fires. Throughout the report, the committee examines what is known of wildland fires and urban fires and uses that information to identify potential emissions, exposures, and health impacts of WUI fires. The committee also recommends that the collection of data is needed to more definitively characterize the variability and overall acute and chronic impacts of WUI fires to the environment and human health. This summary presents an overview of each topic and the proposed research agenda.

FUEL, COMBUSTION, EMISSIONS

Emissions from fires in the WUI can differ greatly from emissions from wildland fires. The fuels burned in WUI fires have different compositions, densities, and quantities of combustible materials than the vegetative biomass combusted in wildland fires. The urban materials and their characteristics impact the combustion conditions, the chemical reaction pathways that dominate during combustion, and the emissions to the air and the residues that remain.

Few data are available on the fuels, combustion characteristics, and emissions, effluents, and residues from WUI fires. Characteristics of these fires can currently be inferred only from the superposition of the characteristics of wildfires and urban fires. Numerous laboratory studies and field efforts have investigated wildland fire emissions, leading to estimates of emissions of multiple individual chemical species and of multiple types of particulate matter. A detailed understanding of the relationships between fuel loading, fire emissions, and fire conditions remains an area of active investigation. The research community’s knowledge of emissions from the combustion of urban materials is derived largely from studies of the toxicity of emissions from enclosure fires (i.e., a fire within a room or compartment inside a building) or from laboratory test methods simulating enclosure fire conditions. These studies have demonstrated how both the material composition and the amount of oxygen available strongly impact the emissions of some toxicants. However, very little is known about urban fuel chemical composition and these fuels’ effects on combustion processes, the types of species emitted, and species’ interactions under different fire conditions.

Integrating the current knowledge of wildland and urban fires into approximations of what may occur in WUI fires can provide directional guidance about the chemistry leading to emissions from WUI fires, but the data needed to characterize that chemistry are presently very sparse. Although WUI fires are increasing in frequency and in magnitude, very little is known about the emissions from WUI fires. Much of the current research on WUI fires has centered on ignition mechanisms and the characteristics of the built environment that lead to destructive fires. Even when data are available for wildfires that affect structures, it is difficult to separate the emissions that are unique to the WUI from those that result from wildland or urban combustion.

Using what is known about the materials in the WUI environment, and combustion conditions, the committee examined how different fuels, the fuel loading and distributions, and a myriad of factors could affect predictions of combustion characteristics and emissions for WUI fires. The WUI fuels differ from the vegetative biomass found in wildlands, particularly in chemical elements and materials of concern, such as halogens, plastics, and metals, which exist in much higher concentrations in the WUI. Potential reaction pathways and resulting emissions, effluents, and residues from WUI fires are postulated. Following the approaches used in wildland and urban fires, emission factors are used to summarize this information, but emission factors for WUI fires are currently speculative. Major chemical classes emitted from urban fires and wildfires are also reviewed to lay the foundation for assessing the human health and environmental impacts.

ATMOSPHERIC TRANSPORT AND CHEMICAL TRANSFORMATIONS

Emissions from WUI fires can substantially, negatively impact human health and quality of life, not only in the vicinity of the fire but also hundreds to thousands of kilometers downwind. This can affect millions of people outside the fire zone, depending on the location of nearby cities. Smoke from major fires sometimes affects air quality on a continental scale.

National monitoring networks provide some data for assessing exposures of routinely monitored air pollutants, such as fine particulate matter, that are associated with emissions from WUI fires; however, data are very sparse on the gas- and particle-phase smoke composition specifically associated with WUI fires, and how it is transformed over short and long distances. WUI smoke composition is dynamic within the fire zone and differs from wildfire smoke because of the different materials combusted in the WUI. The resulting plume composition changes over time (from minutes to days) due to atmospheric chemistry and physical processing, leading to changes in pollutant composition, and in the resulting exposures, downwind of fires. The impact of the unique WUI emissions and chemistry on regional exposures is not well understood. While a dominant route of exposure is inhalation, it is important to also recognize that wet and dry atmospheric deposition can be a source of contaminated water and soil downwind of wildfires and thus also impact exposures through ingestion. Additional WUI emissions known to be water soluble or found in wet deposition may also contribute to water/soil contamination and exposure through WUI-associated deposition.

WATER, SOIL, AND BUILDING CONTAMINATION

Combustion reactions for materials at the WUI (e.g., household components such as siding, insulation, textiles, and plastic, as well as the combustion of biomass) result in significant emission of potential toxicants to the surrounding environment. Although a significant part of these emissions, effluents, and residues are in the gas phase, there are also pathways for the mobilization (or partitioning) of some of these toxicants into nearby buildings, soils, and water streams. There is also the possibility of toxicants in the plume to be deposited downwind of the fire area, as has been observed in long-term studies on the impact of wildland fires. Finally, the active process of firefighting could add contaminants to the immediate area, in the form of flame retardants and other compounds.

All of these processes can impact the immediate conditions after the fire, which are critical as first responders and community residents arrive back in the area to assess damage. There is a dearth of peer-reviewed data documenting the specific impacts of WUI fires on water, soil, and settled dust contamination in built environments, with only a few studies providing critical information on potential impacts.

HUMAN EXPOSURES, HEALTH IMPACTS, AND MITIGATION

The identification of chemicals of concern for human exposure, environmental justice and vulnerable populations, and health impacts from WUI fire emissions is affected by the data limitations described above. The committee assessed potential exposures at multiple scales, from the immediate fire zone, where emergency responders such as firefighters experience direct exposure to heat and high concentrations of fire emissions, to the regional and continental levels, where WUI fire smoke can have an extended impact. Adverse health effects associated with exposure to fire emissions have been measured hundreds of kilometers downwind, such as the 240-km-downwind impacts of the 2018 Camp Fire on California Bay Area residents, to well beyond 1,000 kilometers, such as the 2016 Horse River Fire, which started near Fort McMurray, Alberta, Canada, and resulted in smoke transport and impacts on air quality more than 4,000 kilometers away in New York City. Much of the information presented in the report draws on studies from wildland fires, with limited WUI-specific information. More data specific to WUI fires may provide additional insights to inform research, policies, mitigation practices, and practices that reduce exposure and negative health impacts.

The current literature largely focuses on exposures and health impacts related to smoke inhalation. These include myocardial infarction, ischemic heart disease, dysrhythmia, heart failure, pulmonary embolism, ischemic stroke, and transient ischemic attack. Asthma exacerbations are significantly associated with exposure to wildfire smoke. Exacerbations of chronic obstructive pulmonary disease are also significantly associated with greater contaminant amounts and exposure to wildfire smoke in most studies.

Within exposures to the general public, specific groups may experience greater impacts, such as children, pregnant people, older adults, and immunocompromised people, as well as communities who experience disproportionate environmental and social stressors, such as those of lower socioeconomic status, some non-white racial and ethnic groups, and tribal populations. Heightened vulnerability to wildfire emissions includes higher health responses to a given level of exposure. For example, children are especially vulnerable to air pollution effects due to their small body mass and rapid breathing rates, leading to higher exposure doses than adults. Adults may also experience multiple aspects of vulnerability such as differential exposures, ability to adapt, and health response, which can be related to the availability of language-appropriate and culturally appropriate information and medical care, access to quality health care, and other structural factors. Additionally, low-income communities often do not have adequate cooling in their homes to allow for keeping the windows closed during the summer heat when wildfires are typically present. They also may not be able to afford the purchase of home air cleaners for particle and chemical filtration.

Emergency responders, such as firefighters, are at risk for injury, death, and acute and chronic health impacts of wildfires, as well as mental health effects due to trauma. Outdoor workers, such as farmworkers and utility workers, are at elevated risk for respiratory effects of smoke and associated pollutants, and this may be coupled with preexisting medical conditions. Additional vulnerability factors related to occupational settings may include elevated outdoor air and dust exposures to chemicals and particles, amount of time spent outdoors, and available protective respiratory equipment and training. Post-fire clean-up can also lead to exposure to fire residues for a wide variety of populations.

As in other problem areas that seek to estimate the emissions, exposures, and health impacts associated with WUI fires, there is limited information on human exposures that is specific to WUI fires.

MEASUREMENT SCIENCE

Throughout the report, the committee outlined the diverse data needs associated with understanding the chemistry of WUI fires and their emissions, which range from collecting data on fuel characteristics of heterogeneous structures, to measuring the concentrations, exposures, and health impacts of large numbers of toxicants, many of which will be present at trace levels. Collecting much of these data is challenging, since materials burned in WUI fires are heterogeneous, and identifying their contributions to WUI fire plumes requires fine spatiotemporal resolution.

Data and measurement needs are also interconnected, as shown in Figure S-1. Information about at-risk communities and vulnerable populations can help define the types of structures and potential fuels at the WUI. Data on fuel compositions will determine combustion pathways. The chemical species formed during combustion will determine which atmospheric reaction pathways will be most important. The atmospheric chemistry and transport will determine the toxicants to which communities are exposed and the manner of the exposure. Exposures will determine health impacts. Because of this interconnection, a coordinated approach to data collection, from fuel and emission characterization to exposures, is desirable. Use of consistent measurement methods and collection of data on chemical species over their entire cycle from emission to exposure will enhance the value of all of the data that are collected.

Data and measurements are also needed on various timescales. Retrospective studies of fuels, fires, emissions, exposures, and health impacts, in the field, in laboratories, and via modeling, will be important to improve the understanding of the chemistry of WUI fires. These types of studies will benefit from coordinated databases and information repositories that allow efficient sharing of information and methods. Decision-makers charged with mitigating the impacts of WUI fires, and informing the public with information to mitigate exposure to toxicants, will also need access to well-organized information, but their information needs will be much more immediate, requiring rapid access and a near-real-time response.

RESEARCH AGENDA

WUI fires have the common characteristics of fuel mixes that are distinct from both wildland and urban fires, and a combustion chemistry that is spatially and temporally heterogeneous. These unique characteristics of WUI fires can lead to toxicants and exposures that are largely uncharacterized. The committee identified research that could (1) improve understanding of the fuels and emissions associated with WUI fires; (2) characterize the chemistry,

transport, and transformations of emissions; and (3) quantify exposures and health effects that result from WUI fire emissions. In addition, the committee identified improvements in measurement science and analytics that could make broad contributions to improve research capabilities. Priority research needs within this broad agenda are summarized in Table S-1. Priority research areas were identified based on the information needs of decision-makers, the timeliness and cost effectiveness of research initiatives, the magnitude of the effect being investigated, and the potential impact of broad scientific capabilities. The relative importance of these prioritization criteria were not explicitly weighted; individual chapters describe the criteria used in the choice of priority research areas and also define a broader set of research needs than those summarized in Table S-1. To be most effective, the research agenda should be carried out with multidisciplinary teams who integrate work that ranges from hazard identification and emission characterization to measurement of exposures and health outcomes.

Recommendation 1: To understand the chemistry, exposures, and health impacts of toxicants resulting from WUI fires, researchers and agencies that fund research should implement an integrated, multidisciplinary research agenda. Agencies funding, and investigators performing, research on WUI fire emissions should coordinate their research plans, and they should create widely accessible repositories for data and information relevant to WUI fires.

Implementing an effective research agenda will be challenging. Research has been conducted on emissions from wildland and urban fires for decades, and this research has led to significant advances in understanding of wildland fuels, wildland fire emissions, transformation and transport of emissions, and health impacts of toxicant exposures. There is still very little confidence, however, in making connections such as the impact of fire intensity to emission composition and downwind chemical evolution. Similar experiences are to be expected in research on

TABLE S-1 Research Priorities for Fires at the Wildland-Urban Interface (WUI)

WUI fire emissions. Research on emissions from WUI fires can build on the extensive knowledge base developed for wildland fires, but understanding the impacts of combustion of mixed wildland and urban fuels will require new information on fuels, emissions, transport and transformations, and health impacts of toxicant exposures. Making connections between research areas from emissions to exposure and health impacts will continue to be a challenge. The variability in the types of structures in WUI communities and the types of fire hazards that communities and vulnerable populations are exposed to also spans a wide range, and will add complexity. Building a comprehensive and fundamental body of knowledge on WUI fires will require a long-term effort.

The information needed to understand toxicants emerging from wildfires, structural fires, and WUI fires; their effects; and who they affect spans a broad range of scientific disciplines. Research findings from each of these disciplines need to be continuously communicated across disciplinary boundaries because each step in the chain—from hazard identification to quantification of exposures and health effects—depends on information emerging from other steps. This trans-disciplinary data and information sharing process could begin with information available on emissions from wildland and urban structure fires and be expanded to include information and data from WUI fires. This information sharing process could also include expansion of existing mechanisms for communication with decision-makers and the public, such as www.ready.gov/wildfires.

Recommendation 2: Those implementing research programs should design and implement a multidisciplinary WUI research program that includes the development of tools, resources, and messaging designed to inform a wide variety of decision-makers charged with mitigating wildland and WUI fire impacts. They should also create periodic summaries of policy-relevant research findings, and actionable messaging for decision-makers working with at-risk communities and vulnerable populations.

Policy-relevant research findings and actionable messaging could include recommendations for building materials to be used in WUI communities, strategies for reducing interior environment fire risks, public information regarding the effectiveness of measures to mitigate exposures, and community mappings of toxicant precursors, accessible to decision-makers.

More than 40 million homes in the United States are located at the WUI. Diverse and vulnerable populations will be increasingly exposed to hazards from WUI fires. The development of a multidisciplinary WUI research program, summarized in Table S-1, will have long-term benefits, but immediate action is also needed. The committee identified a number of areas where rapid action could have immediate benefits. These areas are listed in Box S-1. Commitment to both long-term progress and immediate action on improving the understanding of WUI fires would benefit communities throughout the United States and the world.

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