Wildfire activity in the United States is increasing, driven in large part by hotter and drier conditions across North America over the past century.

At the same time, many more fires are occurring at the wildland-urban interface (WUI)—the place where housing and other structures intermix or interface with wildland. An estimated 70,000 communities and 43 million U.S. homes are at risk from WUI fires, up from about 31 million homes in 1990—and, since 1990, about 41% of new housing units have been built in the WUI.

WUI fires carry a number of unique risks, because of their proximity to communities and also the human-made materials and fuels that are burned, such as higher human exposures to smoke and toxic emissions not found in purely wildland fires. Research is needed to better understand these emissions and guide actions to reduce the severity of WUI fire risks and their consequences.

WUI Expansion and Fire Risks

The WUI in the continental United States expanded 52% from 1970 to 2000. Researchers project the WUI to grow more than 10% by 2030. Expansion of WUI land areas has been more rapid in the eastern United States than in the West, but the majority of WUI areas in the East are characterized as low-severity fire regimes compared to just 12% in the West.

Figure 1
FIGURE 1The map shows land at the WUI “interface” or “intermix” as of 2010 (top) and the percent housing units in WUI areas (bottom). Adapted from Stewart et al (2007) using data from Radeloff et al (2018)

High Risk of Loss in WUI Fires

Factors that put WUI communities at higher risk of loss include the density of structures and the unique value of those structures, and a lack of firefighting infrastructure such as roads and sources of water. In the past ten years, wildfires have destroyed tens of thousands of structures and cost many lives as illustrated by stories of some recent WUI fires below.

Some Recent WUI Fires

Fuels and Emissions from WUI fires

Many WUI fires occur in residential areas where the primary fuels are those in and around the home, which can include a diverse array of materials. The elemental composition of those materials has a direct impact on the combustion chemistry and emissions.

Examples of the types and quantities of materials that can be found in the home.
FIGURE 2 Examples of the types and quantities of materials that can be found in the home.

Hover over acronyms below for full emission name.

Material Most commonly released fire emissions

Polyurethane foam in insulation

HCN, CO, NO, NO2, NH3, HCl, H3PO4,
PM, PAHs, VOCs, SVOC, TCPP, TCEP, PCDDs, PCDFs, isocyanates

Polyisocyanurate foam in insulation

HCN, CO, NO, NO2, NH3, HCl, H3PO4,
PM, PAHs, VOCs, SVOC, TCPP, TCEP, PCDDs, PCDFs, isocyanates

Phenolic foam in insulation

SO2, CO, HCl, acrolein, formaldehyde

Extruded polystyrene in insulation


Glass wool in insulation

HCN, CO, NO2, HCl, isocyanates

Oriented strand board (OSB)

HCN, CO, NO2, HCl, acrolein, formaldehyde, PM, PAHs, VOCs, SVOCs, isocyanates

Vinyl siding and/or polyvinyl chloride (PVC) windows


Upholstery on furniture

HCN, CO, NO, NO2, NH3, HCl, H3PO4,
PM, PAHs, VOCs, SVOC, TCPP, TCEP, PCDDs, PCDFs, isocyanates

Vinyl carpet


Polyamide carpet

HCN, CO, NO, NO2, NH3, PM, PAHs, VOCs, SVOCs, isocyanates

Electrical wiring insulation


Acrylic clothing
Residential furniture

HCN, CO, NO, NO2, NH3, PM, PAHs, VOCs, SVOCs, isocyanates
Benzene, toluene, formaldehyde, organophosphate flame retardants

Hover over acronyms below for full emission name.

Car Component Fire Emissions

Door panel

CO, HCl, HCN, NO, PCDDs/PCDFs, VOCs, PAHs, isocyanates

Ventilation system

CO, acrolein, formaldehyde, PAHs, VOCs

Floor material

CO, HCN, isocyanates, PAHs, VOCs


CO, HCN, NO, isocyanates, PAHs, VOCs

Upholstery material

CO, HCN, NO, HCl, SO2, isocyanates, PAHs, PCDDs/PCDFs, VOCs

Electrical wiring




Distribution of WUI Fire Impacts

WUI fires 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. This can affect millions of people outside the fire zone. For example, about 7 million people in the Bay Area of northern California were affected by elevated particulate matter from the Camp Fire, which was more than 240 km away.

Figure 3: Daily fire locations map
FIGURE 3Example of daily fire locations (left) and fire plume projections (right) from the National Oceanic and Atmospheric Administration for September 16, 2021, when fire plume projections from Idaho and California extended as far east as Minnesota.

Contamination of Soils and Water

Although a significant part of pollutants from fires are in air, the toxicants also find their way into nearby buildings, soils, and water streams from runoff. The chemical composition of the runoff may include soot, ash, and other suspended solids; combustion products from the buildings and other materials that were burned; and, if used as a firefighting agent, the firefighting foam.

Figure 4: Potential exposure pathways to WUI fire pollutants
FIGURE 4Distribution of emissions and pollutants from WUI fires. In 2020, one in seven Americans (about 47 million people) experienced dangerous air quality from wildfire smoke

Health Effects of WUI Fires

Many of the chemicals that have been measured in WUI fire emissions have the potential to cause acute, chronic, or delayed health effects through inhalation, dermal, or ingestion exposure. Many of the toxicants are known or probable carcinogens, irritants, respiratory sensitizers, or reproductive and developmental toxicants. Table 3 summarizes the potential exposure routes and health outcomes.

Fibrous asbestos, chrysotile
Cancer; asbestosis; respiratory irritation; pleural disease
Asphyxiant gases
Carbon monoxide (CO), carbon dioxide (CO2), hydrogen cyanide (HCN)
Inhalation Depression of central nervous system and hypoxia; acute respiratory effects.
Dioxins and furans (polychlorinated dibenzo dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs)) 2,3,7,8-Tetrachloro-dibenzo-p dioxin/furan (TCDD/F) Inhalation
Dermal (low penetration into skin by itself, but can cause skin lesions)
Cancer or predisposition to cancer; reproductive and developmental effects; immune suppression; dermal toxicity; endocrine disruption
Flame retardants
Tris(1-chloro-2-propyl) phosphate (TCPP), tris(2-chloroethyl) phosphate (TCEP), tris iso butylated triphenyl phosphate (TBPP), methyl phenyl phosphate (MPP) Inhalation
Neurotoxicity or neurodevelopmental damage;
reproduction and fetal development effects; endocrine and thyroid disruption
Inorganic acid gases Hydrogen chloride (HCl),
hydrogen fluoride (HF), phosphoric acid, SOx, NOx
Inhalation Chemical burns; increased risk of laryngeal and lung cancer
Inorganic and organic metals Lead, lithium, iron, mercury, methylmercury, nickel, cadmium, palladium chloride Inhalation
Neurotoxicity; reproductive and developmental effects, dermal irritation or allergen; respiratory irritation
isocyanates Methyl isocyanate, methylene diphenyl diisocyanate, toluene diisocyanate Inhalation Irritation and pulmonary sensitivity
Organic and other gases Phosgene (COCl2), ammonia (NH3) Inhalation Acute effects; pulmonary edema and irritation
Ozone (O3)   Inhalation Acute and chronic respiratory symptoms including coughing and exacerbation of chronic diseases such as bronchitis and asthma; increased risk of pulmonary infections
Particulate matter (PM)
PM is often classified by size, where the size is based on the aerodynamic diameter in micrometers (e.g., PM2.5); smaller particles penetrate deeper into the respiratory system Inhalation
Cancer; cardiopulmonary toxicant; immunosuppressant; neurotoxicant; reproductive and developmental toxicity
Plasticizers Ortho phthalates including dibutyl phthalate, terephthalates, adipates, benzoates Inhalation
Endocrine disruptors; reproductive and developmental toxicity
Polycyclic aromatic hydrocarbons (PAHs) Benzo[a]pyrene, benzo[a]anthracene, benzo[b]fluoranthene, chrysene, pyrene, fluoranthene, naphthalene, anthracene Inhalation
Cancer; reproductive and developmental (teratogenic) toxicity; kidney and liver damage
Polychlorinated biphenyls (PCBs) 2-Chlorobiphenyl, 2,2-dichlorobiphenyl, 2,4,5-trichlorobiphenyl; PCBs typically occur as a mixture of PCB congeners (i.e., aroclors) Inhalation
Cancer; neurotoxicity; immune suppression; endocrine disruption; reproductive and developmental toxicity; respiratory toxicity
Volatile organic compounds (VOCs) Formaldehyde, acetaldehyde, acrolein, benzene, toluene, ethylbenzene, para-xylene, ortho-xylene, meta-xylene, styrene, naphthalene; complex mixtures of VOCs are classified as total VOCs, and some are not toxic Inhalation
Cancer; reproductive and developmental toxicity; neurotoxicity; respiratory irritation; odorants
Other emission and transformation products that are currently unidentified Per- or polyfluoroalkyl substances (PFAS), including perfluorooctane sulfate and perfluorooctanoic acid Reactive oxygen species, including peroxides (R-O-O-R) and superoxides (O2-) Inhalation
Cancer; respiratory and developmental toxicity

Vulnerable Populations and Health Equity

A growing number of studies have shown that human health vulnerability to wildland fires can be influenced by several factors such as those related to life stage, location (e.g., at the WUI), socioeconomic status, race/ethnicity, occupation (e.g., outdoor workers), and underlying health conditions.

Vulnerable Population Example Pathways to Vulnerability (vulnerability includes increased exposure, inability to adapt, and health response)
  • Higher respiratory rate and more water ingested per body weight than adults (e.g. babies take a breath 40 times per minute compared to 12-20 times per minute for adults)
  • More time spent outdoors
  • Developing organs and immature immune response
  • Inability to discern risks
Older Adults
  • Pre-existing conditions
  • Limited mobility
  • Compromised immune response
  • Social isolation
Pregnant People
  • Physiological changes (e.g., higher respiratory rates, increase in blood and lung volumes)
People with Respiratory and Cardiovascular Disease
  • Fine particle pollution further exacerbates disease and triggers symptoms
Tribal Communities
  • Structural racism
  • Disproportionate health burdens from environmental conditions
  • Existing health disparities
  • Less access to resources by Tribal governments
Communities of Color, and Immigrant, Migrant, and Refugee Communities
  • Structural racism
  • Disproportionate health burdens from environmental conditions
  • Existing health disparities (e.g., higher burden of asthma and cardiovascular disease)
  • Less access to resources (e.g., quality healthcare)
  • Barriers to receiving language- and culturally-appropriate care
Low-Income Communities
  • Fewer resources and means to evacuate
  • Less access to indoor spaces with air cleaners and air cooling
  • Higher burden of asthma and cardiovascular disease
  • Existing health disparities
  • Lack of safety nets for missing work
Rural Communities
  • Less municipal infrastructure, including access to drinking water and safe spaces
  • Lack of extensive evacuation routes
  • Fewer environmental monitoring units available
  • Less access to indoor spaces with air cleaners and cooling
  • Existing health disparities
Unhoused/Homeless Communities
  • Lack of access to multiple basic resources
  • Existing health disparities
People with One or More Disabilities
  • Fewer resources and means to evacuate
  • Inadequate community infrastructure
  • Less mobility and ability to assess risks
Wildland Firefighters and Emergency Responders (e.g., Emergency Healthcare Personnel)
  • Lack of personal protective equipment during firefighting
  • Live and/or work in active fire zone
  • Some states use incarcerated populations for firefighting (already considered vulnerable)
  • Increased duration of exposure and elevated concentrations of smoke-associated chemicals and particles
  • Post-traumatic stress
Outdoor Workers (e.g., Farmworkers, Construction Workers, Utility Workers)
  • Increased exposure to outdoor wildfire smoke and other potential occupational exposures (e.g., pesticides)
  • Outdoor physical exertion increases respiratory exchange
Environmental Remediation Workers (e.g., Hazardous and Solid Waste Removal)
  • First to reenter fire zone
  • Direct contact with chemical hazards
  • Increased exposure to lingering wildfire smoke
Domestic Workers and Day Laborers
  • May participate in ash and debris cleanup as an informal labor workforce with little or no personal protective equipment

Research and Data Collection Needs

Research on emissions from WUI fires can build on the extensive knowledge base for wildland fires, but will require new information. Researchers and funding agencies should implement an integrated, multidisciplinary research agenda and create widely accessible repositories for data and information relevant to WUI fires. Priority research is summarized in Table S-1.

Data are needed on fuel characteristics of heterogeneous structures, and the concentrations, exposures, and health impacts of large numbers of toxicants, many of which will be present at trace levels. Because data and measurement needs are interconnected, 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 collection and research needs for WUI fires are interdependent
FIGURE 5 Data collection and research needs for WUI fires are interdependent. 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 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.

Actions to Reduce WUI Fire Risks

The risks and impacts from WUI fires can be reduced with measures to inhibit the spread of fires in both wildland and urban areas and strategies to minimize exposure to emissions of fires that do occur.

Reducing Impacts of WUI Fires

WUI fire risks can be reduced through building codes and actions to protect home. In California, proposed new building codes are focused on “hardening” the house with ignition-resistant roofs, enclosed eaves, multipane glass, and noncombustible materials on the structure exterior. Table 5 summarizes some recommended policies and steps to reduce the risk of WUI fire.

Applicable area Actions


  • Use land management approaches to reduce fuel loading or otherwise modify fuel characteristics; approaches include grazing, prescribed fire, thinning, or other biomass reduction programs (US Forest Service, 2022)


  • Set the community back from slopes where fire risk may be greater (Colorado Department of Local Affairs, 2016)
  • Cluster ignition-resistant housing to reduce “collective exposure” (Moritz and Bustic, 2020)
  • Maintain protective buffers around communities (e.g., agricultural lands, irrigated parks, or other breaks in flammable fuel) (Moritz and Bustic, 2020)

External to the structure

  • Maintain a defensible space around structures by removing dead or dying vegetation, replacing vegetation with noncombustible materials (i.e., a hardscape), and maintaining space in between trees and between trees and the home (e.g., State of California, 2020; CAL FIRE, n.d.)
  • “Harden” the home by using fire-resistant materials such as stone, brick, glass, concrete, gypsum, stucCO, and cement board, as well as ignition-resistant building practices, such as closing eaves and installing dual-pane windows and exhaust screens (e.g., State of California, 2016)

Inside the structure

  • Use interior furnishings with fire-resistant materials and limit the use of synthetic products based on petrochemicals that can be fuel sources
  • Avoid upholstered furniture, which is often the first item ignited by open flames, and instead use furniture with a fire-barrier material (with no added flame retardants, which can be detrimental to human health) between the cushioning and exterior textile (Zammarano et al., 2020)
  • Choose materials that are inherently fire resistant so that flame retardants are not used and human exposure is prevented (IFSTA, 2016)

Reducing Exposure Risks and Health Effects

The report provides possible interventions to reduce near-field and regional exposure risks, albeit with research needed to enhance exposure mitigation specific to WUI fires. These address workers tasked with managing, responding to, and cleaning up after WUI fires; community and individual exposure; and workers whose with non-fire-related outdoor occupations.

The movement and changes of WUI emissions over space and time.
FIGURE 6 The movement and changes of WUI emissions over space and time.
Dermal Take shower after operational period, or use skin wipes when shower not available
Carry clean change of clothes for after operational period if not returning to base camp
Prevent contamination of backpack (14 day bag with personal items) contents using sealed, impervious bags
Use improved wildland turnout gear to reduce dermal exposure
Inhalation Use respiratory protection if you are a vehicle driver / pump operator
Provide exposure monitoring equipment to firefighters, when appropriate, to help them identify and avoid inhalation hazards (implement with support of safety officers on incidents and risk management personnel at land agencies)
Change cabin air filters in fire department apparatus
Mixed Clean tents and sleeping bags after each assignment
Implement clean cab protocol
Administrative Use job rotations to reduce exposures (fire incident management personnel can rotate and reassign crews and resources after completing job tasks associated with higher smoke exposures to job tasks that have lower expected smoke exposures)
Allow firefighters to patrol for spots rather than stand stationary on the fireline

At the community level, numerous interventions individuals and communities may take also exist with varying levels of efficacy, listed in Table 7.

  Inhalation Reduce activity level and time outdoors. Follow any state or municipality guidance levels or standards for acceptable exposure to smoke, such as the threshold of 151 µg/m3 or less of PM2.5, as provided in California (by the California Division of Occupational Safety and Health) for outdoor workers.
Consider wearing a respirator outdoors, or a properly fitted N95 or P100 mask. Standard dust or surgical masks do not keep out fire smoke. Make sure the mask fits as recommended by the manufacturer. If you are traveling in a vehicle, close the windows and doors and run the air system in recirculation mode. Stay indoors with windows and doors closed, and use an HVAC system in recirculation mode, not bringing in any outdoor air. Use the highest rated minimum efficiency reporting value (MERV) filter in the HVAC system, with a MERV 13 recommended.
Use indoor air cleaners (if available) that contain a high-efficiency particulate air (HEPA) filter for fine particle removal. If the option is available, add a charcoal filter for chemical collection. Place air cleaners in primary living and sleeping areas and ensure that the air cleaner has Association of Home Appliance Manufacturers approval with an appropriately sized clean air delivery rate provided. Do not use ozone-generating air cleaners, since ozone is a strong pulmonary irritant. Other air cleaners may release unintentional ozone during operation; thus, select only products that meet California’s Air Cleaner Regulation AB 2276 of not releasing more than 0.05 ppm ozone (, 2008). If retail air cleaners are not available or are economically prohibitive, consider using a DIY air cleaner consisting of a simple box fan with an attached HVAC filter. Follow assembly and safe operating instructions (Underwriters Laboratories, 2021). Keep the indoor space clean by wiping all surfaces with an antistatic or damp cloth. Use a HEPA vacuum to clean surfaces and upholstered furniture. Consider using PM2.5 personal devices or real-time air monitors for measuring indoor levels, so that mitigative strategies including air cleaning/filtration can be used for high PM measurement events.
Reduce PM2.5 generation indoors by limiting the use of chemical cleaners and aerosols, air fresheners, gas cooking, and processes like frying.
Locate indoor “safe air spaces” provided by the community, especially for vulnerable populations. These may include specially prepared spaces like sports arenas, theaters, and shopping malls. When the outdoor air becomes safer, consider “airing out” your space by opening windows and doors and providing as much air circulation as possible. EPA’s AirNow Fire and Smoke map (https://fire.airnow.gov/ ) is a source of real-time air quality and fire information that can be used to identify when outdoor air quality has improved.
Dermal/Ingestion Keep the indoor space clean and reduce indoor dust by using air cleaners and wiping all surfaces with a static-free or damp cloth. If available, use a HEPA vacuum to clean all surfaces including flooring, countertops, shelving, and upholstered furniture.
Wash hands frequently, especially those of children who have frequent hand-to-mouth activity.
Consider treatment/filtration of drinking water to remove particles and chemical contamination from fire emissions or thermally degraded water lines.

Currently, three occupational standards specifically for wildland fire smoke exist in the United States (Table 8). Common features of these three rules include use of threshold AQI or PM2.5 values for smoke exposure reduction actions; exemption of some workplaces, such as workplaces involved in emergency response; use of direct-read PM2.5 monitors for ambient measurements; and stipulations for instrument accuracy and operation.

Jurisdiction Threshold Value or PM2.5 Concentration Action
State of California AQI ≥ 151 (55.5 µg/m3) Implement engineering and administrative controls; provide respirators for voluntary use
AQI ≥ 500 (500.4 µg/m3) Require respirator use
State of Oregon AQI ≥ 101 (35.5 µg/m3) Develop training and communication programs; provide respirators for voluntary use
AQI ≥ 201 (150.5 µg/m3) Implement engineering and administrative controls; require respirator use
AQI ≥ 501 (500.4 µg/m3) Require respirator use; implement a respiratory protection program
State of Washington 20.5 µg/m3 Develop information and hazard communication plan; encourage use of exposure controls
55.5 µg/m3 Implement engineering and administrative controls; provide respirators for voluntary use at no cost

Risk communication is critical for minimizing harm to residents affected by any disaster. However, risk communication can help only to some extent. For example, while staying indoors is often advised by public health authorities to reduce exposure to regional smoke, staying indoors is not possible for people who have to work or travel outdoors during fire events, and people experiencing homelessness. Findings on the effectiveness of risk communication concerning fires also vary by study location, population, source of the communication, and levels of language- and culturally appropriate information. Trusted sources of information about health risks may come from community-based organizations and trusted social networks in some areas.

Health care providers also play a critical role in advising patients on actions they can take to reduce exposure related to fires. EPA provides a training endorsed by the Centers for Disease Control (https://www.epa.gov/wildfire-smoke-course ) for health care providers to better advise their patients on actions they can take before and during a fire to reduce exposure.

Get the Report