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.
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.
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.
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.
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, |
Polyisocyanurate foam in insulation |
HCN, CO, NO, NO2, NH3, HCl, H3PO4, |
Phenolic foam in insulation |
SO2, CO, HCl, acrolein, formaldehyde |
Extruded polystyrene in insulation |
HF, HBr, CO, PM, PAHs, VOCs, SVOCs |
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 |
HCl, CO, PCDDs, PCDFs |
Upholstery on furniture |
HCN, CO, NO, NO2, NH3, HCl, H3PO4, |
Vinyl carpet |
HCl, CO, PCDDs, PCDFs |
Polyamide carpet |
HCN, CO, NO, NO2, NH3, PM, PAHs, VOCs, SVOCs, isocyanates |
Electrical wiring insulation |
HCl, CO, PCDD, PCDFs |
Acrylic clothing |
HCN, CO, NO, NO2, NH3, PM, PAHs, VOCs, SVOCs, isocyanates |
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 |
Dashboard |
CO, HCN, NO, isocyanates, PAHs, VOCs |
Upholstery material |
CO, HCN, NO, HCl, SO2, isocyanates, PAHs, PCDDs/PCDFs, VOCs |
Electrical wiring |
CO, HCl, PAHs, PCDDs/PCDFs, VOCs |
Tire |
CO, SO2, PAHs, VOCs |
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.
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.
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.
GROUP OF POLLUTANTS | COMMON EXAMPLES | ROUTES OF EXPOSURE | POTENTIAL HEALTH OUTCOMES |
Asbestos |
Fibrous asbestos, chrysotile |
Inhalation Ingestion |
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 Ingestion 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 Ingestion |
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 Ingestion Dermal |
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 Ingestion Dermal |
Cancer; cardiopulmonary toxicant; immunosuppressant; neurotoxicant; reproductive and developmental toxicity |
Plasticizers | Ortho phthalates including dibutyl phthalate, terephthalates, adipates, benzoates | Inhalation Ingestion |
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 Ingestion Dermal |
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 Ingestion Dermal |
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 Ingestion |
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 Ingestion |
Cancer; respiratory and developmental toxicity |
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) |
---|---|
Children |
|
Older Adults |
|
Pregnant People |
|
People with Respiratory and Cardiovascular Disease |
|
Tribal Communities |
|
Communities of Color, and Immigrant, Migrant, and Refugee Communities |
|
Low-Income Communities |
|
Rural Communities |
|
Unhoused/Homeless Communities |
|
People with One or More Disabilities |
|
Wildland Firefighters and Emergency Responders (e.g., Emergency Healthcare Personnel) |
|
Outdoor Workers (e.g., Farmworkers, Construction Workers, Utility Workers) |
|
Environmental Remediation Workers (e.g., Hazardous and Solid Waste Removal) |
|
Domestic Workers and Day Laborers |
|
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.
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.
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 |
Wildlands |
|
Community |
|
External to the structure |
|
Inside the structure |
|
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.
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.
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Destructive fires at the wildland-urban interface (WUI) have burned thousands of acres of residential space in recent years. This report examines the chemical processes that occur during urban wildfires, the identity of resulting chemicals, and what is known about human exposure pathways. The report provides decision-makers with recommendations for additional chemistry research needed to inform their work to mitigate the adverse chemical impacts of wildfires at the WUI.
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