Today, wildfires are becoming larger and more destructive as they burn in the WUI – where human development meets or intermixes with undeveloped wildland fuel. Wildfires are increasing in both frequency and severity due to climate change, the expansion of interface communities, and historical land management practices. 

The economic, environmental, and human health impacts of these fires are substantial. Today, close to one-third of the U.S. population, residing in nearly 50 million homes located in the WUI, is directly at risk of experiencing a wildfire.^1,2 Even more of the population may be indirectly affected by poor air quality issues as the result of wildfire smoke. 

Understanding how wildfire smoke and lingering residues affects indoor air quality, human health, and long-term exposure risks is critical for reducing harm. Research from Chemical Insights highlights how wildfire-related pollutants can linger in homes, settle into dust, and continue to impact occupants even after visible smoke has cleared.

The wildland urban interface (WUI) refers to areas where human development meets or mixes with undeveloped wildland vegetation. These regions are growing rapidly due to population expansion and land use changes. 

WUI areas are particularly vulnerable to wildfires because they combine: 

• Vegetation
• Residential structures
• Infrastructure and synthetic materials 

Unlike traditional wildfires that primarily burn vegetation, WUI fires also consume homes, vehicles, and synthetic materials,³ releasing a complex mix of harmful pollutants into the air. This combination can make WUI smoke more toxic, may lead to more persistent residues and increases both the likelihood of fire spread and the complexity of resulting fire-associated exposures.

WUI fires can be more catastrophic because they can burn through entire towns and neighborhoods and involve urban materials (or fuels) such as homes, cars, and other human-made structures. Compared to the vegetative biomass that combusts in a wildland fire, combustible materials found in the WUI have different elemental compositions and densities,⁴ and are present in different quantities and arrangements. 

For example, chemical elements and materials of concern, such as halogens, plastics, and metals, exist in much higher concentrations in the WUI, resulting in different emissions, exposure outcomes, and health effects. Additionally, materials unique to urban areas, such as polyvinyl chloride or polyurethane, often contain much larger amounts of chlorine or nitrogen, which are known to impact combustion chemistry. 

While all wildfires produce smoke and a large mix of pollutants, including volatile and semi-volatile organic compounds (VOCs and SVOCs respectively) and particulate matter (PM), when a wildfire spreads to include the urban fuels in the WUI, an entirely different mix of pollutants are released. Environmental pollutants come in many forms, including: 

• Atmospheric emissions
• Residues
• Effluents (liquid waste)

WUI fires differ from traditional wildfires because they burn a wider range of materials. As a result, WUI fire emissions are more chemically complex and potentially more harmful to human health than smoke from vegetation alone.  

Compared to traditional wildfire smoke, WUI smoke can contain a different mixture of: 

• PM2.5 Carbon monoxide (CO)
• Volatile organic compounds (VOCs) such as formaldehyde and benzene
• Polycyclic aromatic hydrocarbons (PAHs)
• Metals and other combustion byproducts 

As a wildfire burns, it generates a giant plume of emissions, including particles and chemicals, called a plume. Once these emissions are released into the environment, the plume evolves over time through a process called atmospheric transformation.⁴

Initially, gas-based compounds condense onto other particles in the atmosphere. Then, some compounds react with atmospheric compounds to create new compounds as other compounds photochemically react in the sunlight. The process continues as the plume mixes with other urban pollutants in the air, such as vehicular or industrial emissions.

As the plume moves further downwind, the process slows and ultimately particles begin to fall from the sky, leaving residents in proximity to the plume vulnerable to different types of exposure.

While larger particles may be visible as ash, wildfire smoke is a major source of fine particulate matter (PM2.5). These tiny particles can be harder to detect and can take longer to settle as dust. Fine particulate matter can penetrate deep into the lungs and enter the bloodstream. 

Short-term health effects include: 

• Eye, nose, and throat irritation
• Coughing and shortness of breath
• Headaches and fatigue 

Long-term or repeated exposure may contribute to: 

• Cardiovascular disease
• Respiratory conditions such as asthma
• Increased risk of chronic illness

Wildfire emissions don’t recognize borders. Plumes can travel great distances, even spanning across entire continents. However, those who have been exposed directly to WUI emissions have an increased risk of adverse health effects.⁵ 

Since WUI fires cannot be left to naturally burn out, nearby workers, such as firefighters, emergency response teams, and clean-up recovery crews are increasingly exposed to emissions, smoldering and resulting residues, and dust. Additionally, outdoor workers in surrounding areas, such as farmers and landscapers, may be at a greater risk. 

Certain populations are especially vulnerable, including children, older adults, and pregnant women.⁵ Individuals with pre-existing health conditions are also at higher risk, including those with: 

• Chronic lung or cardiovascular disease
• Compromised immune systems
• A lower socioeconomic status⁵

Even when doors and windows are closed, wildfire smoke can infiltrate homes through gaps in doors and windows, openings such as vents and ducts, and HVAC systems. 

Once inside, pollutants can accumulate in the air, absorb onto surfaces, and settle into dust. This means that indoor environments may remain contaminated long after outdoor air appears clear, extending exposure risks beyond the active fire event.

There are several ways to reduce or avoid exposure to wildfire emissions, including PM2.5, with varying levels of feasibility. Air filtration (or “air cleaning”) is one of the most practical and effective ways to safeguard indoor air quality during and after a wildfire. Air filters are designed to capture pollutants and remove them from the indoor environment. Since PM2.5 is so small, specialty or high-quality air filters are needed to capture it. 

Indoor air can be filtered by: 

• Fitting an HVAC system with a pleated filter with a Minimum Efficiency Reporting Value (MERV) of 13 or higher (if the existing system is designed to support it)
• Using a stand-alone air cleaner with High Efficiency Particulate Air (HEPA) filtration and activated charcoal filtration.
• Using a do-it-yourself (DIY) air cleaner (a furnace air filter[s] attached to an electric box fan) 

Applying higher performance filtration on supply ventilation in homes can reduce outdoor particles by up to 97%, and portable air cleaners can reduce indoor PM exposure by 48% – 78%.⁶

Many people are using DIY air cleaners to remove particles from wildfire smoke. However, since consumers are retrofitting box fans not intended or evaluated for this purpose by the manufacturer, there have been some concerns about their safety during use.

Chemical Insights, with support from UL Research Institutes’ Fire Safety Research Institute, partnered with the U.S. Environmental Protection Agency’s (EPA’s) Office of Research and Development to provide scientific information to help people stay safe during wildfire events. Later, Chemical Insights and UL Standards & Engagement published UL 200A Guidance Document, “Use of Do-It-Yourself Filtration Devices During Wildfires,” which presents evidence-based, actionable procedures to help construct and safely use DIY air cleaners during wildfire smoke events.

We demonstrate how to make and use a DIY air cleaner to protect your indoor air quality during a wildfire smoke event.

After a wildfire event, pollutants can remain in the home as settled dust and surface contamination. 

Key cleanup steps: 

• Wet wipe surfaces to remove residue
• Use HEPA vacuums for dust removal
• Wash fabrics and soft materials
• Replace HVAC filters 

Without proper cleanup, these particles can become resuspended in the air, prolonging exposure.

While people may primarily associate air and water contamination with the combustion products of a wildfire, there is an often-overlooked hazard of residual dust.⁷ During and after a wildfire, even after the sky clears, a hazardous mixture of PM, dust, and ash can infiltrate homes and other buildings for several days or weeks. If not properly removed, this dust can settle on surfaces, contaminate textiles like carpet and upholstered furniture, and even resuspend in the air, presenting a continued exposure risk for residents. The majority of household dust comes from the outside through windows, doors, vents, and on the soles of shoes. Therefore, in the aftermath of a wildfire, it is particularly important to take additional steps to remove settled dust.

The negative health effects from exposure to settled dust may include eye, nose, and throat irritation, exacerbation of asthma, eczema, chronic obstructive pulmonary disease, and cardiovascular disease, such as heart attacks and strokes. Dust is also an important route of exposure to toxicants. Since settled dust is hard to clean up and can remain in environments for an extended period, it presents a long-term (chronic) exposure risk for people, beyond the short-term (acute) risk posed in the immediate aftermath of a wildfire.