Today, wildfires are becoming larger and more destructive as they burn in the wildland urban interface (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. Even more of the population may be indirectly affected by poor air quality issues as the result of wildfire smoke.

WUI fires can be more catastrophic because they 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, 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:

As a wildfire burns, it generates a giant mass of emissions, including particles and chemicals, called a plume. Once these emissions are put into the environment, the plume evolves over time through a process called atmospheric transformation. 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, smaller particulate matter (PM_2.5) may be harder to detect and can take longer to settle as dust.

Wildfire plumes can travel great distances, even spanning across entire continents. While there is much to learn about the health effects from an atmospherically transformed plume at regional and continental scales, more is known about the health effects closer to the initial source.

Certain members of the population may be more vulnerable to exposure than others, such as children, older adults and pregnant women. Additional examples include people who have:

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

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.

While wildfire emissions contain a wide array of hazardous pollutants, PM_2.5 (particulate matter that is 2.5 microns or less in diameter) is a primary concern. When PM_2.5  is inhaled, it can penetrate deep into the lungs, and may even infiltrate into the bloodstream. Studies have directly linked wildfire PM_2.5 to health issues such as exacerbation of asthma, chronic obstructive pulmonary disease, premature death, and circulatory effects, such as heart attacks and stroke.