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The Human Health Impacts of Wildfire Smoke

Ariel view of a WUI community

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.

Wildland Fuels vs. Urban Fuels

Graphic describing the difference between the chemical emissions in wildland fires and WUI fires
Wildfires and wildland urban interface fires produce different pollutant mixtures.

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:

Image of atmospheric emissions

Atmospheric emissions

Residue from a fire


Effluents in water

Effluents (liquid waste)

Exposure to WUI Emissions

Exposure to fire emissions occurs at different scales, impacting how people come in contact with pollutants.
Exposure to emissions occurs at different scales, impacting how humans come in contact with pollutants.

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 (PM2.5) 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.

 In the immediate or near-field, human exposure may occur through:

Outline image of lungs


People may breathe in hazardous particles and chemicals, including suspended dust, ash and volatile organic compounds (VOCs).

Outline image of a stomach


People may swallow settled dust, typically from hand-to-mouth behavior. They may also drink contaminated potable water.

Outline of a hair on skin


People may come into contact with settled dust and ash. They may also shower or wash hands with contaminated potable water.

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 dangerous pollutants, particulate matter is a primary concern.

The Health Impacts of Particulate Matter

Inhalation of particulate matter from wildfire smoke.
Particulate matter (PM) from smoke can enter through airways into the lungs. PM2.5 can enter the bloodstream.

While wildfire emissions contain a wide array of hazardous pollutants, PM2.5 (particulate matter that is 2.5 microns or less in diameter) is a primary concern. When PM2.5  is inhaled, it can penetrate deep into the lungs, and may even infiltrate into the bloodstream. Studies have directly linked wildfire PM2.5 to health issues such as exacerbation of asthma, chronic obstructive pulmonary disease, premature death, and circulatory effects, such as heart attacks and stroke.

How small is 2.5 micrometers? The average human hair is about 70 micrometers in diameter – making it 30 times larger than the largest fine particle.

Air Filtration

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%.

DIY Air Cleaners

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. In 2021, CIRI, with support from the 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. In July 2021, CIRI published a report that presented findings on the evaluation of potential safety risks while operating a DIY air cleaner.

DIY Air Cleaner Diagram
A DIY air cleaner is assembled by taping a MERV 13 filter to the back of a box fan.
Box fan, duct tape and an air filter used to make a DIY air cleaner

How to Make a DIY Air Cleaner

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

Educational Handouts |

Protecting Your Health After a Wildfire: Cleaning Indoor Air

Nov 10, 2023

Application Notes |

Strategies to Protect Air Quality During Wildfires Considerations for Do-It-Yourself Filtration

Aug 13, 2022

Technical Reports |

Wildfire Safety Research: An Evaluation of DIY Air Filtration

Jul 07, 2021

Removing Settled Dust

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. During and after a wildfire, even after the sky clears, a hazardous mixture of particulate matter (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 toxins. 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.

Removing Dust
Removing settled dust with a microfiber cloth.

Educational Handouts |

Protecting Your Health After a Wildfire: Cleaning Settled Dust

Nov 10, 2023

Technical Briefs |

When the Dust Settles: Reducing Chemical and Particle Health Risks Following a Large-Scale Urban Fire

Mar 07, 2023

Technical Briefs |

Health Hazards of Building Dust

Mar 07, 2023

CIRI’s Research Exploring Emerging Public Health Threats from WUI Fires

CIRI is embarking on three, first-of-their-kind research studies to uncover unknown public health risks from WUI fires and define crucial steps to protect the health and safety of people whose homes and communities have the potential to be affected by the catastrophic nature of these fires.

The Effect of Wildfires and the WUI on Indoor Air Quality and Health

CIRI researchers, in partnership with the U.S. Environmental Protection Agency (EPA), are characterizing the indoor and outdoor air’s chemical and particle makeup and assessing related health impacts in a wildfire-prone community. CIRI’s laboratories are developing toxicity methods for evaluating chronic health concerns of fire debris, including residual dust, that can infiltrate homes and buildings. Research findings will lead to processes for managing and reducing public health risks from exposure to WUI fire smoke.

As part of this research, CIRI conducted a pilot study of six homes. We identified more than 475 VOCs in the indoor and outdoor air samples. VOC levels were much higher indoors than outdoors, and formaldehyde levels exceeded recommended exposure levels in half of the homes.

Read the Pilot Study Findings
A huge plume of smoke billowing in the distance, from one of the many wild fires that have been happening in California in the last few years

The Characterization of Atmospheric Contributions of WUI Fire Emissions

CIRI’s sponsored research with West Virginia University is characterizing vapor and aerosol emissions from the combustion of forest biomass with common building materials such as construction lumber and insulations. This first-of-its-kind research simulates real-world WUI fire emissions in geographically targeted regions to analyze emissions that people would likely be exposed to. It fills a gap in knowledge of what emissions result during an actual WUI fire when naturally derived and synthetic materials from the built environment burn together.

Discover More About This Research
Dark Wildfire approaching the city


Air pollution is contributing to the epidemic increase in type 2 diabetes. Scientific studies indicate a link between type 2 diabetes and PM2.5 exposure. This is of serious concern in wildfire-prone areas where PM2.5 is a major emission from WUI fires. Jointly with Duke University and the University of Southern California, CIRI is conducting a residential study on the use of HEPA filtration to improve the cardio-metabolic profiles of residents. The study will also help identify novel biomarkers to predict disease and risk of pollution-induced diabetes.

Person using an air cleaner

Safeguarding the Built Environment

CIRI partners with the Insurance Institute for Business and Home Safety (IBHS), the National Fire Protection Association (NFPA), the U.S. Environmental Protection Agency (EPA), and other organizations to communicate the effects of WUI wildfires on both human health and the built environment.

Mitigation strategies for the built environment involve:

  • Creating defensible space: Providing a protective buffer between buildings and vegetation.
  • Hardening the structure: Utilizing ignition-resistant materials and vent covers.
  • Preventing ember-driven ignition: Continuously removing debris from the roof and base of the building.

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