In response to widespread community concerns, researchers at the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder launched a study to assess the impact of wildfires on indoor air quality. This research, published in ACS Environmental Science & Technology Air, is the first to investigate indoor air pollution immediately following a Wildland Urban Interface (WUI) fire.

The Marshall Fire, which struck Colorado in December 2021, left an indelible mark as the most destructive wildfire in the state’s history. Nearly 1,000 homes consumed, and over 37,000 residents in Boulder County forced to evacuate. While many residents returned to find their homes intact, unpleasant odors and black ash revealed the lingering effects of the disaster.

The Wildland Urban Interface: A Growing Risk

The WUI represents areas where human development meets natural spaces like forests. As urban sprawl increases, so does the risk of wildfires impacting residential areas. Fires in the WUI differ from forest fires due to the variety of materials burned, including furniture, vehicles, and synthetic building materials, which release harmful volatile organic compounds (VOCs).

Key Findings of the Study

Elevated VOC Levels Indoors

The research team installed state-of-the-art air-sampling instruments in a home adjacent to a block of burnt houses in Superior, Colorado. Over five weeks, these instruments measured more than 50 different gases.

Notable Findings:

  • Aromatic VOCs (benzene, toluene, naphthalene): These harmful gases were present at alarmingly high concentrations.
  • Indoor vs. Outdoor Air Quality: Indoor VOC concentrations were significantly higher than outdoor levels, with early levels comparable to urban Los Angeles in the 1990s.
  • Slow Decline: VOC levels decreased rapidly at first but then lingered for weeks, defying expectations.

Lead author Will Dresser expressed surprise at the persistence of these pollutants, noting that prior research suggested VOCs should dissipate within hours.

The “Sponge Effect” of Homes

Researchers hypothesize that homes in the affected area acted like sponges, absorbing VOCs during the fire and slowly releasing them afterward. However, the exact mechanisms and locations of VOC trapping remain unclear.

CIRES Fellow Joost de Gouw commented, “What this means is we don’t understand very well how and where these chemicals get trapped inside a home.”

Impacts on Residents’ Health and Safety

Health Risks of Aromatic VOCs

Aromatic VOCs such as benzene and toluene pose serious health risks, including respiratory issues and long-term effects like cancer. Prolonged exposure to these compounds in indoor environments underscores the need for effective mitigation strategies.

Psychological and Emotional Impact

For many residents, returning to homes that smelled of campfires and were coated in ash added to the trauma of the wildfire. Addressing indoor air quality is not just a health priority but also a step toward psychological recovery for affected communities.

Strategies for Mitigating Indoor Air Pollution

Ventilation: Pros and Cons

Opening windows was one of the first mitigation strategies tested. While this reduced indoor VOC concentrations by over 50%, the improvement was temporary. Once windows were closed, VOC levels rebounded.

DIY Air Cleaners Show Promise

The team also tested home-built air cleaners made of box fans and activated carbon filters. These devices effectively removed VOCs within an hour of operation. However, similar to ventilation, pollutant levels returned after the devices were turned off.

Limitations and Broader Implications

A Single Case Study

This research conducted in one home following a single wildfire event. Factors such as outdoor air quality, wind patterns, and the types of materials burned could lead to different outcomes in other scenarios.

Increasing Frequency of WUI Fires

Scientists predict that WUI fires will increase in both speed and frequency due to climate change and urban expansion. This highlights the urgent need for continued research into post-fire air quality and long-term mitigation solutions.

Guidance for Residents Returning After Wildfires

Consider Air Quality Before Returning

The study’s findings provide valuable insights for residents weighing their options about returning home after a wildfire. Poor indoor air quality can pose serious health risks, even if a home appears undamaged.

Importance of Mitigation Measures

Investing in air-cleaning solutions and maintaining proper ventilation can help reduce VOC levels. However, residents should remain aware that these methods may provide only temporary relief.

Conclusion: A Call for Preparedness and Research

The Marshall Fire underscored the devastating effects of WUI fires and the urgent need to understand their impact on indoor air quality. As fires become more frequent and severe, research like this provides crucial data to help communities recover and adapt.

By identifying the risks and offering practical mitigation strategies, this study empowers residents to make informed decisions about returning home and safeguarding their health.