2021’s Marshall Fire became the most destructive fire in Colorado history, burning nearly 1,000 homes and forcing more than 37,000 residents of Boulder County to evacuate. 

New research by CIRES scientists found compromised indoor air quality in homes near the burnt area for weeks after the fire, similar to pollution levels recorded in urban Los Angeles in the 1990s. According to the researchers, the findings can help individuals weigh their options when returning home after a fire. 

In the aftermath of the fire, some residents found their homes burned to the ground. Others, to their relief, found their homes visibly unscathed. Yet they reported unpleasant odors that lingered indoors and found layers of black ash covering many surfaces. 

In response to community concerns, CU Boulder scientists at CIRES responded quickly to research a topic that had never been studied before: indoor air quality immediately following a fire at the Wildland Urban Interface (WUI), where human development meets forests and other natural spaces. 

Ten days after the fire, researchers set up field instruments in a home bordering a block where houses burned to the ground in Superior, Colorado. Winds had blown smoke directly into the home, and residents complained the air inside smelled like a campfire. 

Will Dresser, lead author and chemistry PhD student at CU Boulder, led the study with CIRES Fellows Joost de Gouw and Christine Wiedinmyer. 

“No study has, in a real-world environment, gone into an indoor space and looked at indoor smoke impacts so close after a fire event,” Dresser said.

Their study, published today in ACS Environmental Science & Technology Air, confirmed what residents were smelling in their homes: Gases harmful to human health were trapped and lingered for weeks following the fire. 

WUI fires have increased in recent decades, yet research looking at the impacts on air quality following these events is limited. The structures and items that burn in WUI fires — cars, roofs, furniture, and carpets — release different, sometimes more dangerous, volatile organic compounds (VOCs).

Researchers set up research-grade instruments in the first-floor room of a home next to a block of burned homes. The instruments sampled indoor air continuously for five weeks, measuring over 50 gases. 

Key findings of the sampling included:

• Aromatic VOCs like benzene, toluene, and naphthalene, which pose risks to human health, were highest at the beginning of data collection. VOC concentrations were much higher indoors than outdoors.  
• Concentrations of aromatic VOCs were similar to those recorded in urban Los Angeles in the 1990s. 
• The aromatic VOCs initially declined rapidly and then more slowly during the five-week study.
• However, the rate at which the VOCs declined was slower than expected.

The results indicate the home acted like a sponge for VOCs during the fire and slowly returned these pollutants to indoor air afterward. However, the study does not reveal where exactly the VOCs were trapped. 

“Based on prior research, we had expected these VOCs to disappear from the home within hours, but it took weeks,” de Gouw said. “What this means is we don’t understand very well how and where these chemicals get trapped inside a home.”

The research team also tested ways to mitigate smells and VOC concentrations indoors.

First, they opened windows to increase the circulation of indoor and outdoor air. When windows were opened, VOC levels dropped. However, after closing the windows, the VOC concentrations went back up. 

Next, they constructed home-built air cleaners made of a box fan and pleated air filters with activated carbon. These filters removed VOCs within an hour of starting the fans — but the VOCs rebounded when the fans were turned off, similar to opening windows. 

Both methods, opening windows and running air cleaners, decreased gases indoors by more than 50 percent. 

While the research is the first of its kind, it’s important to note the results represent a single house following one WUI fire. Different circumstances — poor outdoor air quality, high winds, or new materials burned in the process — could have different environmental impacts. 

Looking to the future, scientists predict that WUI fires will increase in speed and frequency, and Dresser notes it’s important to be informed. The study’s results can help residents weigh their options when deciding whether or not to return home following a wildfire. 

“I think our study brings some numbers and perspective to that story,” Dresser said. “It highlights the importance of these impacts for people returning to areas after these WUI fires.” 

A second CU Boulder study authored by geography professor Colleen Reid focused on health impacts from smoke following the Marshall Fire, learn more about this work published today on CU Boulder Today.