Compounds from wildfire smoke persist inside unburned homes

Early in the afternoon of Dec. 30, 2021, the Marshall Fire erupted in the grasslands outside two Denver suburbs. High-speed winds sent the blaze ripping through neighborhoods. Firefighters could contain it only when winds calmed that evening. The next day, the region’s first significant winter snowfall snuffed out the smoldering ruins of more than 1,000 homes.

Entire blocks had burned to the ground. But amid the ash, some houses stood seemingly undamaged. Residents wondered: Was it safe to return to those homes?

Ten days after the fire was extinguished, a group of researchers from the University of Colorado, Boulder (CU Boulder), rolled their mass spectrometer into one such house and began sampling. They have now published their results alongside another study examining the possible health effects of the air quality inside smoke-damaged homes (ACS ES&T Air 2024, DOIs: 10.1021/acsestair.4c00259 and 10.1021/acsestair.4c00258).

The dataset from the smoke-damaged home contains hundreds of compounds, says atmospheric chemist Joost de Gouw of CU Boulder, leader of the mass spectrometer team. It will take time and more experiments to identify every species, so the group initially focused on well-characterized aromatics. Many are known health hazards. “Benzene is the big one; naphthalene is definitely another one,” de Gouw says. The team identified both in the home, as well as toluene and 51 other compounds.

Higher-than-normal levels of volatile organic compounds (VOCs) persisted for nearly a month inside the house. “The impact of smoke can be retained inside homes for a lot longer than you would expect, so the impact of the fire is not gone after the fire is out,” de Gouw says. Opening a window or using a DIY air cleaner built with a carbon filter dropped the VOC concentration for a while, but once the window was closed or the air cleaner turned off, the VOCs built up again.

The scientists also studied the effects of poor indoor air quality on residents’ health, says Colleen Reid, an environmental epidemiologist at CU Boulder who led the companion health study. They collected data through surveys asking hundreds of residents about their symptoms and their home’s proximity to the fire.

“We found some important physical health symptoms that lasted 6 months to a year after the fire,” Reid says. Headaches were the most common symptom; others included a strange taste in the mouth, a dry cough, sneezing, itchy eyes, and a sore throat. “We don’t know the long-term implications of this,” Reid says, but these data suggest that people should be cautious about returning to smoke-damaged homes.

To create recommendations for people going home after a wildfire, the Environmental Protection Agency relies heavily on scientific literature, says EPA researcher Amara Holder. “We have a lot of information about what to do in the moment when the smoke is happening, but we have very little information about what to do after it’s gone.” These papers provide a unique dataset that may help fill in that gap, Holder says.