Homes that escape wildfires present a less obvious danger

As wildfires cause havoc in Los Angeles, new research reveals the dangers that may lie hidden in homes that appear to have escaped unscathed.

In 2021, Boulder County suffered the worst fire in Colorado history, destroying nearly 1,000 homes and forcing more than 37,000 residents to evacuate. 

Ten days after the fire – known as the Marshall Fire – had been extinguished, scientists at the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder, responded to concerns expressed by people returning to undamaged homes and set up instruments to measure what the blaze had left behind.

They focussed on a house close to a block that had been razed to the ground. While being undamaged by the fire itself, the house was downwind of the blaze and smoke had been blown through it.

Will Dresser, lead author and chemistry PhD student at CU Boulder said: ‘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.’

The team set up in a first-floor room of and sampled the indoor air continuously for five weeks.

They found high levels of aromatic VOCs such as benzene, toluene, and naphthalene, which were much higher indoors than outdoors. 

Initially those concentrations declined rapidly but, as the five-week sampling period progressed, the decline slowed to a rate the team considered was slower than expected.

CIRES Fellow Joost de Gouw said: ‘Based on prior research, we had expected these VOCs to disappear from the home within hours, but it took weeks. We don’t understand very well how and where these chemicals get trapped inside a home.’

They describe the home as acting like a sponge for the VOCs during the fire, slowly returning them to the air afterwards. As an indicator of this, when the building’s windows were opened, VOC levels were seen to fall but once they were closed they went back up.

Similarly, the team ran air cleaners with activated carbon filters which effectively cleaned the room wile in operation but once switch off, the levels rose again.

The team hope that their work can help inform people about the potential risks of returning home after a wildfire. Dresser said: ‘I think our study brings some numbers and perspective to that story. It highlights the importance of these impacts for people returning to areas after these fires.’

The full research can be read here.