Shocking Discovery: Indoor Air Quality Devastated Months After Colorado’s Marshall Fire

In a groundbreaking study released by researchers at the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder, alarming evidence has surfaced regarding the indoor air quality of homes located near the site of the Marshall Fire, which tore through Boulder County in 2021. As a devastating fire that destroyed nearly 1,000 homes and forced over 37,000 residents to evacuate, its aftermath has opened a new chapter in understanding the long-term effects of wildfires.

For the first time, the study reveals that homes close to burned areas exhibited compromised indoor air quality for weeks post-disaster. Researchers found pollution levels reminiscent of those recorded in the polluted air of urban Los Angeles during the 1990s. This important finding aims to guide residents on whether or not they should return to their homes after such catastrophic events.

Residents who returned to their homes found varying degrees of damage; some houses were reduced to ashes while others appeared intact. However, many reported persistent odors and uncovered layers of black ash that hinted at deeper issues. In a timely response to community concerns, CIRES scientists embarked on an unprecedented investigation into indoor air quality soon after the fire, particularly focusing on the Wildland Urban Interface (WUI)—the area where urban settings collide with natural wildlands.

Ten days following the fire's outbreak, researchers deployed high-tech monitoring equipment inside a home located adjacent to a block affected by total destruction. The study quickly validated residents' concerns, as they described their indoor air smelling like smoke. Lead author Will Dresser and his team discovered that harmful gases lingered far longer than expected—raising urgent questions about the quality of air in homes after similar wildfires.

Their study, now published in the journal ACS Environmental Science & Technology Air, reveals that the indoor air contained elevated levels of volatile organic compounds (VOCs) like benzene, toluene, and naphthalene—none of which should be taken lightly. Alarmingly, the levels were significantly higher indoors than outdoors, and initial concentrations echoed the levels of air pollution recorded in Los Angeles over twenty years ago.

The alarming persistence of VOCs within the home indicates that structures can absorb these harmful gases during a wildfire, acting like sponges that release pollutants slowly over time. Scientists had anticipated that these chemicals would disperse quickly, but weeks later, the VOCs were still detected, highlighting a critical gap in our understanding of how pollutants interact with home materials.

To alleviate the high concentrations, researchers tested various intervention methods. They found that simply opening windows significantly reduced VOC levels. Yet, once windows were closed, indoor pollution levels rebounded rapidly. Furthermore, DIY air purifiers made with box fans and activated carbon filters were proven effective for a short time, but pollutants resumed their presence when the fans were turned off. Remarkably, both techniques resulted in a reduction of indoor gas levels by over 50%.

Importantly, while this study marks a significant advancement in understanding indoor air quality after fires, it is based solely on one residence impacted by a single wildfire, suggesting that varying factors—like location, type of materials burned, and meteorological conditions—could yield different outcomes in comparable scenarios.

With the prediction that fires in the Wildland Urban Interface will become more frequent and intense, residents are urged to stay informed about potential health risks associated with returning to their homes after such events. Understanding these findings could prove essential in preparing for the safety and well-being of communities in fire-prone areas.