Months after Colorado's Marshall Fire, residents of surviving homes reported health symptoms, poor air quality

Six months after the Marshall Fire destroyed more than 1,000 houses in Boulder County, Colo., more than half of residents of surviving homes in the area reported physical symptoms— including headaches, sore throats or a strange taste in their mouth— that they attributed to poor air quality, a new CU Boulder study has found.

A companion study showed that the air quality inside one home post-fire equaled that of downtown Los Angeles in the 1990s on a high pollution day, with hazardous gases lingering for weeks.

“Our research suggests that there could be important health impacts for people returning to smoke- or ash-damaged homes after a fire and that we need to have systems in place to protect them,” said Colleen Reid, associate professor of geography and co-author of the studies.

The papers, published this week in ACS Environmental Science & Technology Air, are the first to explore air quality inside smoke- and ash-damaged homes and to assess the health impacts on people who live in them. They come as fires in the Wildland-Urban Interface, like in Paradise, California, in 2018 and Lahaina, Hawaii, in 2023 grow more common.

“A lot of time has been spent studying wildfire smoke – what you get when you burn vegetation. But what do you get when you burn a home, with all its furniture and electronics and cars? Until now, there has been very little known,” said co-author Joost de Gouw, a professor of chemistry and fellow with the Cooperative Institute for Research in Environmental Sciences (CIRES) at CU Boulder.

A record winter blaze

Fueled by 100-mile-per-hour winds and record dry conditions, the Dec. 30, 2021, Marshall Fire burned 1,084 buildings in densely populated areas, took two lives and forced 37,000 people to evacuate.

Once residents returned home, scientists at CU Boulder, just 4 miles from the fire’s ignition, started getting calls.

“At first, they felt really lucky, but when they went into their homes they saw ash everywhere and it smelled differently – like a campfire or chemicals,” said Reid. “They asked: What should we do? We don’t know if it’s safe to go back in.”

With little research offering answers, the scientists began to investigate.

Ten days after the fire, de Gouw’s team erected field instruments in an intact home bordering a block where houses burned down. Over five weeks, they continually measured the presence of 50 gases. 

Meanwhile, Reid and colleagues developed a survey to send to residents within the burn perimeter, as well as a random sample of those within 2 miles.

At six months, 642 people had responded; 413 had responded at the one-year mark.

Some 55% of respondents reported symptoms that they attributed to the fire at the six- month mark, and survey answers depended largely on the condition of their home when they returned home.

For instance, those who found ash inside were three times as likely to report headaches compared to those who didn’t find ash; those who reported an odd odor were four times as likely to report headaches compared to those who did not pick up an unusual scent.

People with the same symptoms tended to cluster together, according to computer mapping analyses. For example, those living near destroyed homes, especially in the direction the wind was blowing the day of the fire, were far more likely to report a strange taste in their mouth.

“These findings are consistent with chemical exposures and suggest that residents of smoke- and ash-damaged homes may have experienced lingering air quality and physical health challenges months after the fire,” said Reid.

Long-term impacts uncertain

The authors cannot say which chemicals caused the health impacts that survey respondents reported. But measurements in one home found high levels of volatile organic compounds (VOCs) like benzene, a carcinogen found in gasoline and Diesel exhaust.

Dust samples also showed high levels of copper, zinc, arsenic and industrial pollutants called polycyclic aromatic hydrocarbons (PAHs), which are known to cause eye irritation.

“If your home survives, and the neighbor two doors down burns, all those melted things can get into the air and find their way into your home” settling into furnishings, carpets and drywall, said Reid.

The authors stress that VOCs are only considered carcinogenic at levels much higher than what they found, and people are regularly exposed to low levels daily through pollution.

Their research found that simple measures, like opening windows and using low-cost, carbon-activated air filters, can substantially improve air quality.

One year after the fire, the number of residents reporting symptoms had declined to just 33% and most said confidence in their home’s air quality had improved.

Until more studies are done, the researchers cannot say whether such exposures can lead to long-term health problems.

Nonetheless, Reid urges anyone going inside a smoke- or ash-damaged home just after a wildland urban interface (WUI) fire to use caution, wearing KN95 masks and gloves.

They hope their ongoing research can lead to clearer guidelines for policymakers and insurance companies about when it is safe for people to return to smoke-damaged homes.

“This wasn’t just any fire. It felt personal,” said de Gouw. “Knowing that we could at least begin to provide some answers to our community has made the work extra meaningful.”