How Should You Clean Your House After It’s Engulfed in Wildfire Smoke?

In the aftermath of January’s Los Angeles wildfires, researcher Elliott Gall got an email from someone who had returned home after evacuating. They wanted to know what to do with their mattresses, which had been exposed to smoke. Should they wash them, air them out, dispose of them? 

He couldn’t give one clear recommendation. “It’s a really difficult question to answer without knowing a whole lot of things and understanding the nature of exposure to that mattress,” said Gall, a materials engineer who studies indoor air quality at Portland State University in Oregon. 

The question, and others like it, are familiar to many air pollution researchers, who struggle to know what advice to give worried individuals about the lingering effects of wildfire smoke. While there’s a lot of research and public guidance on what to do during a fire or smoke event, not much exists on what to do after the smoke has cleared, nor about how long toxins released by wildfires might stick around. In recent years, as wildfires have grown larger and more severe due to climate change and more buildings are exposed to smoke, a small but diligent network of researchers have been working to fill that gap. 

So far, research suggests that wildfire smoke toxins can stick to surfaces after a fire, and then slough off over long periods of time. Canadian researchers recently observed that toxins in wildfire smoke that drifted from Saskatchewan to Alberta settled on glass in a residential backyard, an indication that they can become part of the layer of “urban grime”—pollutants that coat many outdoor surfaces in developed areas. That layer acts as a reservoir of the pollutants that can re-release them when it rains, said study author Iris Chan of McMaster University. It also poses a direct risk to someone touching the grime, a particular concern for crawling children and pets.

That risk is even higher indoors, where there’s hundreds of times more surface area than outside, Gall said, and ventilation doesn’t refresh air as effectively as outdoor breezes and circulation patterns.

To investigate the persistence of toxins inside of houses, Colorado State University researcher Delphine Farmer filled a test house with low levels of smoke and monitored it for six weeks. Her team discovered that volatile organic compounds, or VOCs, from the smoke stayed on surfaces for the duration of the experiment.

And they didn’t just stay there—some VOCs moved off of the surface back into the air. That suggests that even when smoke is gone and no odor of it lingers, the surfaces it touched remain a persistent source of toxins. 

How long different compounds stick around depends on a few chemical properties, Farmer found. First, how volatile the compound is—in other words, how likely it is to move from a solid or liquid to a gas. It also depends on how easily the compound mixes with organic compounds already present on surfaces. Their molecular structure matters, too, as does their size—larger particles are more likely to stick around. 

In Farmer’s test house, the most volatile compounds took only a few hours to slough off surfaces. But the least volatile stayed for the entire six-week experiment, and the team’s calculations suggest that those VOCs could stay put for years.

Unfortunately, “there’s no direct relationship between that time scale and the toxicity,” Farmer said. It may be that some of the most toxic compounds are the ones that come off slowly, and perhaps over a long enough period of time that residents have stopped taking precautions, like running air conditioners or air purifiers. 

Gall’s lab at Portland State ran a similar experiment, except they focused on three common household materials: glass, cotton fabric (specifically bed sheets) and drywall. They added 16 polycyclic aromatic hydrocarbons, or PAHs, directly onto the surfaces, and also burned Douglas fir needles in order to mimic the smoke of the 2020 Labor Day Fires in Oregon. Afterwards, they measured the levels of PAHs, which are known to be carcinogenic. Not only were PAH levels on the surfaces elevated, but the team’s calculations suggested that they would remain high for around 40 days after the smoke had cleared. 

“It was surprising to see the longevity of the compounds on these materials,” Gall recalled. 

Beyond the lab, scientists have observed similar trends in houses actually affected by wildfire smoke. A study after the 2021 Marshall Fire in Colorado found elevated levels of PAHs in the dust that collected in surviving houses six months after the fire. And another study in just one house that was impacted by the fire outside of Boulder showed that VOCs decreased quickly at first, but then their release from surfaces slowed down and continued for many weeks. Interventions like cleaning the air would decrease the level of airborne VOCs temporarily, but, as soon as they turned the cleaner off, the levels went back up. 

In all likelihood, “gasses impregnated every porous surface in the home, and then slowly off-gassed over time,” said Colleen Reid, who studies health and air quality impacts at the University of Colorado Boulder. “And there was such a huge sink of them.”

The Marshall Fire provided a valuable case study, said Reid, because the extremely fast fire caused only a single day of significant smoke; a snowfall helped clear the air the day after the blaze ignited and burned more than 1,000 structures. And yet, even that short duration of smoke was enough to leave behind compounds for months. 

For smoke events that lasted longer, like the Los Angeles fires, Reid suspects that the larger size and duration of those fires would translate to higher concentrations of compounds left behind. And their composition was likely more toxic: the LA fires burned more than 12,000 human-built structures, many more than the Marshall Fire.

“In combustion, it’s pretty well known that the chemistry of your fuel is going to impact the chemistry of your emissions,” said Amara Holder, a mechanical engineer who studies combustion emissions at the Environmental Protection Agency. Many products made by humans contain heavy metals and can emit lead, arsenic, mercury and other toxins when they burn, she said. 

Whether the fuel is natural or human-made, wildfire smoke toxins pose human health risks, but it’s not well known how low levels of these toxins affect health over time. So, in 2022, six months after the Marshall Fire, Reid distributed a survey to people who had returned to their smoke-impacted houses with questions to determine their possible exposure levels: How far they were from a burned building, for instance, or if their house smelled like smoke. 

Every metric of smoke exposure was associated with reports of headaches, though that malady was more common the closer the person was to the burn zone. Close proximity also increased the likelihood of having a strange taste in the mouth. Some symptoms—specifically sneezing, sore throat and headache—were more common in homes that continued to smell differently a week after the fire. 

While the correlations were significant, Reid points out that her team didn’t collect a lot of relevant pieces of data, like how long people were spending in their homes or whether they had cleaned the house. “It’s a first pass,” she said. “We need to do more studies on this.” 

Early studies that focus on wildfire exposure in general—not just these reservoirs of smoke toxins that are left behind—suggest that long-term health effects may include asthma, increased risk of cancer, and premature death. 

Scientists are also studying what practical interventions best remove the persistent toxins left behind by smoke. And the good news is that pretty standard cleaning—things that are easy to add to a “post-wildfire checklist,” according to Gall—can have a big impact. Most toxins can be physically cleared away, “like cleaning a greasy pot,” said Farmer. And, “if we remove those reservoirs that have stuck, then we remove that persistent source of VOCs.” 

Farmer found that a combination of vacuuming, dusting, mopping and wiping down surfaces with a wet rag and a little soap was “incredibly effective.” If a child is playing outside, washing their hands can remove any toxins they pick up from urban grime. Fabrics can be effectively cleaned by washing and drying them. In his study, Gall found that laundering the smoky cotton sheets removed 48 percent of PAHs. 

But it’s important to carefully select the cleaning methods used in a home exposed to wildfire smoke, as some, like sweeping, can kick the toxins back in the air without removing them. 

When and how much to clean is another question that’s currently hard to answer. It seems that keeping up with cleaning is a good idea—the study on urban grime showed that dirtier surfaces were more susceptible to even more dirt.

Farmer, who lives in Colorado and regularly experiences wildfire smoke at her home, will clean the floors of her house after a fire “because that’s easy for me to do,” she said. “I probably wouldn’t go and clean the walls and try to vacuum the ceilings.”

She warns homeowners to be cautious of companies that claim to clean houses of smoke toxins, given the limited amount of science that exists on which they can base their work.

Holder sees the LA fires as an opportunity to continue this research, so that someday the EPA can have more recommendations for what to do in the aftermath of a fire. “This is a disaster that occurs routinely,” she said. “And we need to have more information on how to deal with this disaster.”