Delphine Farmer is a professor of chemistry at Colorado State University.
When wildfire smoke turns the air brown and hazy, you might think about heading indoors with the windows closed, running an air purifier or even wearing a mask. These are all good strategies to reduce exposure to the particles in wildfire smoke, but smoky air is also filled with potentially harmful gases. Those gases can get into buildings and remain in the walls and floors for weeks.
Eliminating these gases is not as easy as using an air purifier or simply opening a window during a clear day.
In a new study published in the journal Science Advances, colleagues and I tracked the life of these gases in a home exposed to wildfire smoke. We also found that the best way to get rid of the risk is among the simplest: start cleaning.
The challenge of smoke particles and gases
In December 2021, several of my friends and colleagues were affected by the Marshall Fire that burned about 1,000 homes in Boulder County, Colorado. The “lucky” ones, whose homes were still standing, asked me what they should do to clean their houses. I am an atmospheric and indoor chemist, so I started looking into the published research, but I found very few studies on what happens after a building is exposed to smoke.
What scientists did know was that smoke particles end up on indoor surfaces – floors, walls, ceilings. We knew that air filters could remove particles from the air. And colleagues and I were just beginning to understand that volatile organic compounds, which are traditionally thought to stay in the air, could actually stick to surfaces inside a home and build up reservoirs – invisible pools of organic molecules that can contribute to the air chemistry inside the house.
Volatile organic compounds, or VOCs, are compounds that easily become gases at room temperature. They include everything from limonene in lemons to benzene in gasoline. VOCs aren’t always hazardous to human health, but many VOCs in smoke are. I started to wonder whether the VOCs in wildfire smoke could also stick to the surfaces of a house.
Tracking lingering risks in a test house
I worked with researchers from across the U.S. and Canada to explore this problem during the Chemical Assessment of Surfaces and Air, or CASA, study in 2022. We built on HOMEChem, a previous study in which we looked at how cooking, cleaning and occupancy could change indoor air.
During our study in CASA, we examined the effects of pollutants and chemicals infiltrating our homes, such as pesticides, smog, and even wood smoke.
Using a cocktail smoker and wood chips, we created a surprisingly chemically accurate proxy for wildfire smoke and released small doses into a test house built by the National Institute of Standards and Technology. NIST’s house allowed us to conduct controlled chemistry experiments in a real-world setting.
We even aged the smoke in a large bag with ozone to simulate what happens when smoke travels long distances, like the smoke from Canadian wildfires that moved into the U.S. in the summer of 2023. Smoke chemistry changes as it travels: Particles become more oxidized and brown, while VOCs break down and the smoke loses its distinctive smell.
How VOCs behave in your home
What we found in CASA was intriguing. While smoke particles quickly settled on indoor surfaces, VOCs were more insidious.
Initially, the smoke VOCs were absorbed by the house, permeating the floors, walls, and other surfaces. However, after the initial smoke dissipated, the house gradually emitted these VOCs over the following hours, days, or even months, with the duration depending on the specific type of VOC.
This release is what we call a partitioning process: During the smoke event, individual VOC molecules in the air attach to indoor surfaces with weak chemical bonds. The process is called adsorption. As smoke clears and the air cleans out, the bonds can break, and molecules “desorb” back out into the air.
The process of partitioning can be observed in the air through the measurement of smoke VOC concentrations. Similarly, on surfaces, the weight of smoke VOCs that settle on highly sensitive balances and are subsequently released can be measured.
Overall, we concluded that this surface reservoir allows smoke VOCs to linger indoors, meaning that people are exposed to them not just during the major smoke event but also long after.
Why worry about VOCs?
Smoke VOCs include well-known carcinogens, and high levels of exposure can induce respiratory and health problems.
Although the concentrations of smoke VOC in our test house decreased over time, they consistently remained above normal levels.
Given that VOC concentrations from other sources, such as cooking and cleaning, can already be high enough in homes to harm health, this additional long-term exposure source from smoke could be important. Further toxicology studies will be needed to determine the significance of its health effects.
How to clean up when smoke gets in
What actions can be taken to eliminate these remaining smoke gases?
Our findings indicate that air purifiers are capable of eliminating only a portion of the VOCs present in the air. However, they are unable to cleanse the VOCs residing on surfaces such as floors and walls. Furthermore, air purifiers are effective solely when in operation, yet their ability to reduce VOCs is not particularly noteworthy.
Opening windows to ventilate will clean the air, if it isn’t smoggy or smoky outside. But as soon as we closed windows and doors, smoke VOCs started to bleed off the surface reservoirs and into the air again, resulting in an elevated, near-constant concentration.
Upon realizing that the smoke VOCs could only be permanently eliminated by physically extracting them from surfaces.
The good news is that cleaning surfaces by vacuuming, dusting and mopping with a commercial, nonbleach solution did the trick. While some remediation companies may do this surface cleaning for you after extreme exposures, surface cleaning after any smoke event – like Canadian wildfire smoke drifting into homes in 2023 – should effectively and permanently reduced smoke VOC levels indoors.
Naturally, we were limited in the number of surfaces we could clean – vacuuming the ceiling proved to be quite challenging! As a result, although surface cleaning did enhance the situation, it did not completely eliminate the levels of smoke VOCs in the house. Nevertheless, our study offers a potential solution for effectively cleaning indoor spaces impacted by air pollutants, whether caused by wildfires, chemical spills, or other incidents.
With wildfires becoming more frequent, surface cleaning can be an easy, cheap and effective way to improve indoor air quality.
This article is republished from The Conversation under a Creative Commons license.
- In:
- Wildfire Smoke
