Home Sweet Home: Study of ‘Chemical Soup’ in our Houses

Microbial traces of everything from coffee to antidepressants make up the invisible environment in our indoor spaces

Chances are very good that as you read this, you are seated somewhere indoors. The surfaces around you are covered in microbes and you are also covered in microbes. All those microbes are busy excreting molecules and responding to the rest of the molecules in the mix. What does all of this mean for your health?

“We are living in a soup of chemistry,” says UConn Department of Chemistry researcher Alexander Aksenov, who is working to understand this microbial and molecular soup in our indoor environments and how it could be impacting our health. He and a multidisciplinary team of researchers, including from the University of California, San Diego, Colorado State University, and the University of Colorado published a paper today in Science Advances exploring these under-studied questions, with some surprising findings that could help inform us how to live healthier lives indoors.

Accounting for our full day, including time spent in cars, on average we spend over 90% of our time indoors, says Aksenov, so the indoor environment is by far the most important for us.

Previous studies show human activities impact our indoor environments, through things like gas stoves, chemical off-gassing, and the type of cleaning solutions we use. These studies usually looked at a limited number of molecules. For this study, the researchers sought to explore the full suite of molecules and microbes within a household environment.

The experiment was performed in a test home in Austin, Texas called the House Observations of Microbial and Environmental Chemistry (HOMEChem). The researchers sampled the house’s surfaces to get a baseline of what microbes and molecules were already present. Then for about six hours each day volunteers occupied and performed scripted household tasks, like cooking, cleaning, eating, or just hanging out. After a month, the researchers took another round of samples.

“My lab focused on the metabolomics, study of complex distributions of small molecules, and we, with the team from UC San Diego, explored chemistry and microbiome of the house,” says Aksenov. “The other research groups studied aspects like aerosols, particle deposition, oxygenation, anything you can measure in the house.”

One thing that completely blew my mind was the most pronounced trace humans left behind was coffee. — Alexander Aksenov, Assistant Professor of Chemistry

Results showed human activities profoundly influence the composition and quantity of molecules around the house.

“We saw that before humans move in, the indoor microbiome was similar to the outdoor microbiome, with species that are a hallmark of things like soil and freshwater sources, for example. Then humans moved in and within a month, the microbiome was rewired entirely to include microbes found on human skin and in the gut.”

They found molecules associated with skin, skincare products, food, animals, and biocides, to name only a few. Then, by applying advanced tools such as molecular networking to the data, Aksenov explains they were able to perform a molecular genealogy of the chemistry within the home, tracing molecules altered chemically either through human or microbial means, from surfaces to their points of origin. One example came as a surprise, says Aksenov.

“One thing that completely blew my mind was the most pronounced trace humans left behind was coffee,” he says. “Even though coffee was not part of scheduled indoor activities, we found multiple versions of a coffee-derived molecule all around the house, including some that originated from coffee and were then modified by microbes, and the epicenter was the coffee machine where it was then spread around by human activities. What are the health effects of those molecules? We have no idea. There are thousands upon thousands of molecules in the house that we attribute as originating from food.”

Microbes have extremely rich enzymatic machinery and produce a large variety of microbially-derived molecules that are very poorly studied, explains Aksenov, meaning there’s a large blind spot in terms of the chemistry that microbes generate. They can produce molecules themselves, or they can take molecules from hosts or from the environment and modify them for their own purposes.

The chemical diversity in the test house increased over the month, across frequently touched surfaces of the house, but especially in the kitchen. The other hotspot, perhaps unsurprisingly, was the bathroom. Our lifestyle choices, especially what we eat, impact the composition of molecules in our immediate surroundings.

Aksenov says another interesting example was capsaicin, the molecule that makes peppers spicy. Somehow, capsaicin was already present, despite the house being thoroughly cleaned before the gathering of the first samples.

“This means that if you’re moving into any building that, even if it has been renovated or cleaned extensively — a hotel room for example — there is a record of the previous inhabitants there already, molecular traces that track the history of everything and everyone before,” says Aksenov. “Capsaicin was localized in the kitchen, but then there is another oxidized version of the molecule that is altered by liver enzymes before being excreted, and that version showed up on all the surfaces that are touched. It was likely produced in humans after ingesting capsaicin and then secreted through the skin.”

The results also gave a glimpse into the health of the volunteers, showing evidence of medications, like antidepressants, steroids, and back pain medicines.

We know microbial communities contribute to our health, says Aksenov, and with increasing evidence of imbalances in these communities contributing to things like asthma and eczema, studies like this one lend insights into our dynamic interactions with our surroundings.

“There are all kinds of health conditions on the rise that we can’t explain that could be related to indoor environments,” Aksenov says. “The important consideration is that we do know that microbes are important players, both those that are part of our microbiome and those in our surroundings. Ultimately, they interact with us through chemistry that you are exposed to by touching surfaces, inhaling, ingesting, and in all kinds of other ways. That’s the chemistry that ultimately affects our health and well-being.”

This is just one of the first steps for this type of research, says Aksenov. Going forward, one question the researchers hope to answer is whether it is possible to influence the chemical soup for the better. Aksenov points out obvious ways to do this, such as having proper ventilation to combat mold growth or moving contaminants outdoors, but he notes these new findings point towards new avenues to be explored.

“Now we know that humans leave a complex imprint, we can explore how the human microbiome interacts with the house, and the chemistry that results,” he says. “Can we influence a microbiome that generates beneficial chemistry for a greater health effect, and if so, how can we introduce that to the house? It’s so multifaceted. I think the main message is that humans, the environment, the chemistry indoors, and microbes exist in a mutual relationship, we all affect each other and now we have scientific evidence showing this.”

This was designed as a large multidisciplinary, community endeavor. It is funded by the Alfred P. Sloan Foundation Microbiology of the Built Environment Program and Chemistry of Indoor Environments Program (G-2017-9944).

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