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Indoor Air can be More Complex than Outdoor Air Systems

Delphine Farmer, an atmospheric chemist at Colorado State University, had spent her whole career exploring the eccentricities of outdoor air—how atmospheric particles and gases move, change, and interact, and how human activities affect the air inhaled by individuals. But when Farmer went indoors, she was forced to reassess her observations.

HOMEChem lead researcher Delphine Farmer, right, and graduate student Erin Boedicker, looking at a droplet-measurement instrument. Image Credit: Colorado State University.

Farmer, who is also an associate professor at the Department of Chemistry, focused on the less-explored area of indoor air and eventually discovered that indoor chemistry can be much more complicated than that of outdoor air systems.

Over two years ago, Farmer and more than 60 colleagues from 13 universities performed a groundbreaking experiment in an effort to map the airborne chemistry of a standard home, exposed to regular home activities like cleaning and cooking.

The effort was called HOMEChem, short for House Observations of Microbial and Environmental Chemistry. The study was headed by Farmer and Marina Vance, a mechanical engineer from the University of Colorado Boulder.

Now, as the researchers go through a large amount of data gathered by them, Farmer and her research team from Colorado State University have published their first-ever significant study from the HOMEChem project.

The study, which was published in the Environmental Science and Technology journal, reveals what the researchers had learned about chemical reactions that took place while they were using a common bleach solution to mop floors.

On the HOMEChem study, her first venture into indoor chemistry, Farmer “became a convert when I heard the statistic that we spend 90 percent of our lives indoors.”

It’s puzzling, really, that all our health outcomes are tied to outdoor air. It made me curious as a scientist when I realized just how little we know about chemistry indoors.

Delphine Farmer, Associate Professor, Department of Chemistry, Colorado State University

Farmer’s team, which includes postdocs and graduate students, is currently involved in collecting potential follow-up studies and crunching more amounts of data.

In the Test House

The HOMEChem researchers, funded by $1.1 million from the Sloan Foundation’s Chemistry of Indoor Environments program, arrived on the ideal location for their experiments: at the University of Texas at Austin, the Test House is a full-size, manufactured “home” that acts as a sort of blank slate for performing scientific experiments.

For most of June 2018, the researchers occupied the house and simulated activities that were performed in a typical Western home. The researchers’ efforts have been described in an overview paper published in Environmental Science: Processes & Impacts.

The team’s experimental run-of-show read mostly like a family’s list of chores. It contained things like wet-mopping floors, scrubbing surfaces with household products, and cooking vegetable stir-fry. There was one session that was also assigned to cooking a regular Thanksgiving meal while simultaneously recording the ensuing emissions.

The researchers recorded all these activities while operating an unlimited number of dollars’ worth of susceptible equipment that can potentially detect everything in the air, right from 1-nm particles to an infinite number of different volatile organic compounds.

Farmer’s research team from Colorado State University included postdoctoral researchers Yong Zhou and Andy Abeleira, and graduate students Jimmy Mattila, Matson Pothier, and Erin Boedicker. The researchers deployed 12 individual instruments for monitoring three wide categories of compounds such as particles, oxidants, and organics.

Anna Hodshire, a postdoctoral researcher and data scientist, has recently joined Farmer’s research team and will be accountable for handling the extensive datasets that were collected by the team over the course of the HOMEChem project.

Bleach Cleaning Results

For the bleach-cleaning analysis, Farmer’s research team recorded the aqueous and airborne chemistry from many consecutive days of cleaning a floor with bleach, diluted according to the specifications of the manufacturer. On certain days, the researchers also noted how that chemistry was impacted when floors were cleaned after a cooking session.

The study showed that the researchers noticed sharp but short-lived spikes in nitryl chloride, chlorine, and hypochlorous acid in the air. These compounds are largely linked, at lower concentrations, with the coastal cities’ outdoor air.

According to Mattila, graduate student and the study’s first author who operated a chemical ionization mass spectrometer during the HOMEChem project, the researchers were surprised to find that the removal and production of inorganic compounds in the air at the time of bleach cleaning are regulated by multi-phase chemistry and not only by the gas phase.

When the bleach present in the mop water is applied to the floor, it would create new compounds by reacting with the molecules present in the walls and surfaces of the house. It was found that such surfaces—and the layer of dirt that builds up in many homes from years of living—can serve as reservoirs for many different kinds of basic and acidic molecules—molecules that can further interact with substances such as bleach.

You would intuitively think that since we’re making these fumes in the air, and there’s other stuff in the air, they’re probably just reacting. It turns out that indoor multiphase chemistry, in the bleach solution and on various indoor surfaces, is what’s actually driving the observations.

Jimmy Mattila, Study First Author and Graduate Student, Colorado State University

In association with researchers at UC Irvine, the team created a model to learn how the surface and aqueous molecules result in secondary chemistry.

When the researchers cleaned the floor after cooking, they also noticed that certain interactions occurred between the cleaning products and the ammonia and nitrogen emissions from the food.

They observed low concentrations of chloramines, which are believed to be compounds dangerous to human health; chloramines are formed when chlorine combines with ammonia. Small amounts of ammonia are also exhaled by humans.

If you look on any bottle of bleach, you’ll see a serious warning not to mix chlorine and ammonia, because it will make a dangerous set of compounds called chloramines. What we found is there was enough ambient ammonia to still make some of these compounds, even without mixing them. Not to the point where it was dangerous, but it was interesting to see that chemistry happening.

Delphine Farmer, Associate Professor, Department of Chemistry, Colorado State University

An evident take away from the research team: Whenever bleach is used for cleaning, it is always better to use a fan or open a window to increase ventilation. The solution should always be correctly diluted; cleaning with concentrated bleach can produce harmful breathable compounds, based on what else is present on the walls or in the air.

A Baseline for Future Studies

The whole HOMEChem experiment was unparalleled in its scope. This analysis was mainly carried out to establish a fundamental understanding of what people at homes, performing regular home activities, can anticipate to inhale.

Among the major takeaways from these experiments as a whole was that mixing different indoor activities results in extremely different indoor chemistry.

For example, we see that cleaning with bleach after you clean indoors with a terpene solution, like Pine Sol, can actually lead to some chemistry you wouldn’t normally see with bleach alone,” added Mattila “That was kind of unexpected, and could be potentially harmful, because it could lead to the production of secondary organic aerosols.”

Since HOMEChem was a measurement experiment, it did not include epidemiologists. The scientists hope that their data will act as a valuable starting point for inquiries into human health outcomes linked to indoor air settings.


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