In the second part of our interview with Brandon Boor of Purdue University, he discusses how his team controls experimental variables during cleaning experiments in order to obtain interpretable data.
Brandon E. Boor, the Dr. Margery E. Hoffman Associate Professor in the Lyles School of Civil and Construction Engineering at Purdue University, presented a talk titled “Indoor Atmospheric Nanoparticle Formation from Scented Cleaning Products” at the American Chemical Society (ACS) Fall 2026 Meeting in Chicago, Illinois, sharing findings from his team’s research on indoor air quality.1–4
In the first part of a multi-part Q&A recapping the talk, Boor discusses the instrumentation and methodology his team uses to measure
In the second part of our interview, Boor discusses how his team controls experimental variables during cleaning experiments in order to obtain interpretable data.
Every experiment follows a defined sequence. For the test house experiments, we begin by establishing the air exchange rate we intend to study. This is controlled by a powered ventilator, and adjusting its set point changes the air exchange rate. We may target a low rate, below one air change per hour, or a considerably higher one, on the order of six to ten.
Once the set point is established, we verify the air exchange rate by tracer gas decay, typically using carbon dioxide injected from a cylinder. This confirms how well ventilated the space is with respect to outdoors. The second decision is what to study: which cleaning product will be purchased and used.
In all our experiments, we obtain products from local retail stores, whether cleaning products, personal care products, or aromatherapy products. Some cleaning products must be diluted before use, while others are applied directly from the bottle.
Indoor ozone levels are largely set by the ventilation rate, since outdoor air is normally the dominant source, unless
We then define how the product will be applied and ensure that the application is consistent across experiments. A wide range of cleaning products is available, many of them heavily scented, so a carefully specified application routine is essential if comparisons between products are to be meaningful.
Once all of this is established, we set up our instrumentation. Some instruments must sample inside the test house. Nanoparticles, in particular, have to be measured in situ, because a long sample line would remove most of them through diffusional deposition. Other instruments are located in an adjacent laboratory and connected by sample tubing, which avoids crowding the test house with equipment that has a large footprint.
We then calibrate the instruments, verifying that each is operating correctly and that all data streams are synchronized. Next, we conduct a background period in the test house to characterize the chemistry of the space when it is unoccupied, since indoor surfaces can be coated with organic films and particles can enter from outdoors through the ventilation system. This establishes the baseline condition of the indoor environment.
The occupants who will perform the experiment then enter the house. We treat this as an occupied background period, because people themselves alter the chemistry of the air. Skin oils on the body and on clothing act as an ozone sink, and we emit VOCs from exhaled breath, from skin, and from the personal care products we wear. Measuring this first establishes the contribution of the occupants themselves, so that the baseline of the test house accounts for outdoor air, indoor surfaces, and the people in the space.
The activity itself, such as cleaning the house, is then carried out for a defined period, after which the occupants leave and we track the decay. Particles that are emitted or formed, along with any gases released during the activity, build up and then decay as they are removed from the air by several mechanisms: ventilation and exhaust pathways, chemical reaction and transformation, and deposition onto indoor surfaces.
Our aim is to track the fate of these contaminants after the activity ends. The experiment concludes at the end of a long decay period, and the three phases together—the background, the activity, and the decay—capture how that activity affected indoor air quality. Because everything is measured continuously and timestamped, we can attribute each feature in the data to a specific event. The combination of these periods allows us to construct a mass balance model and quantify how much pollution the activity produced.
We make every effort to standardize the application. In one study published during the Covid-19 pandemic, we applied a fixed number of sprays of each product onto gridded glass plates using an identical spray bottle, so that the application was as repeatable as possible across formulations. This allows a fair comparison of how different products affect indoor air quality.
For product selection, we purchase off-the-shelf items from retail stores in and around West Lafayette, Indiana. We deliberately span a range of fragrances and formulations: botanical products that use essential oils for cleaning or disinfection, products labeled as antimicrobial, and products labeled as cleaning solutions rather than disinfectants.
The guiding criterion is that every product selected is one an ordinary consumer can purchase and is representative of what is commercially available. We also sample across scent profiles, including lemon and other citrus, lavender, and pine. This has proven valuable, because it lets us assess how everyday household cleaning products affect indoor air quality.
