From VOC–Ozone Reactions to Respiratory Deposition: Unraveling Indoor Particle Exposure

Reflecting on the American Chemical Society (ACS) Fall 2026 Meeting, Brandon E. Boor, the Dr. Margery E. Hoffman Associate Professor in the Lyles School of Civil and Construction Engineering at Purdue University, sat down with Spectroscopy to discuss his team’s research into how scented cleaning products influence indoor air quality.^1–4 In the first part of the Q&A, he explains the advanced instrumentation and measurement approaches used to capture real-time nanoparticle size distributions at the 1–3 nm nanocluster scale, where accurate measurements are particularly challenging.⁵ In the second part, Boor describes how the researchers control experimental variables during cleaning activities to isolate the effects of different products and conditions.⁶

In the final part of our conversation with Boor, he explains the respiratory deposition model his team used to analyze indoor particle exposure.

You translate nucleation and growth rates into human inhalation dose rate comparisons with traffic emissions. What respiratory deposition model or approach did you use to bridge airborne particle measurements to an exposure estimate?

Our objective is not only to characterize the chemistry and physics of a particular activity, but to determine how cleaning alters the abundance of different chemical species and how it drives the formation of secondary organic aerosol.

We also want to connect these measurements to exposure. Indoor air quality matters because we spend the majority of our time indoors, exposed to pollutants generated by cooking, cleaning, and the use of personal care products. Having characterized how these nanoparticles form and how they are emitted, the natural next question is how they interact with the human body. As engineers, we do not study the toxicity or health effects of these particles directly, but we can quantify the rate at which they deposit in the human respiratory system.

Open-source dosimetry models provide a deposition fraction, which gives the probability that a particle of a given size will deposit in a particular region of the respiratory system. Particle size is critical here, governing not only fate and transport in the air but also respiratory deposition.

These models give a size-resolved deposition fraction for the upper and lower airways, which we combine with the measured particle number size distributions from the instrumentation described earlier. Together, these yield the number of particles depositing in each region of the respiratory tract per unit time. This is what allows us to compare different activities and different sources in terms of their contribution to respiratory deposition. When we place those numbers alongside published measurements in traffic-influenced outdoor environments, the deposited dose from a single cleaning event is comparable on a particle number basis.

Indoor environments have many potential VOC and oxidant sources — how did you attribute the nucleation events specifically to cleaning-product terpenes reacting with ozone, rather than other indoor or infiltrating outdoor sources?

Any particles originating outdoors will already be present in the background measurements described earlier.⁵ We often place a filter at the outdoor air intake to remove those particles, so background levels are low.

We measure VOCs, we measure oxidants such as ozone, and we characterize the background thoroughly, so that we know precisely what the state of the environment is before it is perturbed. Once that baseline is established, we carry out the activity, and the response is typically an unambiguous increase or decrease in a measured species. For example, if we apply a personal care product or a cleaning product, we see increases in certain VOCs, and those react with ozone. We simultaneously observe a sharp decrease in ozone, so the behavior of the VOCs and of the ozone is resolved concurrently. These species are known to react to form particles, and because we measure the particles at the same time, we can observe both their nucleation and their subsequent growth. This depends on careful experimental design, a well-characterized background, and precise knowledge of when each activity occurred. When we observe an increase or decrease, we can attribute it. If cleaning is performed in the presence of ozone and particles are detected, we can conclude that those nanoparticles were nucleated through reactions between the emitted VOCs and ozone.

Careful experimental design and proper characterization of background conditions are therefore essential. Both are achievable in a platform such as our zEDGE test house at Purdue, but neither is straightforward in the field, where occupants go about their normal activities in homes, workplaces, and schools. The laboratory work is valuable precisely because we know the background conditions and the timing of every activity. In field settings, many processes occur simultaneously, and it becomes very difficult to attribute a given outcome to a specific activity or source.

References

1. Purdue University, Brandon E. Boor Profile. Purdue University, 2026. https://engineering.purdue.edu/CCE/People/ptProfile?resource_id=124718 (accessed September 8, 2026).
2. Boor, B. E. Indoor Atmospheric Nanoparticle Formation from Scented Cleaning Products. Presented at the American Chemical Society Fall 2026 Meeting, Chicago, Illinois. Available at: https://acs.digitellinc.com/live/37/session/591663
3. Patra, S. S.; Liu, J.; Jiang, J.; Ding, X.; Huang, C.; Keech, C.; Steiner, G.; Stevens, P. S.; Jung, N.; Boor, B. E. Rapid Nucleation and Growth of Indoor Atmospheric Nanocluster Aerosol during the Use of Scented Volatile Chemical Products in Residential Buildings. ACS ES&T Air 2024, 1 (10), 1276–1293. DOI: 10.1021/acsestair.4c00118
4. Jiang, J.; Ding, X.; Tasoglou, A.; Huber, H.; Shah, A. D.; Jung, N.; Boor, B. E. Real-Time Measurements of Botanical Disinfectant Emissions, Transformations, and Multiphase Inhalation Exposures in Buildings. Environ. Sci. Technol. Lett. 2021, 8 (7), 558–566. DOI: 10.1021/acs.estlett.1c00390
5. Boor, B. E.; Wetzel, W. Sizing Up the Nanoscale: Measuring Nanocluster Aerosol in Indoor Air. Spectroscopy Online, 2026. https://www.spectroscopyonline.com/view/sizing-up-the-nanoscale-measuring-nanocluster-aerosol-in-indoor-air (accessed September 16, 2026).
6. Boor, B. E.; Wetzel, W. How to Monitor Indoor Air Quality Effectively. Spectroscopy Online, 2026. https://www.spectroscopyonline.com/view/how-to-monitor-indoor-air-quality-effectively (accessed September 17, 2026).