Quantifying the Infiltration
The study utilized an 18-month window of data, concluding its primary analysis in April 2024. To gather this data, the team deployed a network of low-cost, wirelessly connected sensors across 17 indoor and two outdoor locations. These sensors were specifically positioned to characterize air quality during three distinct types of pollution events common to the region: dust storms, winter inversions, and wildfire season. Each of these events presents a different chemical and physical profile of PM2.5.
Tristalee Mangin, a graduate student in chemical engineering and the leader of the study, explained the methodology behind the data collection. "We look at the ratio of the indoor particulate matter measurements to that of the outdoor particulate matter measurements," Mangin said. "The closer that value gets to one, which means more of the particulate matter is going to be sourced from outdoors versus indoor sources."
By analyzing these ratios across different groupings of HVAC types, the team was able to pinpoint which systems were most susceptible to infiltration. The results were stark: wildfire smoke resulted in four to five times more PM2.5 infiltration into buildings compared to pollution from winter inversions or wind-driven dust.
The hardware used for the study was as innovative as the research itself. The team utilized small monitoring devices designed by Pierre-Emmanuel Gaillardon, a professor of electrical and computer engineering at the University of Utah. These devices were commercialized through TELLUS, a university-affiliated startup. At a cost of just $450 per unit, these sensors are significantly more affordable than traditional compliance-grade monitoring equipment, allowing for a much denser and more comprehensive sensor network. The devices were installed on walls at breathing height to ensure the data reflected the actual air being inhaled by the buildings’ occupants.
How Smoke Differs from Dust and Inversions
One of the most intriguing aspects of the study was the discovery of why wildfire smoke penetrates buildings so much more effectively than other pollutants. According to the findings, outdoor pollution from dust and inversions had a negligible effect on indoor air quality, regardless of the HVAC system in place.
This phenomenon is largely due to the physical nature of the particles. Dust particles, typically kicked up by high winds, are generally larger and heavier. Because of their mass, they are more likely to settle out of the air or be successfully trapped by standard HVAC filters.
Inversions, which are a notorious winter occurrence in Utah’s valleys, present a different scientific puzzle. During an inversion, cold air is trapped near the ground by a layer of warmer air above, leading to a buildup of pollutants. In Utah, these particles are primarily composed of ammonium nitrate, accounting for roughly 60% of the particulate matter.
"It has to do with the thermodynamics of the particles that are in the inversion," Kelly explained. "At indoor temperatures and relative humidities, those particles turn into a gas phase. During an inversion, most of the stuff that’s a particle outside is not a particle when it gets inside. It kind of disappears."
