Modern life is defined by its interiority. On average, individuals spend approximately 80% of their lives inside buildings, whether for work, education, or rest. Within these enclosed environments, the quality of the air we breathe is often taken for granted, assumed to be a filtered sanctuary from the elements. However, the reality of indoor air quality is a complex tapestry woven from the age of the structure, the materials used in its construction, and the constant exchange between indoor and outdoor atmospheres.
New research from the University of Utah has shed light on a critical and often overlooked factor in this equation: the specific type of heating, ventilation, and air conditioning (HVAC) system used to manage a building’s climate. The study reveals that while these systems are designed for comfort and energy efficiency, certain configurations can inadvertently become conduits for outdoor pollutants, particularly during the increasingly frequent wildfire seasons that plague the American West.
The Campus as a Living Laboratory
To understand the nuances of air infiltration, researchers from the University of Utah’s College of Engineering turned their own campus into a massive, "living laboratory." This multi-disciplinary effort saw the College of Engineering team up with the university’s Facilities Management department and experts in occupational and environmental health. By using a diverse range of campus buildings—spanning various architectural eras and mechanical designs—the team sought to determine exactly how outdoor pollution events affect the air inside.
The findings were generally encouraging, suggesting that indoor air quality on the Salt Lake City campus remains high under most circumstances. However, the study identified a significant vulnerability regarding fine particulate matter, specifically PM2.5, during wildfire events. The research demonstrated that while pollution from winter inversions and wind-driven dust is largely kept at bay, wildfire smoke has a unique ability to penetrate certain types of buildings.
The discrepancy in how these pollutants behave is tied directly to the mechanical "lungs" of the buildings. Specifically, the study highlighted challenges associated with commercial HVAC systems equipped with air-side economizers.
The Efficiency Paradox: Air-Side Economizers
Air-side economizers are a staple of modern green building design. This technology is intended to reduce energy consumption by utilizing "free cooling." When the outdoor temperature and humidity reach optimal levels—such as on a crisp summer morning or a cool autumn afternoon—the system uses a series of ducts and dampers to draw in outdoor air rather than running energy-intensive mechanical cooling units.
While this is a boon for energy efficiency and sustainability goals, it creates a direct pathway for atmospheric contaminants. Kerry Kelly, a professor of chemical engineering who oversaw the research, explained that if the outdoor air is thick with smoke, these economizers can pull that particulate matter directly into the building’s circulation. Although the systems are equipped with filters, some of the finest particles from wildfire smoke manage to bypass these defenses and settle into the indoor environment.
"Our buildings are big and they’re complicated, and oftentimes they’ve been added onto and integrated with different kinds of systems," Kelly noted. This complexity makes the management of air quality a moving target. Despite the challenges, Kelly remains optimistic about the future of indoor air management. "I think the management is challenging, but the good thing is it is a very solvable problem. Even simple solutions like portable air filters do a great job."
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."
Essentially, the chemical composition of inversion pollution causes it to evaporate or change state when it enters the warmer, drier environment of a heated building, effectively reducing the concentration of measurable particulate matter indoors. Wildfire smoke, however, consists of solid carbonaceous particles and organic compounds that do not "disappear" upon entry, allowing them to linger and accumulate.
Research Continues to Find Solutions
While the initial study period has concluded, the project remains very much alive. The sensors continue to monitor the campus air, providing real-time data that is visible to the public and the campus community. Next to the devices, statements explain the research and its funding, which was provided by Seed2Soil and Student Community Investment Fund (SCIF) grants.
The long-term goal of the research is to move beyond observation and toward active mitigation. The findings are expected to serve as a roadmap for campus building managers as they develop operational strategies to protect indoor air quality during the "smoke season."
However, implementing a universal fix is easier said than done. The University of Utah campus comprises approximately 18 million square feet of building space, featuring a vast array of architectural styles and mechanical systems. Sean Nielson, an engineer with Facilities Management’s Sustainability and Energy group and a co-author of the study, emphasized that a one-size-fits-all approach is impossible.
"Every building and system has unique features," Nielson said. "Looking at buildings and systems individually is something we’re going to consider in the future and see what we can do to modify that system."
One might assume the simplest solution would be to close the outdoor air dampers entirely on smoky days. However, Nielson points out that building codes and health standards require a minimum amount of outside air to be circulated to dilute and flush out indoor contaminants, such as carbon dioxide and volatile organic compounds (VOCs).
"You still have code minimum requirements for a certain amount of outside air that must be provided," Nielson explained. "Finding the ideal balance between outside air and recirculated inside air is difficult due to the high number of variables."
One potential solution is the implementation of higher-rated MERV (Minimum Efficiency Reporting Value) filters. While these filters are more effective at capturing fine particles, they also create more resistance to airflow, which can strain older HVAC motors and reduce energy efficiency. Furthermore, there are physical limitations to what these filters can catch and how they can be retrofitted into existing, aging equipment.
As the University of Utah continues to refine its building operations, this research provides a vital foundation for balancing the competing needs of energy conservation, mechanical longevity, and human health. The study serves as a reminder that as the climate changes and wildfire events become a more permanent fixture of the landscape, the way we design and manage our indoor environments must evolve in kind.
