As modern life increasingly shifts toward indoor environments, the quality of the air we breathe within the walls of our homes, offices, and classrooms has become a focal point for public health researchers. Statistics indicate that the average person typically spends upwards of 80% of their time indoors. While it is common to assume that four walls and a roof provide a sanctuary from the smog and haze of the outside world, new research from the University of Utah suggests that the reality is far more complex. The study highlights that indoor air quality is not a static condition but a dynamic one, dictated by a building’s age, its proximity to pollution sources, and, perhaps most critically, the specific design of its heating, ventilation, and air conditioning (HVAC) systems.
The research, conducted by an interdisciplinary team at the University of Utah, utilized the Salt Lake City campus as a sprawling "living laboratory." By combining the expertise of the College of Engineering, the Department of Facilities Management, and occupational and environmental health researchers, the team sought to understand the invisible exchange between the outdoor atmosphere and indoor spaces. Their findings offer a nuanced look at how different types of particulate matter—ranging from the heavy dust of desert storms to the microscopic soot of wildfires—interact with commercial ventilation technologies.
While the study concluded that indoor air quality across the campus was generally high, it identified a significant vulnerability during wildfire season. The research demonstrated that certain energy-saving HVAC configurations can inadvertently pull fine particulate pollution, known as PM2.5, into buildings, even while successfully blocking other types of pollutants like winter inversion particles and wind-driven dust.
The Campus as a Living Laboratory
The decision to use the university campus for the study provided the researchers with a unique opportunity to observe a wide array of building styles and mechanical systems in a controlled yet real-world setting. The University of Utah’s main campus is a microcosm of urban architecture, featuring everything from historic structures with older ventilation setups to state-of-the-art facilities equipped with modern energy-efficiency technologies.
"Our buildings are big and they’re complicated, and oftentimes they’ve been added onto and integrated with different kinds of systems," said Kerry Kelly, a professor of chemical engineering who oversaw the research. Kelly noted that while the complexity of these systems makes management a significant challenge, the data gathered by the team suggests that the issues identified are fundamentally solvable. The study suggests that through a combination of mechanical adjustments and simple interventions, such as the use of portable air filters, the risks posed by outdoor pollutants can be mitigated.
The project relied on a dense network of low-cost, wirelessly connected sensors. These devices were deployed across 17 indoor locations and two outdoor stations to capture a comprehensive picture of air movement. The goal was to characterize the behavior of indoor air quality during three specific types of high-pollution events common to the Intermountain West: dust storms, winter inversions, and wildfire season. Each of these events presents a different chemical and physical profile of PM2.5, and they occur at different times of the year, providing a robust dataset for the researchers to analyze.
Tristalee Mangin, a graduate student in chemical engineering and the study’s lead author, explained the methodology used to track the infiltration of these particles. "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 exactly which systems were most susceptible to outside air quality fluctuations.
How Smoke Differs from Dust and Inversions
One of the most striking revelations of the study was the disparity in how different types of pollution enter a building. The researchers found that wildfire smoke resulted in four to five times more PM2.5 infiltration into buildings compared to pollution originating from winter inversions or wind-driven dust events.
This discrepancy is largely rooted in the physical and chemical properties of the particles themselves. According to the findings, outdoor pollution from wind-blown dust did not have a significant impact on indoor air quality, regardless of the HVAC system in place. Dust particles are typically larger and heavier than those found in smoke. Because of their mass, they are more likely to settle out of the air before entering a building’s intake or are easily trapped by standard mechanical filters.
Winter inversions—a common meteorological phenomenon in Salt Lake City where cold air is trapped near the ground by a layer of warmer air—presented a different scenario. Inversion-related pollution in Utah is primarily composed of ammonium nitrate, which accounts for roughly 60% of the particulate matter during these events.
"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." This phase change means that while the outdoor air may look thick with haze, the actual particulate load inside a heated building may be significantly lower because the ammonium nitrate has transitioned from a solid particle into a gas.
Wildfire smoke, however, behaves differently. Smoke particles are incredibly fine and do not undergo the same phase changes as inversion particles when they move indoors. Because they are so small, they can bypass less efficient filters and remain suspended in the indoor air for longer periods.
The Role of Air-Side Economizers
The study identified a specific culprit in the infiltration of wildfire smoke: the air-side economizer. This technology is a staple of modern commercial HVAC systems designed to maximize energy efficiency. Using a series of specialized ducts and dampers, an economizer draws in outdoor air when the external temperature and humidity are favorable—such as on a cool summer or fall morning—to cool the building without relying on energy-intensive mechanical refrigeration.
While this is an effective strategy for reducing a building’s carbon footprint and operational costs, it creates a direct pathway for outdoor pollutants. If the air is thick with wildfire smoke on a morning when the economizer decides to "open the windows," the system can pull a significant volume of particulate matter into the building. Professor Kelly noted that once these fine particles are inside, some are small enough to pass through the system’s filtration units, leading to elevated indoor PM2.5 levels.
To gather this data, the team utilized small, innovative sensors designed by Pierre-Emmanuel Gaillardon, a professor of electrical and computer engineering. These devices were commercialized through a University of Utah startup called TELLUS. At a cost of just $450 per unit, these sensors are a fraction of the price of compliance-grade monitoring equipment, allowing the researchers to deploy a much larger number of devices across the campus. The sensors were strategically installed on walls at breathing height to ensure the data reflected the actual air quality experienced by the buildings’ occupants.
Research Continues to Find Solutions
While the initial paper is based on an 18-month study window that concluded in April 2024, the project is far from over. The monitors remain active across the campus, continuing to feed data to researchers. The work has been supported by Seed2Soil and Student Collective Investment Fund (SCIF) grants, with information about the project posted near the devices to engage the campus community.
The findings are already being used to help campus building managers develop more sophisticated operational strategies. The ultimate goal is to maintain high indoor air quality without sacrificing energy efficiency, particularly during the increasingly frequent wildfire seasons. However, implementing a "one-size-fits-all" solution is nearly impossible given the scale of the university’s infrastructure.
Sean Nielson, an engineer with Facilities Management’s Sustainability and Energy group and a co-author of the study, emphasized the complexity of managing the campus’s approximately 18 million square feet of building space. "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."
The solution is not as simple as merely closing the dampers on smoky days. Building codes 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). "Finding the ideal balance between outside air and recirculated inside air is difficult due to the high number of variables," Nielson added.
One potential tool for improvement is the application of higher-rated MERV (Minimum Efficiency Reporting Value) air filters. While these filters are more effective at capturing fine particles, they can also restrict airflow and put additional strain on older HVAC fans. Nielson noted that there are practical limitations to how these filters can be applied to existing equipment and what specific contaminants they can effectively remove.
As the research continues, the team hopes to provide a clearer roadmap for building managers, not just at the University of Utah but for large institutions across the country. By understanding the specific ways in which different pollutants interact with mechanical systems, they aim to create indoor environments that remain true sanctuaries, regardless of the air quality outside.
