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.
