Does outdoor air pollution affect indoor air quality? It could depend on buildings’ HVAC

We typically spend 80% of our time indoors, where the quality of the air we breathe depends on the age and type of building we occupy, as well as indoor pollution and outdoor pollution sources. But also playing an important role is the kind of HVAC system used to heat, ventilate and cool the building, according to new research from the University of Utah.

Using the Salt Lake City campus as a living laboratory, the College of Engineering teamed up with Facilities Management and occupational and environmental health researchers to explore whether and how outdoor air pollution affects indoor air quality. They found that indoor air quality on campus was generally good.  However, depending on a building’s HVAC system, fine particulate pollution, or PM2.5, from wildfire smoke can infiltrate buildings, while pollution associated with dust events and winter inversions is kept out.

The study found the issue lies with commercial HVAC systems that use air-side economizers. Using special duct and damper arrangements, this technology reduces energy use by drawing air from outdoors when temperature and humidity levels are optimal, such as cool summer and fall mornings. This helps with energy efficiency, but if the air is smoky that day, the system could pull in particulate pollution and some particles make it past the filters, according to chemical engineering professor Kerry Kelly, who is overseeing the research.

Campus as laboratory

“Our buildings are big and they’re complicated, and oftentimes they’ve been added onto and integrated with different kinds of systems,” Kelly said. “So 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.”

The ongoing project deployed low-cost wirelessly connected sensors in 17 indoor and two outdoor locations in an effort to characterize what happens with indoor air quality when particulate pollution is elevated during dust storms, winter inversions and wildfire season—which present different kinds of PM2.5 and occur at different times of the year.

“We look at the ratio of the indoor particulate matter measurements to that of the outdoor particulate matter measurements. The closer that value gets to one, which means more of the particulate matter is going to be sourced from outdoors versus indoor sources,” said study leader Tristalee Mangin, a graduate student in chemical engineering. “We looked at those ratios and then did analyses based on the different groupings of HVAC types.”

How smoke differs from dust and inversions

Wildfire smoke had four to five times more PM2.5 infiltration into buildings than pollution from inversions and wind-driven dust events. However, exceedances of international health guidelines only occurred in buildings ventilated with air-side economizers and were still very rare.

The team used small devices designed by Pierre-Emmanuel Gaillardon, professor of electrical and computer engineering, and commercialized through a U startup called TELLUS. The cost per device was just $450, far less than what compliance-grade equipment would cost. They were installed on walls at breathing height in a variety of buildings spread mostly around the main campus.

According to their findings, outdoor pollution arising from wind-blown dust and inversions did not have much effect on indoor air quality, regardless of the HVAC system. This is likely the result of the nature of the PM2.5 associated with those pollution events.

Dust particles are typically larger and heavier than smoke particles, so they tend to fall out of the air and get caught in filters. Inversion particles are a more complicated story.

“It has to do with the thermodynamics of the particles that are in the inversion. Utah’s particulate matter is primarily ammonium nitrate, like 60% of our particles,” Kelly said. “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.”

Research continues to find solutions

This paper is based on an 18-month study window that ended in April 2024, but the research is ongoing and the monitors remain active and visible. A statement about the research and how it’s funded by a Seed2Soil and SCIF grants is posted next to the devices.

Besides shedding light on HVAC systems’ strengths and weaknesses for keeping out pollution, the findings are expected to help campus building managers craft operational adjustments to maintain good indoor air quality, particularly during wildfire season. However, the campus’s approximately 18 million square feet of building space exhibits great variety that defies a uniform approach, according to co-author Sean Nielson, an engineer with Facilities Management’s Sustainability and Energy group.

“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.”

It will not be a simple matter of just closing dampers on smoky days, but rather adjusting them based on the specific needs of particular buildings.

“You still have code minimum requirements for a certain amount of outside air that must be provided,” Nielson said. “A primary reason for outside air is to dilute and flush out indoor contaminants. Finding the ideal balance between outside air and recirculated inside air is difficult due to the high number of variables.  Applying higher-rated MERV air filtration is one tool that often works to improve air quality for many applications, but there are limitations as to what contaminants can effectively be filtered and how they can practically be applied to existing equipment.

This ongoing research project stems from a collaboration between the KairLab, Facilities Management and the Spencer Fox Eccles School of Medicine. Co-authors include Zachary Barrett, Zachary Palmer and Darrah Sleeth of the U’s Division of Occupational and Environmental Health. These results are to be published in the June 15 edition of Building and the Environment under the title, “Understanding the effect of outdoor pollution episodes and HVAC type on indoor air quality,” and are available now online. Additional support for this research came from the U’s Global Change & Sustainability Center, the Sustainable Campus Initiative Fund, or SCIF, and the Wilkes Center for Climate Science & Policy.