New Study Examines HVAC System Cleaning and Classroom Indoor Air Quality
Every school day begins with the same expectation: that students have a healthy place to learn and educators have a healthy place to work. While much of the conversation around indoor air quality has focused on ventilation and filtration, new research is expanding on another important piece of the puzzle: the condition of the HVAC system itself and the role HVAC system cleaning can play in creating healthier indoor environments.
A recent demonstration study, “Effects of HVAC Cleaning on Indoor Air Quality: A Multi-Season Demonstration of Exposure Assessment in Occupied Public School,” examined indoor air quality conditions in four public elementary schools in the Denver metropolitan area before and after HVAC system cleaning. The study was authored by Nasim Ildiri and Mark Hernandez of the Department of Civil, Environmental and Architectural Engineering at the University of Colorado Boulder.
What the Study Measured
Researchers monitored classroom indoor air quality during occupied school hours using sensors that collected data at one-minute intervals. The study focused primarily on particulate matter, including:
PM2.5, fine particles that can travel deeper into the respiratory system; and PM10, larger inhalable particles often associated with dust, debris, and other airborne materials.
The study compared classrooms served by cleaned systems, with classrooms served by uncleaned units in schools with similar architectural features, occupancy, and grade levels.
What Was Cleaned
The HVAC cleaning was performed by a NADCA Member Company with ASCS-certified professionals, in accordance with ACR, The NADCA Standard.
The work included cleaning air-handling unit components, fans, coil housings, internal surfaces, coils, supply and return ductwork, registers, vents, grilles, and diffusers. The process also included containment measures, negative pressure, HEPA-rated equipment, and other procedures designed to control particulate matter during cleaning. To maintain consistent filtration conditions for comparison, new clean filters of the same size and MERV rating were installed on both cleaned and uncleaned systems.
This distinction is important. The study evaluated standards-based HVAC system cleaning, not a limited or superficial cleaning of visible components.
Key Findings
In the month following HVAC system cleaning, researchers observed measurable reductions in airborne particulate matter during occupied school hours.
Across the four schools, median PM2.5 concentrations were reduced by 36% to 46% after cleaning. PM10 concentrations also decreased, with reductions ranging from 27% to 54%.
The study also looked at longer-term results across the heating and cooling seasons. In schools where researchers were able to compare cleaned and uncleaned classroom groups over time, classrooms connected to cleaned air-handling units showed significantly lower daily mean PM2.5 concentrations during school hours.
Why It Matters
For NADCA members, this demonstration study adds to the growing body of research connecting HVAC system hygiene with measurable building performance and indoor air quality outcomes.
The findings suggest that proper HVAC system cleaning, in accordance with ACR, The NADCA Standard, can help reduce occupant exposure to airborne particulate matter in school environments.
This study provides valuable field-based evidence from real occupied classrooms. Conducted in four elementary schools within a single metropolitan area, it adds meaningful real-world data to the growing body of research on HVAC system cleaning and indoor air quality. As with any field study, the findings should be considered within the context of the study design while recognizing the important insights they provide under actual operating conditions.
For schools, facility teams, engineers, and building owners, the study reinforces the importance of looking at HVAC system cleaning as part of a broader indoor air quality and building hygiene strategy.
For NADCA members, the study provides another credible resource to support accurate, evidence-based conversations about the value of standards-based HVAC cleaning.