Effects of HVAC Cleaning on Indoor Air Quality: A Multi-Season Demonstration of Exposure Assessment in Occupied Public School

In response to recent updates in ventilation performance guidelines, modern building hygiene practices have received increased attention. The Centers for Disease Control and professional architectural engineering societies (e.g., ASHRAE) have issued new recommendations for minimum air exchange rates, including guidance targeted to educational settings and other high occupancy environments. The primary drivers for increased air exchange are to reduce occupant exposures to respirable particulate matter (PM) and carbon dioxide (CO2). However, increased air exchange requires sustained mechanical energy input over baseline conditions. In this context, alternative engineering interventions - including building hygiene practices - have shown potential to improve ventilation effectiveness, while also reducing energy consumption. Recent research has reported ventilation performance benefits associated with systemic HVAC system cleaning in commercial office and educational environments, yet concomitant reductions in PM exposure potential associated with these practices have not been systematically studied. In response, this demonstration study observed patterns of inhalable and respirable PM levels before and after certified HVAC cleaning in public school classrooms through an academic year, during both heating and cooling modes. Indoor air quality (IAQ) parameters from classrooms connected to cleaned air-handling units (AHU) were compared in temporal parallel with classrooms connected to uncleaned AHUs within schools containing similar architectural features, occupancy, and grade levels. As judged by continuous real-time air quality monitoring (1-minute resolution), results from this pilot study (four schools) suggest that certified HVAC cleaning can significantly reduce exposures to both coarse and fine PM concomitant with improved ventilation performance, during typical classroom occupation periods.