Using Size-Resolved PM Measurements to Determine Fine and Ultrafine Particle Infiltration and Filtration Patterns in Older Educational Buildings

JIANING BAO, Nigel Kaye, Ehsan Mousavi, Christopher Post, Andrew Metcalf, Clemson University

     Abstract Number: 476
     Working Group: Reducing Aerosol Exposure with Control Technologies and Interventions

Abstract
Older educational buildings often lack centralized mechanical ventilation and rely on decentralized HVAC systems, such as fan-coil units. This configuration may increase occupant vulnerability to poor indoor air quality (IAQ) due to limited ventilation and filtration capabilities. Fine and ultrafine particles, although contributing little to overall particulate matter (PM) mass, are the most abundant by number in both indoor and outdoor environments. Their small size allows deep penetration and deposition within the respiratory tract, posing significant health concerns.

This study evaluates the effectiveness of an older classroom equipped with fan-coil units as its ventilation system in mitigating occupant exposure to fine and ultrafine particles. Specifically, we examine the size-resolved PM infiltration and filtration characteristics of the indoor environment. Particle number concentrations across various size ranges are measured indoors and outdoors using a Scanning Electrical Mobility Spectrometer (SEMS). To quantify indoor particle dynamics, controlled particle releases are conducted while the classroom is unoccupied. Additionally, CO₂ decay tests are performed to determine the room’s air exchange rates. An aerosol size-resolved box model based on the mass balance approach is used to analyze particle losses across size bins. The model incorporates air exchange, particle deposition, filtration, and penetration processes to derive size-resolved infiltration factors and identify shifts in PM size distributions resulting from HVAC operation changes or the addition of mitigation devices, such as HEPA filters and Corsi-Rosenthal box fans. Furthermore, the model simulates real-world pollution scenarios, including wildfire smoke infiltration and indoor particle generation from occupant activities, to assess indoor exposure risks and evaluate potential mitigation strategies. Results from this study provide practical insights into how building design, operational adjustments, and targeted interventions can effectively manage occupant exposure to fine and ultrafine particles in older educational buildings.