Comparison of Field Approaches for Characterizing Classroom Ventilation Rates (VRs) and Equivalent VRs for PM

Ventilating classrooms with clean outdoor air is essential for reducing exposure to indoor-generated pollutants and maintaining good indoor air quality (IAQ). Routine assessments of ventilation rates (VRs) are necessary to ensure that classroom ventilation systems are operating as designed and delivering adequate outdoor air. Practical approaches available for evaluating classroom VRs include direct airflow measurements, controlled release of tracer gas or particles followed by decay measurement, and estimation based on CO2 concentrations during school hours. In addition, the concept of equivalent clean outdoor airflow rates, or "equivalent VRs" has gained importance, particularly for assessing the removal of fine particulate matter (PM2.5). "Equivalent VRs" represent the total amount of "clean" air delivered through any combination of central mechanical ventilation, natural ventilation, and air-cleaning devices such as HVAC filters and portable air cleaners (PACs). This paper presents a case study conducted in two classrooms at a K-8 school in Stockton, California, comparing VRs and "equivalent VRs" for PM2.5 using common field-based approaches. Results indicate that although the three approaches produced similar results in that they all showed lower VRs in the same classroom as compared to the other, the absolute VR estimates varied substantially across the different methods. These discrepancies highlight the need for caution in selecting a method and interpreting data, as the assumptions underlying each method, such as no leakage in HVAC ducts or sufficiently long stable occupancy periods during school hours, may not always be valid under real-world conditions. The controlled release of tracer gas (e.g., CO2) provided the most reliable VR estimates in this study because the classrooms met the required assumption for this method (i.e., well-mixed single zone) reasonably well. For PM2.5 removal, the "equivalent VRs" (estimated from tracer particle) were higher than the outdoor VRs (estimated with tracer gas), which suggests that air filters (i.e., MERV 13 filter and PAC) increased the total amount of "clean" air and further lowered indoor PM levels. Additionally, results indicate that the MERV 13 filter contributed more to PM2.5 reduction than the PAC in the mechanically ventilated classrooms that were studied.