Ventilation Inadequacy Revealed by Indoor CO2 Dynamics in A Densely Occupied University Classroom Under Controlled and Real-Use Conditions

Abstract

Wang Han Sheng

Indoor air quality (IAQ) in higher-education environments has received increasing attention due to its implications for occupant health, comfort, and cognitive performance. Carbon dioxide (CO2) is widely used as a practical indicator of ventilation adequacy and occupant-related pollutant accumulation. However, empirical evidence quantifying ventilation performance under both controlled and real teaching conditions in air-conditioned university classrooms remains limited, particularly in subtropical regions.

In this study, continuous CO2 monitoring was conducted in a university classroom under three controlled experiments with fixed occupancy and five regular teaching sessions with high occupant density. A mass-balance modeling approach was applied to quantify air exchange rates and human CO2 emission factors. Under controlled conditions, indoor CO2 concentrations increased from background levels to 610–720 ppm, with estimated air exchange rates ranging from 1.60 to 1.79 h−1. During regular teaching sessions, CO2 concentrations frequently exceeded the regulatory threshold of 1,000 ppm, reaching peak values of 1,000–1,450 ppm, despite continuous air-conditioning operation. Although air exchange rates during real classroom use were higher than those observed under controlled conditions, they remained insufficient to offset the substantially increased emission load associated with dense occupancy.

These results demonstrate a systematic ventilation inadequacy in a typical air-conditioned university classroom, where thermal comfort does not necessarily translate into adequate outdoor air supply. The findings highlight the importance of CO2-based ventilation assessment and management strategies for improving IAQ in higher-education learning environments.

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