Post-Occupancy Effective Dilution Rate and Occupied-Period CO2 Exposure in Day-Care Classrooms Based on Long-Term Monitoring and Decay Analysis

Day-care classrooms are sensitive to indoor environments where indoor air quality (IAQ) depends on ventilation, occupancy density, and daily operation. This study evaluated classroom post-occupancy effective dilution rate and occupied-period CO2 exposure in a day-care centre (DCC) in southern Spain under real-world operating conditions. Four classrooms were continuously monitored from January to July 2025 using indoor and outdoor CO2 sensors. Effective dilution rate was estimated from post-occupancy CO2 decay events, while exposure was characterised through cumulative distributions, percentile metrics, exceedance duration, and daily profiles. Results revealed a persistently low post-occupancy effective dilution rate, with values between 0.04 and 0.24 h−1, and a decay-derived equivalent airflow indicator below 1 L s−1 per average child. CO2 exposure varied considerably between classrooms. The most critical rooms reached the 95th percentile (P95) concentrations of 1400–1720 ppm and exceeded 1000 ppm during about 50% of occupied time, while the least affected classroom remained mostly below this threshold. Although a poorer post-occupancy effective dilution rate was generally associated with higher exposure, occupancy density, room conditions, and operational patterns were also associated with differences in indoor CO2 levels. The study highlights that IAQ assessment in day-care classrooms should combine post-occupancy effective dilution rate indicators with exposure-based metrics to identify critical rooms and guide targeted improvement strategies.