Effects of classroom ventilation management and user activities on indoor air quality in Madrid primary schools: A two-year field study

Indoor air quality (IAQ) in schools is influenced by occupancy, ventilation management, classroom activities, indoor sources, and outdoor conditions. This study examined CO 2 , PM 10 , and total volatile organic compound (TVOC) concentrations in nine classrooms from four primary schools in the Madrid region over two consecutive years under real operating conditions. In 2024, usual ventilation practices were documented, whereas in 2025 a structured, seasonally adapted ventilation protocol was implemented in one classroom in each of three schools. Pollutant distributions and their associations with recorded window- and door-opening conditions, occupancy, and pollutant-specific classroom activities were analysed using frequency distributions, boxplots, and selected temporal profiles. CO 2 showed the clearest association with occupancy and air renewal, accumulating rapidly during occupied periods with limited ventilation and decreasing during unoccupied or ventilated intervals. Structured ventilation management was generally associated with lower CO 2 concentrations during scheduled opening periods, although high concentrations still occurred during some long or densely occupied teaching sessions. TVOC and PM 10 showed more heterogeneous responses. Short-term TVOC increases frequently coincided with potentially emitting materials and cleaning activities, whereas PM 10 increases were commonly associated with physical movement and material handling. Associations with ventilation were less consistent for these pollutants. The findings show that CO 2, TVOC, and PM 10 provide complementary information and that low CO 2 does not necessarily indicate low concentrations of other indoor pollutants. Structured but adaptable ventilation should therefore be combined with source control, activity-aware ventilation, and direct observation of classroom operation.