Indoor Radon in School Buildings in Northern Portugal: Insights from Active and Passive Monitoring

Indoor radon is a key indoor air pollutant and a major contributor to population exposure to natural ionizing radiation, with relevance in school buildings where children and staff spend extended periods indoors. This study investigated indoor radon concentrations in two public schools located in a radon-prone area of northern Portugal using a combined short- and long-term monitoring strategy. In the first phase, active RadonEye detectors were deployed for continuous winter measurements in four ground-floor rooms across two schools, with three rooms located in School S1 and one in School S2, providing hourly data to characterize temporal variability. In the second phase, CR-39 passive detectors were installed in the same spaces to obtain integrated long-term concentrations. Active monitoring revealed pronounced spatial and temporal variability, with mean concentrations ranging from 140 to 228 Bq/m3 and peak values reaching 586 Bq/m3. Radon levels generally declined during occupied periods and increased at night and on weekends, indicating the importance of ventilation and occupancy patterns in shaping indoor radon dynamics. Passive measurements ranged from 290 to 300 Bq/m3 in school S1 and from 84 Bq/m3 to 300 Bq/m3 in school S2, confirming higher long-term concentrations in S1. Although most measurements remained below the reference level of 300 Bq/m3, several rooms approached or temporarily exceeded this threshold. These findings highlight the value of combining active and passive techniques to improve radon assessment in school environments and support targeted ventilation and mitigation strategies to reduce exposure and improve indoor air quality.