Indoor air quality in naturally ventilated classrooms and offices in Ireland

• Reduced ventilation patterns associated with prioritising thermal comfort. • Indoor PM 2.5 exceeded guidelines on 48 % of monitored days, correlated with outdoor PM 2.5 . • Distinct temporal variations in pollution levels, particularly PM 2.5 , CO 2 and TVOCs. • Intra-building variation patterns occasionally exceeded inter-building differences. • Measured higher formaldehyde concentrations compared to previous studies. Indoor air quality in naturally ventilated offices and schools has significant implications for occupant health, well-being, and performance. This study conducted comprehensive multi-zone IAQ monitoring across 17 buildings - 8 office buildings, and 9 schools (51 environments in total) in Ireland, examining zonal and temporal variations in key parameters. Real-time monitoring of PM 2.5 , CO 2 , temperature, relative humidity, and total volatile organic compounds was complemented by passive sampling of formaldehyde, BTEX, pinene, limonene, and NO 2 . Results revealed distinct patterns in offices and schools, with CO 2 concentrations in classrooms (median 1319 ppm) significantly exceeding office levels (median 714 ppm) during occupied periods. PM 2.5 and NO 2 concentrations were dependent on outdoor concentrations. PM 2.5 showed a strong correlation with outdoor levels ( r = 0.75) and 48 % of monitored days exceeded WHO 24-hour guidelines. Median formaldehyde concentrations of 19 μg/m 3 and 20 μg/m 3 in offices and classrooms respectively were notably higher than previous studies. In contrast, limonene was detected in 76 % of schools compared to only 16 % of offices. Substantial variation was observed both between and within buildings, with some pollutants showing greater intra-building than inter-building variability. Both indoor and outdoor temperatures emerged as key factors influencing ventilation behaviour, particularly in classrooms where lower temperatures were associated with higher CO 2 concentrations. The results highlight the ongoing challenges in balancing thermal comfort, ventilation and IAQ. Overall, these findings offer insights to guide policy development in naturally ventilated offices and schools, enabling evidence-based policy strategies to mitigate indoor air pollution exposure and enhance occupant health and well-being.