Experimental assessment of the interaction between indoor air quality and thermal comfort in naturally ventilated secondary classrooms in southern Spain

Current European policies focus on achieving climate neutrality by 2050. However, the COVID-19 crisis has disrupted social conditions, reigniting the debate on buildings with high occupancy and static users for long periods, such as schools, given their inadequate health and comfort conditions. In the Mediterranean climate, most school buildings lack suitable ventilation systems, due to either their age or a reluctance to use mechanical ventilation systems. This study provides a quantitative analysis of current behavioural and environmental factors affecting pollutant exposure, covering the gap in the existing literature on simultaneous assessment on indoor air quality conditions (CO 2 , PM 2.5 , PM 10 ), and hygrothermal comfort (temperature and relative humidity) in a post-COVID scenario in existing secondary school buildings in southern Spain. For this purpose, a continuous monitoring of indoor environmental conditions in cooling, mild, and heating seasons is proposed to assess the influence of natural ventilation conditions on indoor air quality and thermal comfort, instead of the short-term monitoring focused on specific periods frequently found in previous studies. The results show a widespread use of natural overventilation through windows, especially in summer (more than 50 % of the occupied hours), to guarantee indoor air quality conditions (with CO 2 below 900 ppm during almost 100 % of the occupied hours). However, in general, this involves clearly compromising thermal conditions (with seasonal average values above 25ºC and 100% of the occupied hours in discomfort during the hottest weeks) and a moderate loss of cognitive performance during more than 97 % of the summer occupied hours. • Simultaneous assessment of indoor air quality and thermal comfort • Long-term monitoring of main indoor environmental and pollutant variables • An excess of hours of over-ventilation is detected, especially in summer • Indoor air quality is generally achieved but to the detriment of comfort conditions • Overheating and a moderate loss of cognitive performance are found in summer