Evaluating the effectiveness of photocatalytic paints in reducing indoor pollen concentrations in schools.

Primary school children in Europe spend six to nine h a day in classrooms, making indoor air quality a critical factor for their health and well-being. This study evaluated the effectiveness of photocatalytic paints in reducing indoor pollen concentrations in classrooms at three primary schools located in different urban and peri-urban areas of Madrid (Spain) during 2023 and 2024. Indoor and outdoor pollen types were characterized, and potential pollen sources were analyzed based on surrounding land use and vegetation. Linear mixed-effects models were used to assess the effects of location, campaign, week, sampling shift and photocatalytic treatment. A total of 45 pollen types were identified outdoors, 30 of which were also detected indoors, indicating a high permeability of school classrooms to airborne pollen. Location, campaign and sampling shift significantly influenced pollen concentrations in both years (all p < 0.001). A significant treatment × period interaction was detected (F = 7.09, p = 0.008). However, this effect was not retained after normalization by outdoor pollen concentrations using indoor/outdoor ratios (F = 2.24, p = 0.136), indicating that outdoor pollen loads remained a major determinant of indoor exposure. An exploratory comparison between land-use and airborne pollen dendrograms revealed identical clustering patterns among schools, suggesting a potential influence of surrounding vegetation on airborne pollen assemblages. These findings highlight the dominant influence of outdoor pollen dynamics on classroom environments and emphasize the importance of incorporating aerobiological information into school indoor air quality management. Since pollen concentrations were consistently higher during the afternoon than during the early morning sampling period, scheduling ventilation during the morning may help reduce indoor pollen exposure, although its effectiveness is likely to vary according to local environmental conditions and pollen dynamics.