In this study, the investigators tested whether portable air cleaners can improve the air quality in primary school classrooms. Poor air quality can affect children's respiratory health and learning. Classroom air quality is a key concern, as children spend a large part of their day there, often with many students in one room. The investigators studied air quality in 180 classrooms in 29 Dutch primary schools. Within each school, classrooms were grouped in threes based on similar characteristics, such as size, number of students, and ventilation system. Classrooms in each group were randomly assigned to receive either: an air cleaner with a filter that traps particles from the air, an air cleaner that uses an electric charge to remove particles from the air, or no air cleaner (control). All devices were tested for safety in a laboratory beforehand. Measurements were taken in all classrooms before the devices were switched on, to provide a refence point. The air cleaners were then operated for nine weeks. After that, air cleaners were switched off for two weeks while additional measurements continued. To measure the effect of air cleaners, special cloths were placed in classrooms to collect dust and tiny droplets from the air, which were later analyzed in a laboratory for bacteria and viruses. This included viruses that commonly cause respiratory infections, such as flu and respiratory syncytial virus (RSV), and bacteria originating from the human skin, nose, or mouth that can spread through the air. Small sensors also tracked tiny particles in the air (particulate matter). The investigators collected additional information on classroom attendance and parent-reported respiratory symptoms to explore possible links. This study provides real-world evidence on how portable air cleaners can affect classroom air quality and may help guide decisions to improve indoor air environments in schools.