2020 No https://pubmed.ncbi.nlm.nih.gov/32080817/ Prokopciuk, Nina Europe Experimental Elementary (K-5) Primary schools Indoor/outdoor aerosol Pollution levels Systematic sources Occasional sources Ventilation The main sources responsible for the elevated levels of indoor PN and PM aerosol concentrations were determined: local canteens in the absence of ventilation (particle concentrations up to 97,500 part/cm3 (CPC)), the school-grader activity during the lesson breaks (275–586 μg/m3), soft furniture and carpets in the classrooms and corridors (~ 200 μg/m3), and in some cases (city center) elevated outdoor aerosol pollution levels (16800–18,170 part/cm3). Elevated aerosol pollution levels were also due to the occasional sources: construction works during lessons (200–1000 μg/m3), scraping the exterior walls of buildings near schools (up to 1600 μg/m3), and the use of petrol-powered trimmers during cutting of green plantings (22500–66,400 part/cm3 (CPC)).