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Aerosol particle number (PNC) and mass concentrations (PMC) were studied in 11 primary schools during the 2017-2018 school years (from September to May) in Vilnius, Lithuania, with the aim to evaluate the main aerosol pollution sources and its levels. Expeditious information on the total aerosol particle concentration over the full range of sizes (from 0.01 to > 1 μm) was estimated using a condensation particle counter (CPC). Particle number and mass concentrations in the size range of 0.3-10 μm were measured and estimated using an optical particle sizer (OPS). The use of aerosol particle size spectra (OPS) in school lodgements facilitated the identification of the main sources of indoor air pollution. 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/cm 3 (CPC)), the school-grader activity during the lesson breaks (275-586 μg/m 3 ), soft furniture and carpets in the classrooms and corridors (~ 200 μg/m 3 ), and in some cases (city center) elevated outdoor aerosol pollution levels (16800-18,170 part/cm 3 ). Elevated aerosol pollution levels were also due to the occasional sources: construction works during lessons (200-1000 μg/m 3 ), scraping the exterior walls of buildings near schools (up to 1600 μg/m 3 ), and the use of petrol-powered trimmers during cutting of green plantings (22500-66,400 part/cm 3 (CPC)).