2019 No https://pubmed.ncbi.nlm.nih.gov/31159138/ Slezakova Europe Observational Epidemiology Study Students Elementary (K-5) Ventilation rates Ultrafine particles Indoor air quality Children Microenvironments Outdoor air Inhalation dose Overall, UFP concentrations showed large temporal and spatial variations. For classrooms (n = 73), median UFP (1.56 × 103–16.8 × 103 # cm−3) were lower than the corresponding levels in ambient air of schools (1.79 × 103–24.1 × 103 # cm−3). Outdoor emissions contributed to indoor UFP (indoor-to-outdoor ratios I/O of 0.0.30–0.85), but ventilation, room characteristics and its occupancy were identified as important parameters contributing to overall indoor UFP levels. Considering specific indoor school microenvironments, canteens were the microenvironment with the highest UFP levels (5.47 × 103–36.4 × 103 # cm−3), cooking conducted directly on school grounds resulted in significantly elevated UFP in the respective classrooms (p < 0.05); the lowest UFP were found in libraries (4.45 × 103–8.50 × 103 # cm−3) mostly due to the limited occupancies. Although students spend majority of their school time in classrooms (66–71%), classroom exposure was not consistently the predominant contributor to school total UFP inhalation dose (29–75%). Outdoor exposure contributed 23–70% of school dose (depending on UFP levels in ambient levels and/or conducted activities) whereas short periods of lunch break accounted for 8–40%.