People spend most of their time indoors (80-90%), which are mainly schools, homes, and workplaces. 1 Thus, investigating the air quality in such environments is of utmost importance. Ultrafine particles (UFPs) (particles smaller than 100 nm) were reported to be present in different indoor environments including homes, schools, offices, and aged care facilities, 2 which could be due to the presence of different indoor sources such as heating, cooking, tobacco smoking, candle burning, hair ironing, copying machines, printers. [3][4][5] These sources are linked to different indoor activities, equipment usage, and household property. 1,6 Outdoor sources contributing to UFPs are mostly traffic and industries. 7 Outdoor UFPs can penetrate indoors and play a major role in exposure to indoor UFPs if a major source is not present indoors. 2 Several studies were conducted to examine the air quality in the primary schools and schools in different countries including Portugal, South Korea, and the USA [8][9][10][11][12] ). Nevertheless, few investigations have been done in preschools to study the exposure of the occupants to fine, UFPs and black carbon. Simultaneous
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