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Article Highlights• Diurnal variation of PM, CO 2 and NO 2 in classroom • PM, CO 2 and NO 2 concentrations during heating and non-heating period • Strong correlation between PM and carbon dioxide CO 2 concentrations • Management strategy for controlling of PM and CO 2 levels inside the schools is needed Abstract Indoor air quality (IAQ) is very important for children health and well-being, since children are particularly vulnerable and sensitive to the presence of air pollutants. This study was performed in two naturally ventilated schools located in the same municipality. The first school is located in an urban area, at a residential-industrial site, while the other school is situated in a rural area. School buildings were chosen based on their urban environment features. The measurements were carried out in heating and non-heating periods in duration of five consecutive working days. The objective of the study was to analyze IAQ in the classrooms with special emphasis on levels and diurnal variations of particulate matter (PM 10 and PM 2.5 ), carbon dioxide (CO 2 ) and nitrogen dioxide (NO 2 ) in occupied and unoccupied school classrooms. In this paper, the CO 2 concentrations were measured at both indoor and outdoor environments. Concentrations of CO 2 higher than 1000 ppm were regularly detected in the classrooms during teaching hours. Indoor concentrations of PM 10 were not exceeded the guideline, daily average, value of 50 µg/m 3 . Concentrations of PM 2.5 exceeded the guideline daily average value of 25 µg/m 3 in both school during heating period. Concentrations of NO 2 did not exceed the guideline value of 200 µg/m 3 . Ventilation rates were calculated and compared with the prescribed limits. In both occupied and unoccupied periods high correlation between CO 2 and PM concentrations was determined.