The paper shows the results of measurements of carbon dioxide concentrations conducted in the school buildings located in two different climates: Bialystok (Poland) and Belmez/Cordoba (Spain). CO2 concentration in first 45 min met requirements of regulations, but with medium occupation of places in classrooms, then the recommended values were excided. We developed a model for CO2 level estimation. This model allowed us to simulated the CO2 concentration for full occupation of classrooms in both countries. In these cases CO2 concentration would exceed maximum value (1000 ppm) during the first hour. Furthermore required air change rates were calculated: for actual occupation they should be in a range 2.5-5 h(-1) to maintain IDA2 and carbon dioxide concentration below 1000 ppm, while in case of full occupation even 6-9 h(-1). Finally, surveys of student satisfaction were performed in situ during the measurements of CO2 level to obtain the dependence of student comfort perception on this concentration. (C) 2016 Elsevier B.V. All rights reserved.