The paper is focused on the investigation of the Indoor Air Quality (IAQ) in two classrooms from a thermal renovated school from Bucharest, related to indoor carbon dioxide (CO 2), ozone (O 3) concentrations or airborne particles (PM2. 5), by using experimental data. The experimental campaign has been conducted over a long period (February-June 2022), allowing the study of indoor air quality under different weather data conditions. The results showed that the use of classroom ventilation is mandatory in order to reach a satisfactory degree of thermal comfort and indoor air quality and even the natural ventilation by windows opening remains a simply and useful tool to lower the indoor pollutant concentrations. However, if the target is to ensure permanent good indoor air quality in these learning spaces, a mechanical ventilation system is more adapted than the natural ventilation, due to the fact it is independent of the weather factors variation (outside temperature and wind velocity and direction).