The research was carried out in a school located in the province of Lublin, built in the 1980s, before and after thermal modernization. During the measurement period, the temperature ranged between 20.8–28.5°C, and humidity – 18–55%. The measurement of the CO2 concentration allowed determining the class of the room. In all cases, the CO2 concentration exceeded 1000 ppm after only 20 minutes of the first class; therefore, the rooms were qualified to the 3rd and 4th class rooms (IDA). During the measurement period, the CO2 concentration in the outside air was 500 ppm for winter and 350 ppm for summer. The dominant bacteria and molds isolated from the tested air samples are: Bacillus lentus, Bacillus licheniformis, Bacillus pumilus, Bacillus cereus, Pseudomonas stut-zeri, Micrococcus ssp., Staphylococcus xylosus, Staphylococcus saprophyticus, Staphylococillus haemillium, Aspergilus acridonium, Aspergilus haemillium Alternaria, Cladosporium, Epicocum, Mucor, Penicillinium. The data showed that the bacteria and fungi levels varied between seasons, with the non-heating season concentrations being higher than those in winter. The bacteria concentrations varied in the range from 1727 to 3670 CFU/m3, while the fungi levels varied from 64 to 489 CFU/m3. The indoor bacteria levels were between two and three times higher than the outdoor levels, while the I/O ratio for fungi was <1.