Introduction: Indoor air quality depends on chemical, biological and physical pollutants. Several different pathogen microorganisms can be transmitted through the air. As a significant pollutant black carbon can contribute to climate change and human health. High concentration of carbon dioxide (CO2) in indoor air can cause malaise, headaches, reduced ability to concentrate and can affect the academic achievement of pupils. Purpose: The aim of our study was to evaluate appearance of microorganisms (total aerobic microbial count, fungi, Staphylococcus sp. and Enterobacteriaceae sp.), aerosol black carbon and CO2 in indoor and outdoor air of kindergartens in Zasavje region. Methods: Microbiological samples were taken with MAS-100 air sampler. The temperature, the relative humidity, the number of people in the room and ventilation types as well as concentration of black carbon (Aethalometer TM) and carbon dioxide (Testo 535) were measured simultaneously with microbiological tests. Samples were collected during all four seasons. Results: The highest mean concentration of total aerobic microbial count in indoor and outdoor air was in summer (1510 cfu m-3), the lowest mean concentration was in autumn (740 cfu m-3). The highest mean concentration of fungi in indoor and outdoor air was in summer (1510 cfu m-3), the lowest mean concentration was in winter (230 cfu m-3). Cladosporium, Penicillium and Aspergillus were the most common detected fungi in indoor and outdoor air. The highest mean concentration of black carbon in indoor air (859 ng m-3) as well as in outdoor air (981 ng m-3) was in winter. The highest measured concentration of CO2 in indoor air was 2570 ppm. The highest mean concentration in indoor air (1716 ppm) was in spring. Discussion and conclusion: The mean concentration of each species of microorganisms in indoor and outdoor air differ from season to season. Because people are the source of Staphylococcus sp. and Enterobacteriaceae sp., concentrations of these microorganisms were higher in indoor compared to outdoor air. Fungal sources, which could contributed to higher concentrations in indoor air, were not present in the rooms. Concentrations of black carbon in air were lower during weekends compared to week days. All measured concentrations of CO2 in indoor air were lower than regulated 3000 ppm limit by legal standard.