The aim of master thesis was to investigate the effect of ventilation rates on concetration of carbon dioxide in the indoor environment, especially in educational institutions and offices. We wanted to determine whether the required quantities of fresh air, as provided by Slovenian law, are sufficient to ensure the concentration of carbon dioxide in the limit values. For this purpose, simulations were performed with a software tool and the results were verified with measurements made in the test room. We also conducted a systematic review of studies on indoor air quality in educational institutions and offices on the impact of carbon dioxide concetrations in the indoor environment on productivity, health and absence. With the results of this master thesis we came to the conclusion that the provision of quality indoor air in absolutely necessary. Minimum prescibed ventilation rate 0,5 h-1 according to the Rules on the ventilation and air conditioning of buildings (O.J. RS, No. 42/2002, chang.) is not sufficient to ensure the quality of indoor air in the model space of the classroom and lecture room. For the calculation of required amount of outside air supply for the model space under consideration the recommended amount of outside air 30 m3/h per person should be used. Similar findings have been made in kindergarten and elsewhere. This recommended value ensures that carbon dioxide concetrations are within the limits that provide the least impact on cognitive performance, productivity and human health, as reported by epidemiological research. At the level of legal requirements, we propose to change the limit values for carbon dioxide concentrations in indoor air to lower values than currently prescribed, which is supported by the findings of other studies.