This article deals with the issue of the indoor environment in terms of optimal conditions of thermal comfort and the satisfactory quality of indoor air. A weekly measurement of the parameters of the indoor environment was carried out in the building of the day rehabilitation centre in order to verify the state of the indoor environment, to determine the processes that create the indoor environment and to define the factors that affect the resulting state of the indoor microclimate. One of the important factors is air quality, where the main factor that influences the indoor environment is the concentration of carbon dioxide, whose value affects the biological functions of the human organism. The analysis of the measured parameters found that the concentration of carbon dioxide in the monitored indoor space is not, as expected, the riskiest factor that may be the cause of dissatisfaction with the indoor microclimate.