The main factors affecting indoor radon concentrations (that is, construction of the building's foundation, type of ventilation used, and pressure differences between indoor and outdoor air) have been studied in this work. Construction in a hillside and mechanical exhaust ventilation, as well as location in the radon-prone areas of Finland increase the indoor radon concentration. The pressure differences and ventilation rates did not affect the indoor radon concentration statistically significantly. In most of the buildings investigated, the radon exhaled from the construction materials proved to be only a minor source of radon. Monitoring with the integrated method may overestimate the occupational exposure to radon. If the radon concentration observed in the initial integrated measurement is high, more accurate continued measurements are recommended. This stydy demonstrated that the need for radon surveillance and mitigation is out utmost importance especially in workplaces in the radon-prone areas of Finland. Mitigation is even more important if the workers live in a similar or higher risk area in which their workplaces are located.