Radon is the most significant source of exposure to ionizing radiation from natural sources, and it is the major cause of lung cancer worldwide after smoking. The Directive 2013/59/Euratom mandates European Union member states to establish radon action plans and reference levels for indoor radon concen-trations, and this implies measuring indoor radon both at workplaces and dwell-ings, in particular in areas classified as “radon priority areas”. This study was con-ducted within an area classified with a high susceptibility index of mainland Portugal. Radon concentrations were measured at 65 workplaces of 16 facilities and 16 dwellings of workers from the facilities. The radon concentration values were exceeded in 73% of the workplaces (>300 Bq/m3), and in 34.5% of the cases, the workers should be classified as Category A (dose > 6 mSv/y). The annual ef-fective dose limit (20 mSv/year) was exceeded in 7% of cases. Gamma dose rates were generally low. Differences in radon levels were observed between workplac-es and dwellings, although the distance between the facilities is under 11 km, as well as within different floors of the facilities (higher at the ground floor). Correc-tive measures (e.g. air renewal, ventilation, depressurization) should be imple-mented to reduce the radon concentration below the reference level and limit ex-posure to less than 6 mSv/year.