For over ten years STUK (The Radiation and Nuclear Safety Authority, Finland) has performedsystematic indoor radon mapping with municipal health authorities. In the most radon-proneprovinces (Uusimaa, Kymi and Hme) there are about 480,000 low-rise dwellings. It is expectedthat in 51,000 homes the action level of 400 Bq/m3 is exceeded, of which about 6,000 have beendetected. In the rest of Finland the numbers are: 820,000 low-rise dwellings, 17,000 expectedcases of exceeding the limit, of which 800 were detected, respectively.