The high values in Sweden (and partly perhaps Finland) was partly(there are also geological reasons) due to a popular popular brand of concrete (called Blue Concrete colloquially) being made with Alum Shale that happened to contain a large amount of Uranium (and thus the radiation/decay will eventually produce Radon gas).

This combined with Sweden being a fairly cold country (with people keeping their windows shut) was a recipe for disaster with the further combined effect of smoking and radon gas leading to quite a few fatalities.

These days multi-family housings are regularly inspected to make sure that Radon gas amounts aren't too high (even if the required limits are fairly high in an international context).