The radioactivity is due to the local geology. Underground water dissolves radium in uraniferous igneous rock and carries it to the surface through at least nine known hot springs. These are used as spas by locals and tourists. Some of the radium precipitates into travertine, a form of limestone, and the rest diffuses into the soil, where it is absorbed by crops and mixes with drinking water. Residents have unknowingly used the radioactive limestone as a building material for their homes. The stone irradiates the inhabitants and generates radon gas which is usually seen to promote lung cancer. Crops contribute 72 µSv/yr to a critical group of 50 residents.