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It is known that the decay products of the U-238 are findable in nature in certain materials (such as tufa, limestone, etc.), therefore some populations are naturally exposed to radiations originated by building materials, houses and isotopes present in drinking-waters. It is evident that, for example, the residents of Campania or Lazio will be involved, on such conditions and during their whole life, in the action of natural radiations of decay products of Uranium and consequently of Radon, which is originated and, as gas, also findable in the air of those indoor areas, at a concentration inversely proportional to the relevant ventilation factors. Furthermore, it is known that this isotope is always found in thermal waters too and sometimes at levels which make them classify as radioactive and with therapeutic properties still subjects of studies and researches. Actually, presence of Radon in warm waters is particularly evident in some basins (the Baths of Badgastein in Austria) and is also findable in some italian thermal towns such as Merano Laurisia, Abano, Baths of Lucca or the island of Ischia itself. All underground waters and especially the warm ones, including, those of Ischia basins, hold radioactive isotopes, since their composition is related to the nature of the underground, which they come from, and is strongly influenced by the kind of rocks present in that area, by the granulometry of the geological strata or by existent fractures, by flow speed and finally by solubilization capacity. On such conditions it is evident that all waters, during their scouring, will be enriched by Radon 222 (a gaseous, therefore more soluble, element); just because of its physical state, it will be more present in external environment in case of warm waters such as those ones now examined. Consequently the environmental state from a radioecological point of view of the utilized waters has to be considered, as well as any other thermal resort, fundamentally due to Radon (Rn) and to the relevant decay products. As a matter of fact, whereas from the family of Uranium come Radon (Rn 222), Lead (Pb 210) and Polonium (Po 210), these last two, respectively with halving-times of 22.3 years and 138.4 days, from the family of Thorium (Th 232) originates Radon (Rn 220 in past times defined "Toron" according to its "forebear") and Lead (Pb 212 with a halving-time of 10.6 hours).(ABSTRACT TRUNCATED AT 400 WORDS)