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The radon risk of a specific building could be estimated fairly reliable by a good knowledge of the geogenic radon potential of the building ground and the house characteristics, whereas the radon hazard of a region is more difficult to achieve. In Germany, an interpolated soil gas radon map in the scale of 1:1.000.000 offers a nationwide planning instrument to identify areas where high radon concentrations in the ground are to be expected. The mapped geological units can be regarded as proxies standing for a complex series of subsoil geochemical and physical properties including the assigned radon activity concentration in soil gas. However, the size of a grid element (3 km 3 km) and the scale-determined generalization of geology with solely 22 geological units do not admit the use of this summary map for local predictions. Since the interpolation algorithm is guided by the areal distribution of geological units, the number of supporting measurement points and involved units significantly depends on scale.
To minimize the efforts and costs of house builders and communities, a top-down strategy was developed, based on the generalization of geological units for different scales. For federal states a scale of 1:100.000 and for communities a scale of 1:50.00 or 1:25.000 are recommended. The interpolation procedure and the generalization technique are discussed, giving examples of radon prediction maps for different scales. Using geological information to deduce a geogenic radon potential for planning purposes in different scales is especially appropriate, when - like in large-area states like Germany - only a relatively small part of the total house stock has been measured for indoor radon.
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