ORIGINAL ARTICLE Indoor radon mapping and its relation to geology in Hungary Miha ´ly Minda Æ Gyo ¨rgy To ´th Æ Istva ´n Horva ´th Æ Ivan Barnet Æ Krisztia ´n Ha ´mori Æ Eszter To ´th Received: 5 February 2008 / Accepted: 7 April 2008 / Published online: 26 April 2008 Ó Springer-Verlag 2008 Abstract Indoor radon mapping may show stronger dependence on geological formations if the measured homes are one-storied houses with no basement. In Hun- gary, 17,244 homes were investigated on the yearly average of indoor radon concentrations; among these homes, there were 6,154, one-storied, no-basement houses. In Hungary, 21 geological units were created relevant for indoor radon index characterized by lithology, the position of the ground water table, and the gas permeability. Maps were drawn of different topography (counties, grid, geo- logical units) and different values (maximum, mean, indoor radon indexes). A kind of standardization of houses was that only the one-storied, no-basement ones were chosen, but from geological point of view some more information was gained when the wall materials (bricks or adobe) were also taken into account. (‘‘Adobe’’ is made of clay and straw in Hungary, and not burned as brick, just dried on sunshine). Enhanced indoor radon values can be observed on the bedrock of Cenozoic volcanic rocks and their eroded materials deposited on the local alluvial valleys. Another group with relatively increased indoor radon values can be connected to granite bodies. The grid method is useful for covering large state or even continental areas. For practical public use and detailed radon risk mapping geological or administrative unit-systems could yield more reasonable and useful results. Keywords Indoor radon  House-structure  Geology  Hungary Introduction The Joint Research Center (JRC) of the European Union intends to map Europe concerning the indoor radon, using the measured yearly averages of radon activity concentra- tions. This action gives rise to the problem of factors influencing indoor radon levels. It was recognized almost at the beginning of radon surveys that the highest radon levels are mainly caused by geological formations under the houses (Samuelsson 1989; Ha ´mori et al. 2006; Keller et al. 1992; Mose et al. 1992). But some parameters of the house structure also can influence the radon levels (Arvela 1995; Gunby et al. 1993; Levesque et al. 1997). In the upper level flats of many-storied buildings the radon levels may exceed a few Bq/m 3 if the building materials contain radium in a concentration larger than the average (Somlai et al. 1998, 2005). The radon levels of rooms on the ground floor can be influenced in different degrees in different countries. For example, Gunby et al. (1993) took into account a number of factors of house structure, but not the existence or lack of basement, while Kemski et al. (2006) in Germany found that the role of the basement under the measured room was found to be 15%. In Hungary, ground floor rooms with no basement under them were grouped whether they are in one-storied houses M. Minda (&)  K. Ha ´mori  E. To ´th RAD Laboratory, Boronkay, Kossuth te ´r 1, Va ´c, Hungary e-mail: minda.mihaly@gmail.com G. To ´th  I. Horva ´th Geological Institute of Hungary, Stefa ´nia krt. 14, Budapest, Hungary I. Barnet Czech Geological Survey, Geologicka ´ 6, 152 00 Prague 5, Czech Republic 123 Environ Geol (2009) 57:601–609 DOI 10.1007/s00254-008-1329-6