Indoor and rock 222rn levels in Israel - an integrated approach to radon risk assessment in the south-east mediterranean countries

Analysis of a comprehensive database containing all available indoor radon measurements which were carried out in Israel between 1989 and 1994 (total of 31,842 assays) coupled with Geographical Information System (GIS) capabilities and geophysical data, shows a significant correlation between indoor radon levels and its levels in the underlying bedrock. This correlation enables the characterization of radon-prone areas from two complementary viewpoints, and allows obtaining a statistically well founded estimation for the radon levels which the populations in such radon-prone areas are exposed to. The indoor data were grouped according to zip codes into distinct municipalities; only those in which there were at least 20 measurements were taken into account for further handling (total of 119 inhabited regions). A log-normal distribution has been fitted for every set of data, calculating the expected percentage of measurements exceeding 200 Bq/m3 (the action level for Israel) for each municipality. Based on alpha-track measurements of radon in rock pore-gas, all rock units in Israel were divided into three categories, according to their potential for radon emanation: less than 10,000 Bq/m3; 10,000-50,000 Bq/m3 and higher than 50,000 Bq/m3. Almost all rock units which exhibit high radon potential belong to the Mount Scopus Group - chalks, limestones and cherts of Senonian age, containing appreciable amounts of phosphorites with high uranium contents. These rocks are exposed over approximately 20% of Israels area and are typical to all southern and eastern Mediterranean regions, with some heavily populated cities built on them. Figure 1 demonstrates the indoor data for 119 municipalities on a background of bedrock radon potential emanations. Some non-parametric statistical tests were executed on the data (i.e.: joint frequency distribution via 2X2 crosstabulation; Wald-Wolfowitz runs test), in order to find out whether indoor radon concentrations measured in dwellings residing on bedrocks belonging to different radon-potential categories could be derived from the same population. The results show a significant distinction between two populations: the average radon level in buildings located on the Mount Scopus Group (or on areas in which these rocks are covered by a thin layer of alluvium) is 79 Bq/m3, compared to 50 Bq/m3 for all other regions. Estimating a 75/25% ratio for personal indoor/outdoor stay, these values correspond to exposure levels of 61 Bq/m3 and 40 Bq/m3, respectively. These finding may suggest an increase in lung cancer cases due to radon progeny for residents of radon-prone areas in the southeast Mediterranean region.