Radiation Measurements 40 (2005) 509 – 512 www.elsevier.com/locate/radmeas Radon levels in the volcanic region of La Garrotxa, Spain C. Baixeras a , ∗ , J. Bach b , K. Amgarou a , V. Moreno a , Ll. Font a a Grup de Física de les Radiacions. Edifici Cc, Departament de Física, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain b Unitat de Geodinàmica Externa. Departament de Geologia. Edifici Cs, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain Received 27 August 2004; accepted 1 December 2004 Abstract A preliminary survey in the city of Olot, the main town of the volcanic region of La Garrotxa, showed that dwellings built on volcanic formations present higher indoor radon levels than dwellings on non-volcanic materials. The soil of the area is not especially rich in radium. However, some of the volcanic materials present very high permeability and therefore radon entering the houses might have travelled over long distances. In this paper we present indoor radon values measured in a larger survey carried out during April–July 2004. The influence of the volcanic materials found in the preliminary survey has been confirmed. The results obtained suggest the possibility that radon comes from the degassification of mantle through active faults. The values obtained in working places do not constitute a relevant radiological risk for workers. © 2005 Elsevier Ltd. All rights reserved. Keywords: Radon; Radon survey; Indoor radon; Working places; Volcanic materials 1. Introduction The unique characteristics of radon (radioactive noble gas) make it very attractive for earth science studies, as it can migrate over significant distances within the earth and the atmosphere, be measured with accuracy, and be used as a tracer for many environmental processes of interest. Many examples of applications on earth sciences can be found in Durrani and Ili´ c (1997). However, it is also well known that the inhalation of radon progeny indoors is the largest sin- gle source of radiation exposure to population (UNSCEAR, 2000). In Spain, according to the 96/29/EURATOM Direc- tive of the European Commission, the regulations of pro- tection against ionizing radiation (Real Decreto 783/2001, 2001) establish for the first time the need of monitoring those working places that might have high radiation levels from natural radionuclides. ∗ Corresponding author. Fax: +34 93 581 1508. E-mail address: carmen.baixeras@uab.es (C. Baixeras). 1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2004.12.024 In this context, in the period May–September 2002 we carried out a preliminary survey in the city of Olot, the cap- ital of the volcanic-origin region of La Garrotxa, located in the northeast of Spain. The geological structure of Olot con- sists on a folded Tertiary substratum over which seat recent volcanic formations (between 350,000 and 10,000 years) (Guérin et al., 1986) and sedimentary deposits. In general, volcanoes are made of pyroclastic deposits, and valleys are covered with lava flows. Somewhere the volcanic materials are alternated with lake deposits because lava flows caused the formation of natural dams and sediments filled them too. Pyroclastic materials have intergranular-origin permeability and lava flows have fissure-origin permeability whereas lake deposits and tertiary substratum have a small permeability. In this preliminary survey a total number of 21 dosimeters were exposed in both private houses and working places. The results obtained suggested a clear influence of the soil type on indoor radon values: higher concentrations were ob- tained in dwellings built on the volcanic materials. In this paper we present the results obtained in a larger survey that has been designed to improve the statistics and