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and inside homes (> 2,000 Bq/m3). For this reason, we felt the need to deepen our knowledge on the radon present in the Etnean area, focusing in particular on indoor radon pollution that, as widely recognized, is among the main causes of cancer largely (but not exclusively) of the respiratory system. Firstly, since 2005 we made a broad surface survey that revealed very high radon emissions from soils near active faults on Etna. Typical background soil activity on Etna were <1,000 Bq/m3, whereas in areas of stronger soil degassing, activity values up to ~60,000 Bq/m3 were measured. Furthermore, since late 2015 we have performed continuous indoor radon monitoring inside seven houses, some of which located close to degassing faults on the eastern, southern and south-western flanks of the volcano. Indoor radon concentration varied according to the season of the year, but above all, they changed according to the geology and tectonic setting of the substratum of the monitored houses. In one case, indoor radon concentration reached 3,549 Bq/m3 and remained > 1,000 Bq/m3 for several consecutive months, highlighting a potential health problem for those living in such environments. In other cases, the construction features of the houses and/or the materials used seemed to play an important role in the mitigation of indoor radon accumulation, even in the presence of intensely degassing soils. These preliminary data demonstrate the need to deepen the studies, extending indoor radon measurements to other urban areas, in order to monitor the health hazard for the Etna population, amounting to about one million people.
Introduction
Radon (222Rn) is a short-lived decay product of uranium (238U). It is a noble gas, invisible, odorless, tasteless, and chemically inert. Radon is widespread in the Earth's crust, with activity concentrations ranging many orders of magnitude, chiefly depending on the original uranium content, on the physical characteristics of the rocks and on the ways of its motion through the rocks and soils. It is eight times heavier than air and this fact, together with its short half-life (3.8 days), limits its diffusivity to nearly 2 m in soil and 2 mm in water (1, 2). Therefore, high activity concentration in radon emissions at the topographic surface is produced by convective flow of gases that facilitate the transport of radon from greater depth within soils.
Faults and fractures of the Earth's crust are the easiest paths for radon to move through the rocks and get to the surface because these zones are generally characterized by a higher porosity than surrounding rocks. Indeed, faults are likely the locations of high soil degassing and elevated radon activities (3–7).
In recent years, many in-soil radon measurements were undertaken on Mt. Etna volcano (Italy), one of the most active volcanoes in the world (8) (Figure 1). The typical background level of in soil radon activity concentration on Etna was found to be <1,000 Bq/m3 (9–11), whereas in areas of stronger soil degassing radon activity was up to 60,000 Bq/m3 (12). Anomalous soil degassing is particularly strong in the east and southwest flanks of the volcano, that are characterized by continuous tectonic deformations and gravitational collapses (9, 13–15). The collapsing flanks are bordered by numerous active faults (10, 16–19), sometimes revealed from radon in-soil investigations (11, 12, 20–22), and many of these faults cross urban areas (Figure 1).