Thesis title: Building materials as a source of indoor radon exposure
The population exposure to indoor radon is a leading cause of lung cancer (EPA 2003, WHO 2010). established to be a Group 1 and Group A human carcinogen, according to the classification used by the International Agency for Research on Cancer (IARC 2012). In particular, exposure to the decay products of radon (used in brief to refer to the isotope 222 of Rn) and thoron (used to refer to the isotope 220 of Rn) represents, on average, approximately half of the overall effective dose from natural sources suffered per year by the global population, i.e. 2.4 mSv year−1 (UNSCEAR 2008). The UNSCEAR Report from 2008 reports effective doses due to radon and thoron inhalation of 1.15 mSv year−1 and 0.1 mSv year−1, respectively. Due to the relatively low outdoor concentration, most of the exposure to radon occurs indoor; strong evidence of the association existing between the exposure to radon at home and lung cancer has been reported in literature (Lubin, Wang et al. 2004, Darby, Hill et al. 2005, Krewski, Lubin et al. 2005).
Radon is a naturally occurring radioactive noble gas. In nature, there are three radon isotopes: radon-222, radon-220 and radon-219 generally referred to as radon, thoron, and actinon respectively. The most abundant is the radon-222 which has a decay half-life of 3.823 days (Chisté and Bé 2007). This isotope is produced by the alpha decay of radium-226 (decay half-life of 1600 years) and it belongs to the natural decay chains of uranium-238 (decay half-life of 4.5 billion years), one of the so-called “primordial radionuclides” present in undisturbed Earth’s crust since its formation. It is worth noting that radon is the only gaseous element of uranium-238 decay chain, and so it is able to exhale from the Earth’s crust to outdoor, accumulating in closed spaces. As the uranium-238 is present everywhere in Earth’s crust – with a mean concentration of 3 ppm worldwide – radon gas exposure poses a potential risk to every place on the planet.
Radon can enter indoors spaces from three different sources:
i. directly from soil due to radium-containing rocks still in the crust;
ii. indirectly from crustal materials no longer incorporated in crust but contained in building materials;
iii. via radon-carriers utilities such as air, water and natural gas;
The relative contribution of each of these entry patterns obviously depends on specific circumstances, i.e. building characteristics (including building materials, construction typology and floor level), morphology and composition of the underlying soil, ventilation features, occupancy patterns and living habits of occupants. A review of radon sources, transport mechanisms and influencing parameters has been carried out in Chapter 1. The results are the homogenization of the current theoretical knowledges about radon generation and the standardization of both parameters and assumptions influencing the modelling. A large review has been carried out to report the typical values of most of the parameters regulating or influencing the radon generation. Finally, a systematic and comprehensive description of radon transport in porous media by considering several contributions of different authors has been provided.
Chapter 2 specifically deals with building material contribution to indoor radon activity concentration. Council Directive 2013/59/EURATOM ( European Commission 2014) requires Member States to consider any source of radon ingress – including building materials – when preparing the national action plan to address long-term risks from radon exposure. In particular, Member States are explicitly required to identify (and/or develop) strategy, including methods and tools, to identify building materials with significant radon exhalation rate.
Pertaining to building materials, several measurements of radon exhalation rate have been reported on literature through the years (references are available in the text), almost all of them on building material’s samples. A systematic collection of radon exhalation rate measurements carried out in Europe on about 20000 samples of structural and superficial building materials has been recently published (Nuccetelli, Risica et al. 2017) and updated (Trevisi, Leonardi et al. 2018). Generally these measurements have been carried out through the so-called “dynamic” or “accumulation method”, firstly proposed by IAEA Technical Report 474 (Ishimori, Lange et al. 2013). The data analysis highlighted some critical issues, such as the use of different units to express radon exhalation rate, the use of different measurement techniques and the general lack of information about density and thickness of samples. In many cases these differences of type and amount of information make difficult a reliable comparison of the obtained data. In the light of these considerations, a protocol for measuring the radon exhalation rate from building material’s sample has been proposed in Chapter 2.1. Moreover, the impact of thoron interference on radon exhalation rate measurements has been experimentally assessed in the same Chapter, developing different configurations of the “dynamic” or “accumulation method” differing from radon detectors, mode of operation, tube length and filters.
However, as firstly highlighted by Sahoo, Sapra et al. (2011), the difference between radon exhalation from samples and that from walls has not been investigated enough. As a result of this lack of knowledge, measurements of radon exhalation from building materials samples have been commonly used to assess the effective dose attributable to radon exhaled from walls in indoor environment (e.g., Stoulos, Manolopoulou et al. 2003, Mahur, Kumar et al. 2008, Shweikani and Raja 2009, Ujic, Celikovic et al. 2010, Kumar, Chauhan et al. 2014, Saad, Al-Awami et al. 2014) even though different geometries and boundary conditions characterize the two scenarios. If the one-dimensional (1-D) geometry better describes the radon exhalation from a building element, mainly walls, it is not suitable to the radon exhalation from a sample of building material, the latter better modelled as a three dimensional (3-D) phenomenon.
Currently, three main possibilities exist to provide reliable values of radon exhalation rate from a wall.
i. The first possibility is the mathematical modelling. This approach is the only feasible to design the interventions to prevent the radon accumulation indoors. In section §2.3, a systematic review of the models describing the radon migration is reported, and the corresponding solutions were derived for any reasonable boundary conditions sets. However, the formulations derived to assess the radon exhalation rate from building structures depends on environmental conditions and building characteristics and they might require a wide set input parameters as well as significant computational capabilities. In the same section, the numerical impact of adopting the simplified formulation – instead of the exact ones – to assess the radon exhalation rate was quantified by varying the main building structure parameters.
ii. The second possibility is to estimate the radon surface exhalation rate from a wall as related to that from a building material sample. This approach has been proposed by Sahoo, Sapra et al. (2011) and recalled by Orabi (2018). It relies on a mathematical model specifically developed to correlate the two measurements by combining the 3-D model of the radon flux from the building material sample and the 1-D model used to assess radon exhalation from a wall made up by the same material. The 1-D model considered to assess radon exhalation from a wall was derived by Jonassen and McLaughlin (1980) under simplified assumptions, essentially it is valid only when advective contribution is negligible and both wall surfaces are free to exhale into a radon free space. These assumptions are not suitable to describe every scenario occurring in dwellings, but they must be considered when using Sahoo’s model.
iii. The third possibility to provide reliable values of radon exhalation rate from a wall is in-situ measuring radon exhalation rate directly from the wall surface, i.e. through the so-called accumulation method described by ISO 11665-7 (International Organization for Standardization 2012a). To the author knowledge, no specific apparatus has been developed and documented in literature due to the method's inner difficulties, mainly ensuring the airtightness and, the repeatability of the measurements. An innovative apparatus has been designed, constructed, and commissioned at the “Laboratory of Radioactivity of the Italian National Institute of Health” to measure the radon exhalation rate in-situ from vertical surfaces of existing building walls. It is described in section §2.2. The apparatus is non-destructive, completely self-standing, and easily transportable. The sealing mechanism has been demonstrated to assure airtightness similarly to destructive setups. The apparatus introduced is the only available solution to measure the site-dependent contribution of building materials to the indoor radon concentration. The apparatus presented has been successfully used to measure the radon exhalation rate from walls made of Italian Tuffs and to assess the impact of superficial finishing on the radon exhalation rate from a wall.
Once collected radon exhalation rate measurements on Italian tuff samples – through “dynamic” or “accumulation method” – and directly on-site from walls made of the same building material – through SIREN apparatus, the two measurements have been compared; the radon exhalation rate from the sample resulted to significantly underestimate the actual radon exhalation rate occurring from a wall. Moreover, the model proposed by Sahoo was used to estimate the radon exhalation from the wall using the measurement on the sample as input, the result is an overestimation of radon exhalation rate from the wall for two building materials considered, while for a third one – characterized by a very high radon exhalation rate – the model resulted accurate. Finally, the measurements with SIREN apparatus on the walls were compared with the results of mathematical models applied to the case-specific scenario; the models reported a radon exhalation rate value of the same order of magnitude of the actual one measured with SIREN apparatus, proving to be useful tool to assess contribution of building materials to indoor radon activity concentration. In section §2.4 a review of the models proposed in literature to assess the indoor radon activity concentration from all the sources has been reported.
Chapter 4 was focused on building materials’ contribution to outdoor gamma radiation, especially in urban areas. The data of the first national survey on outdoor gamma radiation in urban areas were analysed and the impact of building materials to external gamma dose rate in urban environments was assessed.