Development of radon transport model in different types of dwellings to assess indoor activity concentration
Introduction
The healthiness of indoor environments is increasingly evaluated to try to safeguard the health of occupants both in the home and in the workplace. Radon is a natural radioactive gas and has been classified as a major natural pollutant in buildings and must be included in any assessment aimed at estimating indoor air quality (US EPA, 1988; ICRP, 1993; European Commission, 2013). Radon is present in all terrestrial environments in varying quantities in three isotopes; the most abundant is the 222Rn having a decay half-life of 3.823 days. This isotope, produced by 226Ra, belongs to the natural decay chains of 238U, which is one of the main elements constituent the Earth's crust (Ambrosino et al., 2020e; Malanca et al., 1991). The 222Rn (later called Radon) has been studied extensively over the past few decades for (i) its use as an environmental tracer (see for example more recent papers Ambrosino et al., 2018a, 2019a, 2020b; Sabbarese et al., 2017b, 2020), and (ii) the hazards associated with its indoor concentration in human living environment (Ambrosino et al., 2018b; Bochicchio et al., 2009; Kumar and Chauhan, 2014; Sabbarese et al., 2017a). In a built environment, Radon originates mainly from soil through cracks in the ground and building substructure by diffusion and advection and, secondly, from building material and in small quantity from the air and water used in the home (Ambrosino et al., 2020d; Szajerski and Zimny, 2020). The Radon contribution exhaled from building materials can be even higher, when NORM (Naturally Occurring Radioactive Materials) are used for production of dwellings (Leonardi et al., 2018). Radon emission from soil, and in general from a medium, is influenced by several factors: porosity, granulometry, permeability, relative humidity and temperature (De Martino et al., 1997, 1998; Monnin and Seidel 1997; Morawska and Philips, 1993; Semkov and Parekh, 1990; Thu et al., 2020). Understanding the characteristics of soil and building material enables estimation of the indoor Radon concentration in dwellings and associated human health risks (Ajayi et al., 2018). Radon is well-known to be the main cause of lung cancer just after cigarette smoking (Frutos et al., 2019). The residential exposure risk due to the Radon in dwellings can considerably increase for indoor workers or people with sedentary activity (Ambrosino et al., 2020f; Müller et al., 2016). The recent updates of EU Basic Safety Standards in 2013/59/EURATOM directive, established the risk related to Radon inside buildings and radioactivity of building materials: the reference levels for Radon indoor air has been established in 300 Bq/m3 (European Commission, 2013; Frutos et al., 2019). Nowadays it is more and more common practice to require appropriate certificates confirming safe Radon levels in buildings (Szajerski and Zimny, 2020). The continuously growing awareness of Radon risk among scientists and general population results in elaboration of new active systems for reduction of Radon activity concentration in buildings (Ambrosino et al., 2020c; Hung et al., 2018).
In this framework, the goal of the present work is the assessment of the Radon activity concentration in different types of dwelling studying the soil gas entry rates into buildings. Most of the papers in literature show a lot of measurements performed with this aim (Guo et al., 2004; Kumar and Chauhan, 2014; La Verde et al., 2018b; Righi and Bruzzi, 2006; Stanley et al., 2019). The importance of the present work is being able to carry out evaluations of indoor Radon before making extensive measurements, also to make a possible assessment of areas and/or buildings with significative risk of presenting high levels of Radon. This goal is poorly treated in literature, and only very few works deal partially with it, studying only selected problem connected with the simulation of Radon flux from fractured rocks, specific soils composition, insulation materials (Ajayi et al., 2018; Riley et al., 1999; Savović et al., 2011a, b; Skubacz et al., 2019; Szajerski and Zimny, 2020). There is absence of papers that compare different types of buildings to evaluate a relative Radon risk index depending on the soil type and building materials. The modeling technique implemented to simulate the Radon entry rates into buildings is the finite difference method (FDM) (Nikezić et al., 2008; Savović et al., 2011a) for predicting Radon flux considering the mechanisms responsible for gas-phase contaminant transport. The studied model does not concers any air exchange physics within the building or between indoors and outdoors but only considers diffusion/advection between floor materials and the ground in order to obtain a reference maximum level of possible activity concentration. With mechanism for air exchange between indoors and the outdoors, indoor radon concentrations can be reduced effectively up to ~40–50% (Syuryavin et al., 2020).
Section snippets
Background
The basic concept in measuring Radon concentration involves observations of Radon through the porous medium being studied (Fournier et al., 2005). This medium separates two chambers that have different Radon concentrations. In such configurations, Radon moves from one chamber to another, primarily in one direction only (Gauthier et al., 1999; Sasaki et al., 2006). The problem of Radon transport can be treated using a plane sheet model (Savović et al., 2011a, b). In such conditions, the Radon
Research approach
The aim of the present study is to simulate the increase in activity concentration of indoor Radon in different types of dwelling, following the equation governing the Radon production and transport into the soil and its entry into buildings (§2.1). Fig. 1a shows the different types of buildings analyzed, coming from Italian campaigns of Radon monitoring carried out in the ‘80-‘90s (Sciocchetti et al., 1985), and, then, highlighted in the ‘National Radon Plan’ by Italian public institution for
Results and discussion
The Radon activity concentration at each floor of the different types of dwelling (0, I, II, III, IV) is computed. The solutions of the system composed by equations (2), (4) and the boundary conditions (8) are of the type of equations (5), (6), with different parameters D, ε, k, μ and S according to the medium that Radon passes through. For brevity, they are not reported. The numerical solutions are reported. A constant 222Rn activity concentration at the input side, from 6 m depth of ground
Conclusions
The present work aims to evaluate the Radon activity concentration within the different floors of a building. The growing awareness of the impact of Radon on human health has increased the demand to know the amount of radon with which one lives in a confined environment. Support for a pre-assessment is provided on the basis of the type of building, the underlying soil and building materials. Radon gas, naturally produced in the Earth's crust, migrates within the rock mass by diffusion and/or
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors are grateful to the University of Campania “Luigi Vanvitelli” for the financial support provided through the VALERE PLUS 2018 PROGRAM.
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