**1. Introduction**

Numerous studies have shown that there is a clear correlation between indoor radon exposure and the risk of developing lung cancer [1,2]. Radon gas is considered to be the second leading cause of lung cancer after tobacco, and is responsible for between 3 and 14% of deaths caused by this disease in the first world [3,4] and the main source of ionizing radiation for the population [5–8].

The World Health Organization (WHO) recommended a reference level of 100 Bq/m3 annual average radon concentration to initiate action plans to minimize health hazards due to indoor radon exposure. However, if this level cannot be reached under the countryspecific conditions, the chosen reference level should not exceed 300 Bq/m3, which represents approximately 10 mSv per year [4].

The interest in radon exposure maps is because the concentration of radon in buildings varies according to their geographical location. This variability is due to a large number of factors that affect the presence of radon indoors. These maps will be a useful instrument for applying the requirements of European legislation [5], which must be implemented in member states at all administrative levels: national, regional and local, to the radon problem. An overview of indoor radon mapping in Europe [9,10] showed the heterogeneity of the data: each country used different sampling strategies, measurement techniques, and representations of the data obtained.

In 2013, the EU-BSS (European Basic Safety Standards) required the design and implementation of National Radon Action Plans in the member states to identify areas of risk for the presence of radon gas in homes and workplaces. It establishes that the indoor radon concentration level in homes and workplaces be set at 300 Bq/m3, and requires the

**Citation:** Fernández, A.; Sainz, C.; Celaya, S.; Quindós, L.; Rábago, D.; Fuente, I. A New Methodology for Defining Radon Priority Areas in Spain. *IJERPH* **2021**, *18*, 1352. https:// doi.org/10.3390/ijerph18031352

Academic Editors: Shinji Tokonami and Ikuo Kashiwakura Received: 17 December 2020 Accepted: 28 January 2021 Published: 2 February 2021

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radon priority areas (RPA) to be delimited. The BSS defines the RPA as an area where it is expected that in a significant number of houses the average annual radon concentration exceeds the national reference level [5].

The problem was that different interpretations of RPA were introduced in each country [11–17]. In Europe, both mapping methodologies and the definitions of RPAs are diverse [18–26].

The concept of radon potential, soil radon potential, or geogenic radon potential is used by the different member states to define and delineate the Radon Priority Areas. Projects currently under development, such as the EURAMET MetroRADON [27–29], or the European Atlas of Natural Radiation [30–35], try to homogenize concepts, mapping methodologies that permit a clear definition of these areas.

The Spanish Nuclear Safety Council (CSN) published a radon potential map in 2017 [36,37]. The concept used in Spain, and defined by the CSN to identify "radon priority areas", is that of radon potential. The CSN defines these areas using the 90th percentile to generate a cartography of the radon potential map of Spain. The variables used by CSN are the national 222Rn concentration database measurements in homes, geological information (lithostratigraphies), and exposure rates to terrestrial gamma radiation.

The CSN generated the radon potential map by combining these three variables: Radon measurements in homes were grouped by lithostratigraphic unit and level of exposure to gamma radiation, and units with homogeneous radon levels were obtained from these data groups. For these units, the 90th percentile (P90) of the radon concentration distribution was considered to be a limit with higher than 90% confidence, and the units were represented by rank in 5 categories based on radon levels from the use of the 90th percentile: P90 > 400 Bq/m3; P90 301–400 Bq/m3; P90 201–300 Bq/m3; P90 101–200 Bq/m3 and P90 < 100 Bq/m3. A radon concentration value is calculated using the 90th percentile, meaning that 90% of the values in an area are below that value, and 10% are above it. The CSN identified the areas of Spain where there are a significant percentage of homes with radon concentrations with a given probability of exceeding 300 Bq/m3 [36,37].

This paper sets out a mapping methodology that improves the definition of radon priority areas in Spain using the same variables used by the CSN (222Rn concentration measurements in homes, lithostratigraphies, and gamma radiation exposure data), but with a different approach. In addition, following the steps taken by the European Commission-Joint Research Centre (EC-JRC) in the production of the European Atlas of Natural Radiation and the European Radon Map [30–35], their reference coordinate system (the GISCO-LAEA projection) will be used and the 10 km × 10 km cell system to represent the data obtained. This cell system ensures the confidentiality of the radon samples taken in private homes and harmonizes the maps from the different countries.

#### **2. Materials and Methods**

#### *2.1. Input Data*

In order to produce a Spanish radon potential map with this new methodology, we used the following data sources:
