Since its very first moments on the Earth, Mankind has been continuously exposed to ionizing radiations from environmental radioactivity, consisting in three kinds of contribution: cosmogenic, primordial and anthropogenic. Among them, the one produced by naturally occurring primordial sources provides the largest percentage to the human exposure. The most important primordial source is constituted by radionuclides, such as 40K and the uranium and thorium decay families (238U, 232Th and 235U series). Many of the intermediate radioisotopes in these chains are metals and chemically reactive. For this reason, once formed by the predecessor's decay, they remain confined in the rocks, where they were generated, unless dispersed into the environment due to the erosion mechanisms made by weathering processes.

The only exception is represented by radon, the heaviest chemical noble element, which occurs in the gaseous state at Standard Temperature and Pressure (STP) conditions and naturally does not create chemical bonds. It tends, therefore, to migrate within the rock materials in the soil, where it has been formed (emanation process), and transported in the near-surface soils by fluid carriers (water, air, CO2, CH4), through advective and diffusive displacements, favoured by the soil mechanical characteristics, (porosity, structure and particle size) and the environmental conditions reaching the atmosphere (exhalation process) or spatial locations distant from the source (Hosoda at el., 2012; Stavitskaya et al., 2019).

The most abundant naturally occurring radon isotope is 222Rn (half-life of 3.82 days) and, also from the human health protection point of view the most relevant together with its short-lived alpha emitting progeny. In fact, regarding its relevance as hazard for humans, this radionuclides easily bind to the aerosol present in the indoor air, and, once inhaled, deposit on the bronchial tree, where emitting alpha ionizing radiation turns to be harmful for basal cells of the bronchial epithelium, critical targets for any cancerous changes (Ismail and Jaafar, 2011).

Inhalation of radon and its decay products contributes to about the 50 % of the individual annual dose due to natural radioactivity and is considered the most important cause of lung cancer, after smoking (WHO, 2009). Because at the moment there is no known threshold value below which radon exposure carries “zero Risk” of lung cancer to humans, there is a general interest and concern to minimize the exposure of the population to levels as low as possible.

Besides its relevance as a harmful hazard for people health, radon has such peculiar physical features that make it extremely useful in environmental and earth science (Baskaran, 2016). Radon assessment has been used like one additional tool (seismic precursor) to register the possible occurrence of seismic events by means of its possibly occurring activity concentration anomalies (Buttafuoco et al., 2007; Iovine et al., 2018), associated to land movements due to volcanic eruptions and occurring in geothermal areas; in fact, gases can be released can be generated and pressurized by hot regions such as volcanos and stress can be generated by the build-up of strains that precede earthquakes and volcanic eruptions (De Lauro et al., 2009; Falanga et al., 2019).

The main exposure to high radon activity concentrations generally occurs in basement or/and poor ventilated environments, where, once exhalated in the air from a source (soil is the most relevant one), radon can accumulate reaching levels harmful for the occupants. Building materials play a secondary role and in some cases water and gas supplies represent a further non-negligible source. Outdoor radon level is in the range of 5–15 Bq/m3 range, due to the effectiveness of mixing processes in the atmosphere (Nero, 1989; Nero and Nazaroff, 1984, 1988, UNSCEAR, 2000).

But, even in presence of a relevant source of radon production, the total indoor activity concentration mainly depends on the ability of the gas to penetrate inside a closed environment. Radon migration depends on two independent processes: the advection mechanism driven by a pressure gradient and the molecular diffusion, due to gas concentration gradient. The parameters affecting the transport process are essentially the diffusion coefficient and the permeability of the medium. Instead, the entry into the building depends not only on the transport phenomenon but also on the interface properties between the soil, where the transport occurs and the internal environment. Therefore, the accumulation inside the building is the result of the balance between the entry rate and the gas removal rapidity (essentially related to the ventilation rate and the radioactive decay).

So, if the relevant influence of the soil on radon concentrations detectable in buildings is more than evident, it is also clear that building materials (influenced by the radium content, structure and water content) also can play a strategic role due to the exhalation phenomenon from them. It is estimated, in fact, that for a single standard house in masonry and in a temperate climate, the radon coming from the soil beneath the basement, constitutes 60 % of the total, while the diffusion from building materials and the infiltration of radon with the extract air, can play a not negligible role (superior to 20 % and 18 % respectively); other indoor radon sources like water and natural gas supplies turn out to be relatively less important.

For such reasons radon activity concentrations are expected to decrease on the upper floors where the largest contributions is expected to be from building materials or, sometimes, from particular pathways of air circulation (stairwell, elevator, cables...) inside the building which starting from the ground can reach the upper floors (chimney effect).

All this assumed, the relative contributions of the various sources of radon and parameters affecting the processes are strongly variable from one building to another. Nevertheless, it could be a good starting point to analyse all data collected in a specific area to identify firstly the peculiar geological features of the local area and then categorize the buildings, according the traditional local system, in order to recognize the more exposed type to the problem of the radon accumulation (Mancini et al., 2014a).

The interest to identify buildings vulnerable to radon accumulation, because of its impact on people health, is, also, moved by the requirements of the European Directive 2013/59/EURATOM, which states all the sources of radon entry into buildings have to be considered and taken into account under a point of view truly interdisciplinary. In the same spirit expressed by the Directive, the Joint Research Centre (JRC) of the European Commission has been working to the project of a European Atlas of Natural Radiation. The Atlas is a collection of maps of Europe displaying the levels of natural radioactivity caused by different sources such as indoor radon, cosmic radiation, terrestrial gamma radiation, terrestrial radionuclides in soil and bedrock (Tollefsen et al., 2017; Ciotoli et al., 2017; Watson et al., 2017; Bossew et al., 2020).

For such reasons an Interdisciplinary Strategic Programme of Applied Research for the Assessment of Naturally Occurring Radioactive Materials in Natural and Anthropic Environments, called RAD_CAMPANIA, with particular attention to radon, has been started in Italy at the University of Salerno, in collaboration with the interUniversity Centre for Applied Research on the Prediction and Prevention of Major Hazards (CUGRI), and the Regional Agency for the Environmental Protection of Campania (ARPA Campania) (Guida et al., 2013; Cuomo et al., 2015; Mancini et al., 2018).

This 10 years work, so far, has shown that the environmental monitoring in workplaces or housings, can, no more, be limited to integrated measurements, but must be necessarily capable to identify which is the main source (soil, building material, water or natural gas) responsible for the accumulation of indoor Radon. This is needed in order to design and perform the most appropriate mitigation remedies, in new and existing buildings, reducing the maximum levels to the reference limits prescribed by the legislation and compliant with the requirements of energy-efficient modern buildings, as it has been underlined by Yarmoshenko et al. (2014).

RadCampania project produced numerous and different outcomes, from the study of the interactions between natural water bodies (surface and groundwater) to the modelling of the radon entry and accumulation in buildings. In this work the aspects related to geogenic feature of the soil and characteristics of buildings are discussed and analyzed.

Thanks to the adaptability and flexibility of the developed methodology the procedure is appropriate in any areas and results can be considered as valid support in areas with similar geological features, instead.

Once collected all data and information, the first step was to categorize the complete dataset (Tables 1–2). Note a the different number of investigation sites per province because it is proportional to the extension of the territory and the presence of a great variability of the soil (non-homogeneous geology). So, the larger province of Salerno was investigated with a greater number of investigations in the soil and indoor air.

Instead, in order to elaborate more in-depth data pursuing the aim to investigate the main features responsible of the accumulation of radon indoor, the analysis was restricted to data related to indoor radon and the soil. Data related to water were not considered because not significative for this purpose.

So, this kind of analysis involved 192 data, 96 in the soil gas and 96 in buildings performed in the province of Avellino, Benevento and Salerno. Soil gas measurements were performed in the nearby of the investigated building in order to then verify a possible correlation between the geogenic prognosis and the actual measured indoor values. In fact, if radon concentrations in soil gas can be regarded as a natural hazard parameter which is independent on construction type, state and age of existing houses, indoor radon instead is very influenced by the building-specific characteristics lifestyle of inhabitants.

As previously stated, the aim of this final report is to evaluate possible correlation between indoor radon concentration and other features. For this kind of analysis, data were catalogued according to the zone (urban, metropolitan, rural), the type of foundation (direct contact, crawl space, private, upper floor) and the type of material (reinforced concrete, tuff, stone) (Tables 3–5) and, then, combined according to the main materials used for walls and the type of foundation, Table 6.

Results confirm how urban areas are more exposed to the indoor radon accumulation, maybe because of the outdoor air pollution people are not accustomed to ventilating naturally their house and tends to live in well closed and isolate flats. So, in urban areas the problem is mostly attributable to behavioural factors related to the city lifestyle.

As regards the structural features it is not possible to define sharply a type of foundation or material which increase with evidence indoor levels. In fact, even if tuff is a material with higher radium content than others, data show how higher concentrations were revealed in buildings made in concrete and with aerated foundation (crawl spaces). This result highlights how the maintenance of the interface soil-buildings (in terms of presence of cracks) plays the most important role in the accumulation phenomena. This consideration could be also confirmed by the comparison of Tables 3 and 4. As it could be observed (Tables 1–2) the province showing a higher radon concentration from soil (Salerno) is not the same showing higher indoor radon concentrations (Avellino). The maintenance of a buildings is also connected by the socio-economic and cultural context. For example, detached and semidetached houses show generally a bad maintenance state instead of multifamily building because of the refitting costs.

At least another important consideration could be done in relation to the obvious reducing of the indoor radon levels from lower to upper floor. As expected, the contribute of the main source of radon, the soil, tend to decrease dramatically (in our case less than 4 times) because of the ventilation exchanges indoor-outdoor and indoor-indoor.

Regarding the contribution to the indoor radon activity concentrations provided by the building materials its direct evaluation from the activity of the naturally occurring radionuclides contained in walls, ceilings and structural elements, which is expressed usually, both in the regulatory bodies and in the scientific literature, in terms of the activity concentration index (I) or the radium equivalent activity (Ra-eq), is extremely difficult if not impossible (Keller et al., 1987) .

Therefore, for such reasons, in the framework of the Buildings sub-project a simplified approach, the Salerno Indoor Radon Emanation Model (S.I.R.E.M.) procedure, to assess indoor radon activity concentrations based upon on-site experimental evaluations of radon emanation by building materials, has been formulated with the aim to realize a tool not only with a scientific valence but also useful and precious for its applications to real situations (Mancini et al., 2012, 2014b).

Results evidence that to map the radon potential from soil cannot be the only tool, for authorities, to manage the radon issue. The maintenance of the interface soil-building plays one of the most important tool. So, mitigation system and action for new and existing building should focus firstly on this structural part. Apartments and closed spaces sited in the basement and ground floor are very exposed to the radon issue, so the control of indoor concentrations should focus on the lower rooms of a building. The respect of the limits in those places is precautional enough to assure, as first level of analysis, the same at the upper levels, even if in urban areas this assumption could not be respected because of the low ventilation rates which increase the effect of the contribution due to exhalation by building materials.

The categorization of buildings (according the foundation type, materials, number of floors) show how it is possible to identify a peculiar behaviour for each construction-type and so try to build up a performance indicator for different kind of construction, in term of response to the potential radon from soil. So, it could be very interesting to propose in a future work, an indicator of the building performances as useful tool, for authorities, to recognise constructions potentially more exposed to high indoor radon activity concentrations and for professional to support the design of constructions. Statistical analyses could be useful for this purpose, too.