# The Relationship between Radon and Geology: Sources, Transport and Indoor Accumulation
## 2 The Origin of Radon
### Rocks and Soil Migration Factors  
#### 2.2.1 Permeability of Soil and Rocks  
Permeability is the ability of soil and rocks to allow the passage of water and gases [31]. Soils and rocks with higher permeability, such as sandstones and limestones, have larger pores and fissures, allowing radon to move more easily [32]. On the other hand, soils and rocks with low permeability, such as clays and shales, tend to retain radon and limit its mobility [33]. The permeability of the soil and rocks plays a crucial role in the movement of radon through the environment and can have a significant impact on the concentration of radon in indoor environments [34]. In areas with high permeability, radon can seep through the soil and enter buildings, increasing the risk of exposure [35]. In contrast, in areas with low permeability, radon can become trapped in the soil and not reach indoor environments, reducing the risk of exposure [36]. The permeability of soil and rocks in a given area is an important factor when identifying high-risk areas and developing effective mitigation strategies [37]. Therefore, the combined measurements of soil permeability and concentration of uranium and thorium in rocks and soils allow assessment of the concentration of radon and the risk of exposure in a particular region [38]. For example, granitic rocks, despite exhibiting low porosity, may display high permeability if highly fractured. On the other hand, volcanic rocks can have high porosity due to the presence of vesicles, yet their permeability is often constrained by a lack of connectivity among these pores [39; 40]. Clays, siltstones, and mudstones typically present low permeability, largely owing to the small size of their pores and a lack of interconnectivity among them [41]. Lastly, limestones can exhibit a wide range in permeability, from very low in microcrystalline limestones to very high in fractured limestones or those with substantial intergranular porosity [42; 43]. There are several relevant studies about this subject. One of the first research studies, which was published by [PERSON] et al. (1991), is a comprehensive and seminal review about radon's behaviour in the geological environment [32]. The authors explored the general geochemistry of radon, highlighted the factors that control its emanation from minerals and rocks into disperse phases and outlined the importance of the permeability of the host rock and soil in the process of radon diffusion. The study emphasises that the emanation of radon depends not only on the uranium concentration of the source, which acts as the progenitor of radon in the decay series, but also significantly on the nature of the host mineralogy and the permeability of the host rock and soil. The permeability of these hosts is critical as it determines how easily radon can move and diffuse through the rock or soil. A rock or soil with high permeability provides easier pathways for radon to escape, thus enhancing its diffusion. These authors underlined the role of weather, which can profoundly affect the concentration of radon in soil gas. However, they noted that variations due to the geological substrate are often greater, which underscores the overarching importance of geology in radon behaviour. The authors concluded the study by presenting guidelines to identify areas of high radon emanation, which is especially relevant given that radon becomes a problem when it collects in buildings and underground structures. Examples of different types of porosity are presented in the study by [PERSON] and [PERSON] (2008), and they provide valuable insights into the factors that influence the permeability of rocks, emphasizing the role of microstructure, defined in terms of pore and crack structures [44]. The permeability of porous materials is determined not only by porosity but also by the shape and arrangement of pores and the amount of clayey component. Importantly, only effective porosity, represented by interconnected open pores, influences permeability as it allows the passage of water.