# The Relationship between Radon and Geology: Sources, Transport and Indoor Accumulation
## 2 The Origin of Radon
### Accumulation of Radon in Indoor Environments  
#### 2.3.1 Radon Diffusion from the Soil  
The process of radon diffusion is strongly influenced by the porosity of the soil and the permeability of rocks, both of which are crucial elements in facilitating the mobility of this gas. Soil porosity, referring to the amount of free space between grains, determines the ease with which radon can move. More porous soils allow for quicker radon diffusion. The permeability of rocks, which is the ease with which a fluid can traverse them, also plays a significant role. Highly permeable rocks such as sandstone and limestone facilitate radon diffusion, whereas less permeable rocks such as clay and shale tend to restrict it. The diffusion coefficient, a parameter quantifying the movement of radon through these mediums, is influenced by various factors, including soil porosity, rock permeability, and soil moisture. In practical terms, dry and sandy soils generally exhibit a higher diffusion coefficient, allowing radon to move more freely, while clayey and moist soils possess a lower diffusion coefficient, thereby limiting the mobility of radon. The study by [PERSON] and [PERSON] (2020) examined the impacts of various factors on the radon emanation and diffusion coefficients [71]. These elements included temperature, grain size, moisture content, main element make-up, and soil pH. Their research revealed that grain size and moisture content had a large impact on the emanation and diffusion coefficients. The radon emission coefficient rose as particle size decreased, while the diffusion coefficient fell. However, despite fluctuations in grain size in soils with larger particle sizes, both coefficients remained mostly unchanged. The study also analysed soil particles of five different sizes and discovered that when moisture content rose, the emanation coefficient rose along with it, while the diffusion coefficient fell. Depending on the variety of grain sizes, the radon emission coefficient had consistent values at various moisture concentrations. When moisture content rose, the researchers saw a sharp rise in radon emissions from smaller grain sizes. For soil grains with a size between 0.1 and 0.5 mm, the saturation emanation coefficient ranged from 0.47 to 0.23, while the associated saturation moisture contents ranged from 16% to 4%. It is interesting that the study discovered that the soil's pH and iron concentration appeared to have a substantial impact on the radon emission coefficient. Additionally, the study's calculations for the effective diffusion coefficient agreed with those made by an earlier model. The study demonstrated that the radon diffusion coefficient exhibits Arrhenius behaviour in its temperature dependence. According to the underlying principles of diffusion, which indicate that diffusion rates rise with temperature, the diffusion coefficient also rises with temperature. A study, from [PERSON] et al. (2003), utilized high-resolution gamma-ray spectroscopy to examine natural radioactivity, elemental concentrations, and dose rates in various rock types, predominantly from Cyprus (Table 2) [72]. The results show that samples originating from the Troodos ophiolitic complex generally contain lower radionuclide concentrations than those of sedimentary origin. The latter, particularly when combined with phosphates, carbonates, and silicates, show higher concentrations of elements in the uranium and thorium series. However, thorium, uranium, and potassium activity and elemental concentrations in the soil samples studied were found to be within normal ranges and significantly lower than those observed in other countries. The mean absorbed dose rate in air outdoors was significantly below the world-average value, implying that Cyprus's inhabitants are exposed to radiation levels considerably lower than those reported globally. However, as the study only measured outdoor dose rates, additional research is necessary to determine indoor dose rates. Another study, presented by [PERSON] et al.