# The Relationship between Radon and Geology: Sources, Transport and Indoor Accumulation
## 2 The Origin of Radon
### Accumulation of Radon in Indoor Environments
#### 2.3.3 Building Materials  
Although the main source of radon in indoor spaces is the soil and underlying rocks, some building materials can also release small amounts of radon [82]. For example, concrete and certain types of bricks and stones can contain traces of uranium and radium, which can generate radon over time [83]. The extent to which building materials contribute to indoor radon levels depends on several factors, including the type of material, its age, and the level of radionuclides present [84]. While the contribution of building materials to indoor radon levels is generally minor compared with that of soil and rock, it is still important to consider when assessing the overall risk of radon exposure [85]. Building materials that contain high levels of radionuclides, such as granite or other natural stones, can be a significant source of radon if used in large quantities in a building [75]. A recent study presented by [PERSON] et al. (2020) examined the exhalation and emanation of radon and thoron from a variety of building materials from the Iberian Peninsula [86]. The authors concluded that both gases can be released from building materials into indoor atmospheres. This research found correlations between the exhalation rates of these gases and their parent nuclide exhalation concentrations (radium/thorium). Notably, zircon exhibited the highest concentration values of \({}^{226}\)Ra and \({}^{232}\)Th as well as the highest exhalation and emanation rates. Certain granites also demonstrated an annual effective dose that exceeded the 1 mSv\(\cdot\)y\({}^{-1}\) exposure limit recommended for the public by European regulations. In their study, [PERSON] et al. (2007) carried out radiometric analyses on various building materials from the Gujarat region in India, drawing attention to the elevated concentrations of natural radionuclides, particularly in fly ash samples and silica fume [87]. These higher concentrations notably contributed to an increased gamma dose in such materials. Considering the local cement industry's use of silica fume in manufacturing, the researchers correlated these higher levels of radioactivity with elevated radionuclide content in cement products, especially white cement. However, the annual effective doses and radium equivalent values for these materials were within the recommended levels. Interestingly, [PERSON] et al. observed no linear relation between the emanating factor or mass exhalation rate and the radium content in the analysed samples. This may be attributed to how radon, a decay product of radium, is released from crystal lattices through various mechanisms. The researchers found an approximate linear correlation between the mass exhalation rate and the emanation factor, possibly due to the standardization of the sample's physical properties through powdering and drying prior to analysis. The study concluded that precautions must be taken to mitigate radon emission in dwellings constructed with fly ash bricks. Furthermore, [PERSON] et al. advocated the adoption of their method to estimate the emanation factor, providing an index to select building materials regarding potential radon levels in dwellings. The researchers argue that although in situ values are ideal, they are difficult to obtain, hence the necessity of their method and the importance of considering factors such as radon content, bulk density, porosity, moisture content, and air exchange rate when predicting indoor radon levels. In Table 4 are summarized the results obtained by [PERSON] et al. (2007). In these results, some of the materials, fired clay brick for instance, showed relatively low values for all three metrics, with a \({}^{226}\)Ra concentration of 16 \(\pm\) 5 Bq\(\cdot\)kg\({}^{-1}\), an exhalation rate of 6.1 \(\pm\) 3.4 mBq\(\cdot\)kg\({}^{-1}\)\(\cdot\)h\({}^{-1}\), and an emanation factor of 7.8 \(\pm\) 3.4%.