# The Relationship between Radon and Geology: Sources, Transport and Indoor Accumulation
## 2 The Origin of Radon
### Accumulation of Radon in Indoor Environments
#### 2.3.6 Rn Accumulation
Spanning almost two decades, these studies present an insightful progression in the understanding of radon behaviour and collectively highlight the significant strides made in understanding radon dynamics in Spanish caves, the potential health risks, and the broader implications for various research fields. The research chronology reveals a progression from initial monitoring and dose calculations to more complex studies of seasonal variations, spatial differences, and the influence of environmental factors. The accumulated knowledge underscores the importance of continuous monitoring and targeted interventions to mitigate health risks, contributing to the discourse on the safe and sustainable use of these unique environments. \begin{table}
\begin{tabular}{c c c} \hline
**Country** & **Cave** & **Equilibrium Factor** \\ \hline \multirow{6}{*}{Hungary} & Gellert-hill System & 0.94 \\  & Szemlo Hill Cave & 0.50 \\  & Pal Valley Cave & 0.48 \\  & Therapeutic Cave & 0.51 \\  & Vass Imre Cave & 0.53 \\ \hline \multirow{6}{*}{USA} & Carlsbad Caverns & 0.44 \\  & Crystal Cave & 0.90 \\  & Jewel Cave & 0.19 \\  & Lehman Cave & 0.81 \\  & Oregon Cave & 0.66 \\  & Round Spring Cave & 0.98 \\  & Wind Cave & 0.46 \\  & Howe Caverns & 0.67 \\ \hline \multirow{2}{*}{Italy} & Grotta Grande Vento & 0.69 \\  & Grotta di Quinzano & 0.55 \\ \hline \multirow{6}{*}{Czech Republic} & Katerinska Cave & 0.75 \\  & Punkvevni Caves & 0.86 \\  & Sloupsko-sousvske Caves & 0.87 \\  & Balcarka Cave & 1.94 \\ \hline \multirow{2}{*}{Japan} & Akiyoshi-do Cave & 0.70 \\  & Taisyo-do Cave & 0.71 \\  & Kagejiyo-do Cave & 0.52 \\ \hline Slovenia & Postojna Cave & 0.56 \\ \hline \end{tabular}
\end{table} 
Table 6: Some values of the equilibrium factor measured in caves and presented by [PERSON] (2005), using as references the previous works by [PERSON] et al. (1971), [PERSON] et al. (1978), [PERSON] and [PERSON] (1978), [PERSON] et al. (1980), [PERSON] (1990), [PERSON] et al. (1993), [PERSON] (1996), [PERSON] et al. (2001) and [PERSON] et al. (2003) [97; 121; 122; 123; 124; 125; 126; 127; 128; 129]. One of the earliest studies, by [PERSON] et al. (2005), investigated the radon concentration in Altamira Cave over a complete annual cycle [130]. The research, which aimed to calculate the annual effective dose for guides and visitors, reported radon levels ranging from 186 Bq\(\cdot\)m\({}^{-3}\) to 7120 Bq\(\cdot\)m\({}^{-3}\), with an annual average of 3562 Bq\(\cdot\)m\({}^{-3}\). Notably, the effective doses did not exceed international recommendations, highlighting the relative safety for tourists and cave guides. This study established the groundwork for subsequent research in this area, with its focus on continuous monitoring and dose calculations. Six years later, [PERSON] et al. (2011) conducted a similar annual cycle of radon concentration measurements in different halls of the Nerja Cave [131]. Unlike the previous study, this research aimed to detect seasonal variation patterns and relate them to various meteorological factors. This research expanded our understanding of the complexity of radon behaviour in caves, contributing to an emerging discourse on the implications of radon concentration variations for cave tourism. [PERSON] et al. (2013) carried out an insightful study in a shallow volcanic cave, using radon concentration to study hygrothermal variations [132]. This pioneering research established a link between radon activity and weather and microclimate conditions, which has substantial implications for diverse research fields, including volcanic and seismic activity monitoring, cultural heritage conservation, indoor air quality control, and the role of subterranean ecosystems as trace gas reservoirs. Two years later, [PERSON] et al. (2015) expanded the geographic scope of the investigation to include several caves in Mallorca [133]. This study was significant for identifying caves where radon concentration exceeded the recommended action levels and posed health risks.