## 3 Results and Discussion
Office buildings also showed higher concentrations compared to residential buildings since the doors and windows were closed after working hours and during weekends, and this allowed radon accumulation originating from basements into higher floors. For an evaluation of differences among/within buildings and inter-site comparison, we have normalized AM and GM radon concentrations (Figure 5b). Normalized AM and GM concentrations are generally around 20%-30% for all buildings except R1a and R1b (above 40%) that is a basement floor. Also, it can be observed that in the same building and on different floor, normalized ratio is decreased (R1a,b vs. R1d). With the normalized radon concentrations, it can be observed that there is no statistically significant difference in the distribution of radon measurement among buildings/floors and different geological sites except site R1. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Type of Building** & **AM (Bq/m3)** & **GM (Bq/m3)** \\ \hline Family house & \(38.3\pm 24.9\) & \(32.1\pm 2.5\) \\ Residential building & \(53.5\pm 39.8\) & \(44.2\pm 2.6\) \\ Wine cellar house/weekend & \(51.7\pm 37.8\) & \(40.7\pm 2.3\) \\ cottage & \(101.0\pm 48.7\) & \(87.0\pm 2.5\) \\ \hline
**Floor** & & \\ \hline Basement & \(70.9\pm 38.8\) & \(61.2\pm 2.2\) \\ Ground floor/1 st floor & \(42.5\pm 30.8\) & \(32.8\pm 2.9\) \\ \hline
**Overall** & & \\ \hline This study (Northern Croatia) & \(54.9\pm 34.3\) & \(45.1\pm 2.6\) \\ Croatia [5, 6] & \(68\pm 95\) & \(50.0\pm 2.3\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Average indoor radon concentration values measured in different building types and floors compared to overall results and national indoor radon levels in Croatia [5]. Radon concentrations for Northern Croatian region shown in Figure 1b [6] are set to be between 51–100 Bq/m3. Figure 5: (a) Radon AM and GM concentrations. (b) Normalized AM and GM radon concentrations. In rocks and soils, radon is usually captured inside the mineral matrix but it is released to the interstitial space between grains when the radium decays. Radon moves more rapidly through permeable soils, such as coarse sand and gravel, than through impermeable soils, such as clays [23]. It is important to note that radon released from the aggregate building materials (sand, gravel, crushed rock, clay) could also enter a house (walls, foundations) through the same migration pathways as radon produced by the surrounding rocks and soils [24]. The amount of radon depends on the amount of uranium present in the bedrock at the site, the type and permeability of soil surrounding the disturbed zone and underneath bedrock, and the soil's moisture content [25]. In the current phase of the research it is hard to conclude how much radon exhalation from alluvial Holocene deposits (sands, gravels, clays) and coarse-grained Neogene clastic sediments contributes to the total radon concentration in the indoor basement at these locations. Indoor radon concentrations in this study mainly vary with the occupancy and it is evident that the ventilation has a significant effect on the reduction of concentration (Table 2). Results show that people living in basements may be exposed to higher level of radon than people living in ground/first-floors. Also, we noticed that office buildings' indoor radon concentrations are higher than residential buildings, mostly due to the ventilation effect. It can be concluded that results of average radon concentrations (Table 3) were mostly comparable with national average radon concentrations conducted in one-year study, presented in Figure 1b [5,6]. In general, average radon concentrations are within maximal allowed values (300 Bq/m\({}^{3}\)) according to the Euroatom Directive. However, in the worst-case scenario with no ventilation, the maximum radon concentrations exceed 300 Bq/m\({}^{3}\), as presented in Figures 2 and 4. Radon concentrations within a home vary among rooms and so the actual radon concentration relevant to a person will also vary, depending on the time spent in different rooms.