## 4 Results and Discussion  
Figure 3 displays the most representative radon accumulation curves obtained during the 10-day measurement period using the Rad7 system for the following lithotypes: Lecce stone (a), Modica stone (b), Ignimbrite Campana (c), and Viterbo tuff (d). The corresponding radon growth trends for Comiso and Mendicino stones were previously presented in [29]. Additionally, the fitted effective decay constant is presented in Table 1, for all of the investigated samples. Significantly, a comparison of fitted effective decay constant values, i.e., (0.023 \(\pm\) 0.001) h\({}^{-1}\), (0.027 \(\pm\) 0.001) h\({}^{-1}\), (0.017 \(\pm\) 0.001) h\({}^{-1}\), (0.050 \(\pm\) 0.004) h\({}^{-1}\), (0.081 \(\pm\) 0.011) h\({}^{-1}\), \begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **\(\mathbf{\lambda}\) (h\({}^{-1}\))** \\ \hline Lecce stone & 0.023 \(\pm\) 0.001 \\ \hline Modica stone & 0.027 \(\pm\) 0.001 \\ \hline Ignimbrite Campana & 0.017 \(\pm\) 0.001 \\ \hline Comiso stone & 0.050 \(\pm\) 0.004 \\ \hline Mendicino stone & 0.081 \(\pm\) 0.011 \\ \hline Viterbo tuff & 0.018 \(\pm\) 0.001 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The fitted effective decay constant for all investigated samples. Figure 3: The most representative radon growth curves to equilibrium, through _Method 1_, for the Lecce stone (**a**), Modica stone (**b**), Ignimbrite campana (**c**), and Viterbo tuff (**d**). and (0.018 \(\pm\) 0.001) h\({}^{-1}\), for the Lecce, Modica, Ignimbrite Campana, Comiso, Mendicino, and Viterbo natural stones, respectively, with the radon decay constant, i.e., 0.008 h\({}^{-1}\), highlights the significance of taking into account bound exhalation and leakage current. Indeed, their contributions, estimated at approximately 0.015 h\({}^{-1}\), 0.019 h\({}^{-1}\), 0.009 h\({}^{-1}\), 0.042 h\({}^{-1}\), 0.073 h\({}^{-1}\), and 0.010 h\({}^{-1}\) for the Lecce, Modica, Ignimbrite Campana, Comiso, Mendicino and Viterbo natural stones, respectively, cannot be neglected. Figure 4 reports the most representative radon concentration curves characterized by a linear increase during the initial 24 h measurement period for all the investigated lithotypes. Figure 4: The most representative radon growth curves to equilibrium, obtained applying _Method 2_, for Lecce stone (**a**), Modica stone (**b**), Ignimbrite campana (**c**), Viterbo tuff (**d**), Comiso stone (**e**), and Mendicino stone (**f**). Table 2 summarizes the calculated values of the specific radon exhalation rate, together with its uncertainty, for each of the six natural stone samples, as well as the corresponding u\({}_{\text{test}}\) values. Significantly, exhalation rates varied among stone types due to mineral composition, porosity, and geological origin [68]. For example, the high exhalation from Viterbo tuff reflects its volcanic nature [59]. These variations are relevant for heritage sites where such stones are prevalent. Furthermore, compared to studies in other regions [37; 38; 68], the radon exhalation rates from the investigated stones are moderate to low. For example, granite samples can reach values over 1 Bq h\({}^{-1}\), whereas our highest value (Viterbo tuff) is 0.072 Bq h\({}^{-1}\). Finally, the u-test results, reported in Table 2, consistently support the statistical equivalence of the two data collection protocols, with all values being lower or equal to the critical threshold of 2. Figure 5 reports a comparative bar chart to visually contrast the exhalation rates measured by both protocols. Going on, Figure 6 illustrates the simulated radon accumulation curves derived from the Markkanen room model (based on Equations (5) and (6)).