## 5 Conclusions  
In this study, the radon exhalation rate of several natural stones of historical and artistic relevance, i.e., Lecce stone, Modica stone, Ignimbrite Campana, Comiso stone, Mendicino stone, and Viterbo tuff, were evaluated using the closed chamber method (CCM) in combination with the Durridge Rad7 system for alpha spectrometry of short-lived radon progeny. Two different analytical approaches were applied: _Method 1_, based on an extended 10-day measurement period, and _Method 2_, relying on a more rapid 24 h data acquisition. To determine the accuracy of the two methods, a statistical u-test was performed. Additionally, the Markkanen room model was used to simulate the indoor radon concentration associated with each stone type, providing insights into potential radiological risks. Based on the obtained results, it can be found that:  
1. \(\mathrm{u}_{\mathrm{test}}\) values indicated good agreement between the two methods, supporting the validity of the shorter, more time-efficient method for estimating radon exhalation rates. Thus, the faster 24 h method has been validated for use in practical applications, offering time-efficient assessments for construction materials. This has implications for quick screening in construction and heritage preservation--areas where fast decision-making is crucial for worker and public safety and compliance with safety standards.
2. The predicted indoor radon concentrations, for all samples, remained well below the regulatory limits set by Legislative Decree No. 101/2020, as well as the WHO recommended limit and the EPA action level, thereby excluding any significant health concerns under typical indoor conditions.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Sample** & **Simulated Activity Concentration (Bq m\({}^{-3}\))** \\ \hline Lecce stone & 25.4 \\ \hline Modica stone & 5.7 \\ \hline Ignimbrite campana & 9.3 \\ \hline Viterbo tuff & 37.3 \\ \hline Comiso stone & 4.1 \\ \hline Mendicino stone & 4.1 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Indoor activity concentration values simulated using the Markkanen room model.  
These results underscore the value of investigating radon emissions from natural building materials, particularly those with cultural and architectural significance. Such assessments are crucial for enhancing radiological safety, informing construction practices, and ensuring compliance with regulatory frameworks. Moreover, the confirmation of a faster and reliable analytical protocol, along with the implementation of a validated room model, offers practical tools for future studies and risk assessments in the domain of environmental radiation protection.  
Conceptualization, F.C. and V.V.; methodology, F.C. and S.M.; validation, D.M. and M.G.; formal analysis, L.P. and F.M.; investigation, F.C., G.P. and V.V.; resources, F.C., D.M. and V.V.; data curation, F.C. and S.M.; writing--original draft preparation, F.C. and L.P.; supervision, D.M., M.G. and V.V. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data in this study are available upon reasonable request to the corresponding author.  
The authors declare no conflicts of interest.