## 1 Introduction  
Human exposure to ionizing radiation comes from both natural and artificial sources. According to the 2008 UNSCEAR report, the global average annual radiation dose is about 2.4 mSv from natural sources and 0.6 mSv from man-made activities [1]. Natural radiation includes external exposure from cosmic rays and terrestrial radionuclides present in soil and construction materials, as well as internal exposure from inhaling or ingesting naturally occurring radionuclides found in air, water, and food [2, 3, 4, 5, 6]. Artificial sources are mainly related to medical procedures that use ionizing radiation for diagnostics and treatment, such as radiology and radiotherapy [7, 8, 9, 10, 11, 12, 13]. Nuclear accidents, such as those at Chernobyl and Fukushima, have also released significant quantities of radionuclides into the environment, including \({}^{131}\)I, \({}^{137}\)Cs, \({}^{134}\)Cs, and \({}^{90}\)Sr [14; 15; 16; 17; 18; 19]. Among natural sources, radon--specifically \({}^{222}\)Rn, due to its greater isotopic abundance--is of particular concern, as it is the main contributor to internal exposure from natural radiation [1]. This noble gas is colorless, odorless, chemically inert, and continuously produced in the decay chain of \({}^{238}\)U [20; 21; 22; 23; 24; 25]. It can diffuse through soil, rocks, and water [26; 27; 28]. The ground and its underlying geological formations account for about 69% of radon emissions [29; 30]. Geological features and weathering processes strongly influence the geogenic radon potential (GRP), i.e., the natural ability of the ground to generate and release radon gas into the environment, primarily influenced by the uranium content of rocks and soils, their permeability, and geological structure [31; 32]. Understanding these factors is essential for predicting radon release and planning land use or building design strategies that reduce indoor accumulation, thereby enhancing safety [33]. In addition, construction materials sourced from quarries may emit radon, posing potential health risks to workers and building occupants. Many historic structures use locally quarried stones, yet little research exists on their radon emissions, in particular for Southern Italian heritage structures [34; 35; 36; 37; 38; 39; 40]. Notably, radon is classified as a carcinogen, and long-term exposure to elevated concentrations has been clinically linked to an increased risk of lung cancer. The World Health Organization identifies radon as the second leading cause of lung cancer after smoking [41]. This hazard arises mainly from its radioactive decay products--\({}^{218}\)Po and \({}^{214}\)Po--which emit alpha particles. While radon itself is inert, its progeny is reactive and can adhere to airborne dust, allowing them to be inhaled and settle in the lungs. Once deposited in lung tissues, they can cause cellular damage that significantly heightens cancer risk [42; 43]. Recent epidemiological studies have shown that even low-level radon exposure can significantly increase lung cancer risk [44]. Given these health implications, effective monitoring and mitigation of radon levels are critical components in view of the definition of efficient radiation protection strategies. In light of this, the European Union adopted Directive 2013/59/EURATOM, which was subsequently implemented in Italy through Legislative Decree D.Lgs. 101/2020 [45]. The directive emphasizes the importance of thoroughly investigating all potential indoor radon sources, including those arising from building materials. Within this context, the present study investigates the accuracy of two different analytical approaches for measuring radon exhalation rates from building materials. The research focuses on six types of stone widely used in Southern Italian heritage architecture--namely Lecce stone, Modica stone, Ignimbrite Campana, Comiso stone, Mendicino stone, and Viterbo tuff.