Systematic Methodology for Mitigating Radon Risk and Enhancing Fruition of Underground Sites: The Case-Study of Herculaneum’s Theatre

Radon exposure poses a significant health risk in underground cultural heritage sites, where limited ventilation and prolonged visitor presence can lead to high radon exposures. While previous studies have concentrated on monitoring and mitigation strategies, few have developed a comprehensive approach that ensures both safe and sustainable site use. This research introduces an innovative methodology that integrates periodic/seasonal radon risk assessment with risk-informed access management based on periodic monitoring and time tracking. This approach is based on: (i) periodic monitoring to obtain representative concentrations; (ii) the calculation of permissible stay durations using a dose-based framework; (iii) implementation via access registration (badges) and procedural measures; and (iv) the application of mitigation measures when concentrations exceed limits (otherwise, the dose is evaluated in accordance with the applicable reference levels). This strategy was implemented and validated at the Roman Theatre in Herculaneum, a unique case study characterised by complex architectural constraints (as the theatre is completely underground) and high cultural significance. Results from years of monitoring, along with ongoing campaigns, demonstrate that this methodology not only reduces radon-related health risks but also enhances visitor experience. This integrated framework provides a replicable model for balancing conservation, safety, and accessibility in underground heritage sites.