The eastern master fault of the Siena Graben (central Italy), known as Rapolano Fault, is a place of migration of a large amount of CO₂-rich gas from deep geothermal reservoirs. Under particular topographical and meteo-climatic conditions, large emissions of carbon dioxide can represent serious hazards to human and animal life. Carbon dioxide is a carrier gas for radon (Rn), the concentrations of which are high in the soil despite the low U content of the overburden. CO₂ pressure also drives the upwelling of thermal waters producing at the surface, in some places, radioactive (²²⁶Ra rich) travertine formations, inducing higher soil-gas Rn levels. Three main types of hazards, mechanical, toxicological and radio-ecological, may occur depending on the upwelling fluid phases (gas alone or gas plus water). Gas distribution and partitioning in different micro-environments (e.g., soil pores, groundwater, ground-atmosphere interface) should be taken into account when planning hazard-assessment surveys.