Rong Cave is one of the more important caves in northern Vietnam’s Dong Van Karst Plateau Geopark (part ofthe Global Geoparks Network), because its subterranean lake provides agricultural and domestic water for neighbor-ing communities. Maintenance and utilization of Rong Cave’s water reservoir, as well as touristic cave use, requirefrequent human access to Rong Cave. Depending on the availability of seasonal drip water and the water level of thelake, the abundant clay-rich sediment in the back portion of Rong Cave and possible seepage of gas from deeper stra-ta along geologic faults provide seasonally elevated concentrations of radon in cave air. Based on repeated measure-ments over 10 months in 2015 and 2016 of the concentrations of radon isotopes ($^{222}$Rn and $^{220}$Rn, also called thoron) with a portable SARAD® RTM 2200 instrument (SARAD® GmbH, Germany), the human total annual inhalation dose was estimated according to the UNSCEAR (2000) algorithm. The result indicates that the radon-related radiation ex-posure is insignificant for short-term visitors but may reach ~1.8 mSv a-1 for tour guides and ~25 mSv a$^{-1}$ for cave utility workers. The latter values exceed the IAEA-recommended safety threshold of 1 mSv a$^{-1}$ (IAEA, 1996). We recommend radiation monitoring for cave utility workers and tour guides. Prolonged human presence in Rong Caveshould be avoided during periods of seasonally elevated radon concentrations.