Indoor and outdoor ²²²Rn and ²²⁰Rn and their progeny levels surrounding Bayan Obo mine, China

More than half of the total natural ionizing radiation dose received by the human population is caused by radon and thoron (Rn and Tn) and their progeny. To estimate the level of radiation due to radon and thoron and their progeny, an investigation was conducted in a residential area near the world’s largest open-pit mine of Bayan Obo in Inner Mongolia, China. The concentration of Rn, Tn, and their decay products in air and soil were studied by using AlphaGUARD, RAD7, and ERS-RDM-2S for a discrete period of time in three different locations. The average indoor concentration of radon and thoron was 62.6 ± 44.6 Bq/m³ and 108.3 ± 94.5 Bq/m3 respectively, and the outdoor concentration was 12.9 ± 6.3 Bq/m³ and 55.8 ± 18.5 Bq/m³, respectively. Relatively high concentrations were recorded in the area near to the mine, with a significant increasing trend observed in indoor thoron concentration. A prominent hotspot in thoron concentration was found in a single-story house with values 747 ± 150 Bq/m³. The equilibrium equivalent thoron concentration (EEC_Tn) varies from 0.48 Bq/m³ to 2.36 Bq/m³ with an arithmetic mean of 1.37 ± 0.64 Bq/m³, and comparatively higher than EEC_Rn. Concluding that the mining activity at Bayan Obo mine is significantly increasing the level of indoor thoron and its progeny in surroundings. It is suggested to further systematically investigate the indoor Rn and Tn progeny concentrations in the residential dwellings of the Bayan Obo mining area, and ²³²Th content of the building materials, to provide a basis for calculating the radiation dose.