Abstract: To study the influence of exploitation of Bayan Obo ores on indoor ²²²Rn, ²²⁰Rn and γ radiation levels, measurements of indoor ²²²Rn,²²⁰Rn concentrations were performed using solid state nuclear track detectors for one year, and indoor γ radiation level was performed using TLD for 3 months. The investigation shows that ranges of concentration and averages of indoor ²²²Rn, ²²⁰Rn and γ radiation level are 16.7-125.3 Bq/m³ and 39.3 Bq/m³, 30-153.3 Bq/m³ and 51.2 Bq/m³ and 120-283 nGy/h and 149 nGy/h respectively. Thusly, the paper estimates that the exploitation and utilization of Bayan Obo ores has caused an increase of indoor γ radiation level. The levels of ²²²Rn, ²²⁰Rn and γ in Baiyan Obo area are higher than in Baotou. The concentration of indoor ²²⁰Rn in Baotou Iron and Steel Co. Ltd. (Baogang) is no higher than in other districts of Baotou, but the concentrations of indoor ²²²Rn and the γ radiation levels in Baogang area, Baiyan Obo area and the rooms with slag building materials are higher than in other districts. With the years' increase of the buildings built, the concentrations of ²²²Rn and ²²⁰Rn are increasing notably, and the γ radiation levels have no obvious trend noticed.

Key words: Baotou, Bayan Obo, indoor, ²²²Rn, ²²⁰Rn, γ, NORM