Radiological risk assessment of indoor 222Rn and 220Rn exposure in the seismically active Kopili Fault Zone on the eastern wedge of the Shillong Plateau, India.

Abstract

This study investigates indoor levels of ²²²Rn, ²²⁰Rn, and their progeny in houses located within the seismically active Kopili Fault Zone on the eastern wedge of the Shillong Plateau. Single-entry pinhole dosemeters and deposition-based progeny sensors with LR-115 (II) nuclear track detectors were used for the measurements, which were carried out over a 3-month exposure period during the winter season (December to February). The ²²²Rn concentrations ranged from 135.3 ± 9.5-457.5± 17.3 Bq m^-3 (average: 213.4 Bq m^-3), while ²²⁰Rn ranged from 101.4 ± 26.7-305.8± 34.2 Bq m^-3 (average: 178.2 Bq m^-3), exceeding WHO and ICRP thresholds. Significantly, certain Assam-type houses even exceeded the ICRP's upper reference level of 300 Bq m^-3. These elevated concentrations could be attributed to the uranium-enriched Precambrian Basement rocks underlying the region, the frequent seismic activity that characterizes this geologically sensitive area, and the inadequate ventilation in the houses. The equilibrium equivalent ²²²Rn and ²²⁰Rn concentrations (EERC and EETC) were calculated for both the 'attached' and 'unattached' progeny fractions. The ratio of EETC to EERC varied from 0.04 to 0.10, with an average of 0.07, which is consistent with the globally reported range of 0.01-0.5. The total effective inhalation doses (EIDs) during the exposure period from ²²²Rn, ²²⁰Rn, and their progeny ranged from 0.6 ± 0.2 to 1.6 ± 0.3 mSv, with an average of 0.9 mSv. To reduce health risks from prolonged exposure to radioactive gases in this earthquake-prone region, improved ventilation and structural modifications are strongly recommended.