Estimation of Indoor Radon, Thoron and Progeny Concentration in District Bijnor, Uttar Pradesh, India

Indoor exposure to Radon (²²²Rn), Thoron (²²⁰Rn), and their progeny is one of the main contributors to natural radiation dose received by humans. In this work, indoor levels of these radionuclides were investigated in residential houses of Bijnor district, Uttar Pradesh, India. The study mainly focused on understanding how concentrations change with seasons, how they are distributed across dwellings, and how different types of houses influence their levels. For measurements, passive twin-chamber single-entry pinhole dosimeters coupled with LR-115 type II solid-state nuclear track detectors (SSNTDs) were used. Monitoring was carried out over a period of one year, divided into four seasons of nearly 90 days each. The track densities obtained after exposure were converted into concentrations of 222Rn, 220Rn, and their progeny using standard calibration relations. A clear seasonal variation was observed in both 222Rn and 220Rn concentrations (Table 1). 222Rn values ranged from 16.20 ± 1.17 to 44.31 ± 1.93 Bq/m³. whereas 220Rn varied between 6.44 ± 0.74 and 16.38 ± 1.17 Bq/m³. The highest concentrations were recorded during Season 2, while the lowest appeared in Season 4. This pattern is likely linked to differences in ventilation and prevailing atmospheric conditions during these periods. Similar behavior was noted for the equilibrium equivalent concentrations of radon (EERC) and thoron (EETC). The distribution pattern of measured concentrations (Fig. 4 and Fig. 5) does not follow a symmetric trend. Instead, the majority of dwellings fall within lower concentration ranges, while relatively fewer exhibit higher values. This positively skewed distribution reflects the influence of indoor environmental conditions, particularly variations in ventilation practices, occupancy behavior, and structural characteristics of the dwellings. Since the monitoring was conducted in well-defined seasonal intervals of 90 days (September–November, December–February, March–May, and June–August), the observed variability can be reasonably associated with seasonal changes in air exchange rates and indoor confinement conditions.When results were compared across different dwelling types (Fig. 6 and Fig. 7; Table 2), noticeable differences emerged. Houses with mud flooring showed higher average concentrations, 57.55 Bq/m³ for 222Rn and 19.01 Bq/m³ for 220Rn than those constructed with brick and cement. This suggests that direct contact with soil and the nature of building materials play an important role in determining indoor levels. The annual mean concentrations were estimated to be 28.56 Bq/m³ for 222 Rn and 10.45 Bq/m³ for 220Rn. These values remain well within the limits recommended by international radiation protection guidelines. Based on these observations, indoor radiation levels in the study area can be considered safe, with variations primarily controlled by seasonal changes and housing characteristics.