In this study, measurements of indoor radon ((222)Rn), thoron ((220)Rn) and their equilibrium equivalent concentration (EEC) were carried out in 96 dwellings from 22 different villages situated in Hamirpur district, Himachal Pradesh, India, by using LR-115 type II-based pinhole twin cup dosimeters and deposition-based progeny sensors (DRPS/DTPS). The annual average indoor (222)Rn and (220)Rn concentrations observed in these dwellings were 63.82 and 89.59 Bq/m(3), respectively, while the average EEC (attached + unattached) for (222)Rn and (220)Rn was 29.28 and 2.74 Bq/m(3). For (222)Rn (f Rn) and (220)Rn (f Tn), the average values of unattached fraction were 0.11 and 0.09, respectively. The equilibrium factors for radon (F Rn) and thoron (F Tn) varied from 0.12 to 0.77 with an average of 0.50, and from 0.01 to 0.34 with an average of 0.05, respectively. The annual inhalation dose due to mouth and nasal breathing was calculated using dose conversion factors and unattached fractions. The indoor annual effective doses for (222)Rn (AEDR) and (220)Rn (AEDT) were found to be 1.92 and 0.83 mSv a(-1), respectively. The values of (222)Rn/(220)Rn concentrations and annual effective doses obtained in the present study are within the safe limits as recommended by the International Commission on Radiological Protection for indoor dwelling exposure conditions.
Keywords: DCF; EEC; Effective dose; Equilibrium factor; Unattached fraction.