Indoor radon/thoron levels and inhalation doses to some populations in Himachal Pradesh, India H. S. Virk* and Navjeet Sharma Department of Physics, Guru Nanak Dev University, Amritsar 143005, India. E-mail: virkhs@yahoo.com Received 29th August 2001, Accepted 10th October 2001 It is well established that some areas of Himachal Pradesh (H.P.) state of India situated in the environs of the Himalayan mountains are relatively rich in uranium-bearing minerals. Some earlier studies by our group have indicated high levels of radon (w200 Bq m 23 ) in the dwellings. It is in this context that an indoor radon/thoron survey has been carried out in selected villages of four districts in the state of H.P. This survey has been conducted as a part of a national, coordinated project using twin chamber dosemeter cups designed by the Environmental Assessment Division (EAD), Department of Atomic Energy, Govt. of India. The track-etch technique is used for calibration of plastic detector LR-115 type-II which are employed for recording alpha tracks due to radon/thoron and their daughters. Year long radon/thoron data have been collected for seasonal correlations of indoor radon/thoron in the dwellings. The indoor radon levels have been found to vary from a minimum value of 17.4 Bq m 23 to a maximum value of 140.3 Bq m 23 . The indoor thoron levels vary from a minimum value of 5.2 Bq m 23 to a maximum value of 131.9 Bq m 23 . The year average dose rate for the local population varies from 0.03 mSv h 21 to 0.83 mSv h 21 . The annual exposure dose to inhabitants in all the dwellings lies below the upper limit of 10 mSv given in ICRP-65. Introduction The measurement of radioactivity in the human environment, in general and in the Himalayan ecosystem, in particular, is of special interest to mankind. It has been established that radon is a causative agent of lung cancer when present in high concentration e.g., in uranium mines where some case studies are reported. 1–3 The health hazard of radon is principally due to its short-lived daughters, especially 218 Po and 214 Po. During recent years, several reports have appeared in the literature regarding the ever-increasing interest in monitoring radon in the indoor environment all over the world. 4–10 It is well established that some areas of Himachal Pradesh (H.P.), situated in the environs of Western Himalaya are quite rich in radioactive minerals. 11–12 Some of these sites in H.P. state were exploited by the Atomic Minerals Division of Department of Atomic Energy (DAE) Govt. of India. An earlier survey reported very high values of indoor radon in dwellings around some of these sites. 13 High values of soil-gas radon varying from 13300 Bq m 23 to 75400 Bq m 23 were recorded by our group in a previous spot survey. 14 Based on these facts, we decided to monitor radon inside dwellings in some of the villages falling in four districts of H.P., specifically Una, Hamirpur, Kullu and Kangra, as part of a national coordinated radon project sponsored by DAE. Hamirpur and Una districts lie in the middle and lower Siwalik Himalaya while Kangra valley is enclosed between the middle Siwaliks and the Dhauladhar range of Western Himalaya. Siwalik sediments contain, in general 3–10 ppm of uranium which is much higher than the world average of 2.1 ppm in greywackes and 1.5 ppm in arkoses. 15 Uranium anomalies were also reported in river waters flowing through Western Himalaya. 16,17 Due to this compelling evidence of high radioactivity reported in soil-gas and water channels of H.P., it has become obligatory to undertake this survey for indoor radon monitoring for estimation of radiation inhalation dose delivered to inhabitants due to radon/thoron and their progenies. Experimental technique We have used twin chamber dosimeter cups to measure the concentration of radon and thoron separately (Fig. 1). Plastic detector (LR-115 type II) films 2.5 6 2.5 cm 2 were exposed in these cups, the latter being obtained from Environment Assessment Division, BARC, Trombay. Each cup has two chambers, each with a height of 4.5 cm and diameter 6.2 cm. The detectors were fixed at the bottom of each chamber, the mouth of one chamber being covered with glass fibre filter paper and the other with a semi-permeable membrane. These cups also have provision for exposing the detector in bare mode on the outer side of the cup. The detector that is placed in the chamber covered by a membrane records alpha tracks due to radon ( 222 Rn), the membrane only allowing radon to pass through it, suppressing the thoron to less than 1%. The other detector, covered with filter paper, records tracks due to alpha particles from radon and thoron. The bare detector records the tracks due to alpha particles from radon, thoron and their progeny and is used to determine radon and thoron progeny concentration in mWL ; . From this data the inhalation dose due to radon, thoron and their progeny can be calculated. JEM: 76271_4.3d 9/11/01 12:19:08 Rev 6.06e/W (Aug 31 2000) The Charlesworth Group, Hudds 01484 517077 Fig. 1 Rn–Tn discriminating decimeter. DOI: 10.1039/b107793q J. Environ. Monit., 2001, 3, 1–5 1 This journal is # The Royal Society of Chemistry 2001