---

Background: Decay products of radon and thoron present in indoor environment are the most important sources of radiation from natural sources which affect human beings, since general public spend at lest 80% of their time in indoor. Materials and Methods: Air samples were collected for measuring the concentration of radon and thoron daughter products from various indoor environments during four different seasons of the year from the Gudalore taluk of Nilgiri Biosphere using high volume air sampler. The concentration of decay products of radon and thoron were estimated using an alpha counter. Results: Studies have revealed that the annual average potential alpha energy concentrations of radon and thoron progeny in dwellings of Gudalore were recorded as 3.54 and 2.65 mWL respectively. It was also observed that the potential alpha energy concentration (PAEC) values for radon progeny undergo dramatic change with the change of season perhaps due to different aerosol contents in the air. The maximum 222Rn (3.93 mWL) and 220Rn (3.10 mWL) progeny concentrations were observed during winter months and the minimum values (222Rn 3.14 mWL and 220Rn 2.20 mWL) were observed during summer. Conclusion: Studies have shown that the dwelling with mud wall registered high value of radon and thoron progenies (5.49 mWL and 3.88 mWL). While low values were observed in dwellings with vinyl floor. High concentrations of radon and thoron progeny were observed between 3.00 to 5.00 Hrs, while low values were observed at 14.00 hrs. Studies suggest that radiation emission from radon and thoron daughter in the study area were remained well below the recommended level. Iran. J. Radiat. Res., 2010 8 (3): 149-154