Natural radioactivity levels in indoor environment in India

Inhalation dose rate due to radon (222Rn) and its progeny contribute more than 50% of the natural background radiation exposure. International data base on 222Rn in the environment has been well documented in the periodic publications of UNSCEAR. In India, external gamma radiation levels have been well mapped by earlier measurement. Country wide survey of outdoor radiation levels using thermo luminescent dosimeters carried out has revealed a national average value of external gamma radiation dose as 775μGy/y including the cosmic ray component. Of this, 48.7% contribution is from 40K and the remainder is from the thorium (33.6%) and uranium series (17.7%). Good data base on the concentration of 238U, 232Th and 40K in geological materials is also exists for the country. Thus there exists a good data base on total external gamma radiation levels across the country. Since the contribution due to 222Rn and its progeny is estimated to be about 50% of the total natural background radiation, there is a need to supplement the external exposure with the inhalation dose component. As a part of the on going survey of mapping of the country for 222Rn, Bhabha Atomic Research Centre has initiated national survey of mapping the country for indoor 222Rn in dwellings around normal background radiation levels. Measurements in about 1772 dwellings of different types of construction scattered over 126 locations in India using cup with membrane mode dosimeter based on Solid State Nuclear Track Detectors (SSNTDs) were carried out. LR-115 Type-II, 12 μm thick (pelliculable) films were used as detector. 222Rn levels varied from 6.3 to 156.0 Bq m-3 with an overall geometric mean (GM) of 23.0 Bq. m-3 (GSD 2.61 ). Annual effective inhalation dose rate due to 222Rn and its progeny in Indian dwellings using UNSCEAR (2000) dose conversion factors gave a GM value of 0.61 mSv/y. Contribution due to 222Rn gas and its progeny; while that due to 222Rn progeny works out to be 86.5% of the total inhalation dose rate. Contribution due to 222Rn progeny works out to be 21.4 times that of 222Rn gas. Any possible relation between indoor 222Rn levels and dwelling types has been examined by comparing the relevant means statistically. Most significant difference is found between mosaic and wooden floor dwellings having plastered whitewash walls. Combination of whitewash walls and mosaic floors leads to high levels of 222Rn. In this paper the concentrations due to 222Rn levels in dwelling in normal background regions of the country are presented. Paper discusses the methodology used and the results obtained. Levels are also compared with the reported values in literature. (author)