Residential Radon Concentrations in Kathmandu Valley Using Solid State Nuclear Track Detectors

Measurement of radon is very important in dwellings because of its radiological impact on public health. Radon contributes more than half of the total ionizing radiation dose. It is known from the recent surveys in many countries that radon is the second cause of lungs cancer after smoking. In this context, we have measured radon ( 222 Rn) concentration in different dwellings of Kathmandu valley, Nepal. The time integrated method using LR-115, type II plastic track detectors, was employed for the measurement based on Solid State Nuclear Track Detector (SSNTD). In addition, radon concentrations in the bedroom and kitchen were also measured. The overall concentration of radon in Kathmandu valley varied from 8±2 to 787±134 Bq/m 3 with the average value of 80±15and annual effective dose varied from 0.14 to 13.54 mSv per year. The radon concentration was found more in the dwellings of highly urbanized areas and in the poor ventilated dwellings of Kathmandu Valley. Introduction Radon and its progeny constitute the most important natural radiation exposure not only in mining but also in many dwellings. After smoking, radon represents the second most important cause of developing lungs cancer (Szacsvai, 2013; UNSCEAR, 1994; IAEA/AQ/33, 2013).The main sources of radon are soil and rocks, however, it is present in trace amounts almost everywhere because of its parent radioactive element uranium which is commonly found in the earth’s crust. Radon belongs to the noble gas column in the periodic table with a fairly long halflife of 3.8 days. Three natural isotopes of radon occur; Radon ( 222 Rn), Thoron ( 220 Rn), and Actinon( 219 Rn) emerging from the radioactive decay of Uranium ( 238 U), Thorium and the Actinium series respectively (Sathish, 2011). Radon emanates mainly by diffusion processes from the point of origin following alpha decay of 226 Ra in underground soil and water, building materials used in the construction of floors, walls, ceilings, natural gas used for cooking, etc. The concentration of radon in the atmosphere varies depending upon the place, time, height above the ground and meteorological conditions (Kant, 2004). Generally, all building materials contain certain amount of uranium and radium. So the exhalation of radon from these materials to the inside of the house can be a source of residential radon. Outdoor air can also play a role for the radon entering inside the dwellings through open doors and windows, cracks and fissures in the buildings, etc. (Ahmed, 1994). Also, the concentration of radon and its decay products show large fluctuations in the indoor atmosphere due to the variations of temperature, pressure, nature of building materials, wind speed, occupants’ behavior, etc. (Al-Khalifa, 2006). When radon gas is inhaled, densely ionizing alpha particles emitted by deposited short-lived decay products of radon ( 218 Po and 214 Po) can interact with biological tissue in the lungs and disrupt the DNA of these lung cells. The damaged DNA is potential enough to lead to cancer. This DNA damage, associated with radon, can occur at any level of exposure because a single particle can genetically damage a cell (Mehra, 2006; BEIR VI 1999; WHO 2009). It has been pointed out that indoor radon exposure is also tentatively linked with the risk of leukemia and certain other cancers, such as melanoma and cancers of the kidney and prostate ( Henshaw, 1990). Keeping the radiation hazards of radon in mind, it is quite important to make a systematic study of the indoor radon concentration. For this purpose, radon measurements have been carried out in a number of dwellings of Kathmandu valley. The nuclear track detector technique a fairly reliable method for the integrated and long term measurement of indoor radon activity ( Subba Ramu, 1992). In this work, we have used the SSNTD technique for the assessment of indoor radon ( 222 Rn) and its progeny concentration. Materials and Methods Study Area: Kathmandu Valley at a Glance Kathmandu valley is comprised of three different districts; Kathmandu, Bhaktapur and Lalitpur. It lies between the latitudes 27o 32’ 13” and 27o 49’ 10” north and longitudes 85o 11’ 31” and 85o 31’ 38” east. It is located around1, 300 meters above sea level. The climate of Kathmandu valley is sub-tropical cool temperate with maximum of 35.6°C in April and minimum of –3°C in January and 75% annual average humidity. The average rainfall is 1,400mm, most of which falls during June to August (Dangol, 2009). The Kathmandu valley is surrounded by the high rising mountains such as Shivapuri (2,732 m) in the north and Phulchoki (2,762 m) in the south. The rugged topography of the mountains with steep slopes reflects the geological structure of the valley. The basin is in the middle part of the lesser Himalaya, and bounded by the hill ranges Mahabharatlekh to the south and Shivapurilekh to the north (Upreti, 2001).The surface of Kathmandu valley is generally broad and almost flat except towards the boundaries of the valley, where rivers are deeply incised. Well developed terraces, formed by erosion from rivers, are common in the valley. The Kathmandu valley infilling consists of three million year-old fluvial and lacustrine sediments, consisting mainly of gravel, sand, silt, clay, peat, lignite and diatomaceous earth, etc., delivered mainly from the mountains in northern parts of the basin. Mines and minerals found in Kathmandu Valley are quartzite, dolomite, pegmatite, gneiss, schist, slates, limestones and marbles. The soil of the basin of Kathmandu valley is mainly alluvial soil, residual soil, and alluvial fan deposit ( Shrestha, 2004).