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ORIGINAL ARTICLE
Year : 2016  |  Volume : 39  |  Issue : 1  |  Page : 38-43  

Estimation of indoor and outdoor effective doses and lifetime cancer risk from gamma dose rates along the coastal regions of Kollam district, Kerala


Department of Physics, Center for Advanced Research in Physical Sciences, Fatima Mata National College (Autonomous), Kollam, Kerala, India

Date of Web Publication1-Jul-2016

Correspondence Address:
P J Jojo
Department of Physics, Center for Advanced Research in Physical Sciences, Fatima Mata National College (Autonomous), Kollam, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.185180

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  Abstract 

The exposure of human beings to ionizing radiation from natural sources is a continuously inescapable feature of life on earth. Direct measurement of absorbed dose rates in air has been carried out in many countries of the world during the last few decades. Such investigations are useful for the assessment of public dose rates. Indoor and outdoor gamma dose rates were evaluated along the coastal regions of Kollam district, Kerala, through direct measurement using portable gamma dosimeter, and analysis of soil sample for activity of 238 U, 232 Th, and 40 K concentration was carried out using gamma spectroscopy. Indoor and outdoor exposure rates, the annual effective dose (AED), and lifetime cancer risk of residents along the coastal regions of Kollam district, Kerala, were evaluated. The reduction coefficients were also calculated for the region. The mean indoor effective dose due to background gamma along the coastal region of Neendakara panchayath was found to be 7.56 mSvy−1 which is larger as compared with the worldwide average of the AED of 0.48 mSv y−1  and the outdoor mean effective dose of 4.83 mSvy−1 . Estimated excess lifetime cancer risk (ELCR) from indoor AED equivalent ranges from 22.56 to 26.46 × 10−3 and ELCR from outdoor ranges from 14.95 to 16.65 × 10−3 . Excess average lifetime cancer risk estimate from all the values is found to be 20.56 × 10−3 , which is larger compared with the resulting worldwide average 0.25 × 10−3 .

Keywords: Effective dose, excess lifetime cancer, gamma dose, reduction coefficient


How to cite this article:
Monica S, Visnu Prasad A K, Soniya S R, Jojo P J. Estimation of indoor and outdoor effective doses and lifetime cancer risk from gamma dose rates along the coastal regions of Kollam district, Kerala. Radiat Prot Environ 2016;39:38-43

How to cite this URL:
Monica S, Visnu Prasad A K, Soniya S R, Jojo P J. Estimation of indoor and outdoor effective doses and lifetime cancer risk from gamma dose rates along the coastal regions of Kollam district, Kerala. Radiat Prot Environ [serial online] 2016 [cited 2019 Sep 15];39:38-43. Available from: http://www.rpe.org.in/text.asp?2016/39/1/38/185180


  Introduction Top


The natural terrestrial gamma radiation dose rate is an important contribution to the average total dose rate received by the world's population. The knowledge of radionuclide distribution and radiation levels in the environment is important for assessing the effects of radiation exposure to human beings. [1] There are two main contributors to natural radiation exposures: High-energy cosmic ray particles incident on the earth's atmosphere and radioactive nuclides that originated in the earth's crust and are present everywhere. [2] The cosmic sources include radiations from extra-terrestrial origin. Terrestrial background ionizing radiations are essentially derived from 40 K, and radionuclides belonging to 238 U and 232 Th series present in the earth crust. [2],[3] These radionuclides are common in rocks and soil, water, plants, and air that make up our planet and in our building materials. [2],[4] The variation of terrestrial radiation is typically larger than that of cosmic radiation. [3] There are regions in the world where the outdoor terrestrial radiation exceeds substantially the average value due to the enrichment of certain radioactive minerals leading to the formation of what are known as high background areas. The presence of high background areas has been reported in several countries such as China, Iran, Germany, USA, Brazil, and India. [5]

In general, the health impact of exposure to radon ( 222 Rn) inhalation by humans in indoor environment is a major public concern worldwide. The exposure is due to the emanation of radon gas from the decay chains of radioactive thorium ( 232 Th) and uranium ( 238 U), which are present in soil layers and indoor construction materials especially granite. [6] The dose rate depends on the geology and geographical conditions and appears at different levels in the soil of each region of the world. [7],[8],[9] Radon enters a home through the lowest level in the home that is in contact with open ground such as cracks in solid foundations, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, and the water supply. [10] The indoor radiation environment differs from outdoors. The International Atomic Energy Agency (IAEA) provides the reduction coefficient which is the ratio of indoor and outdoor ambient dose equivalent rates for evaluating indoor exposure doses. [9] The exposure doses of residents are evaluated using the reduction coefficient for radiation levels in houses and buildings. The provided reduction coefficient is 0.4 with a range of 0.2-0.5 for wooden houses; for concrete and brick houses, the coefficient is 0.2 with a range of 0.04-0.4. These values are evaluated based on the European house style and radioactive contamination (IAEA, 2000).

The main objective of this study was to measure the indoor and outdoor radiation dose rates and calculate annual effective dose (AED) received by the people along the coastal regions of Kollam district, Kerala. Excess lifetime cancer risk (ELCR) was calculated for each designated location by standard method. [11] The values obtained were compared with that of the world average as well as with the other values reported in literature. A brief review of radiological survey in some areas as carried out within and outside the country by some researchers with result obtained is shown in [Table 1].
Table 1: A review of radiological survey in some areas as carried out within and outside the country

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  Materials and methods Top


Selection of the measurement sites

The gamma background radiation measurements were performed both indoor and outdoor at 10 designed locations along the coastal regions of Kollam district.

Dose rate measurement

The ambient gamma absorbed dose rates were measured in all sampling locations using geometric mean (GM) tube-based gamma dosimeter (POLIMASTER PM 1405). This device can detect gamma rays in the energy range of 0.05-3 MeV and the dose rate measurement range is 0.1 μSv/h to 100 mSv/h. The gamma radiation levels were measured both inside and outside the dwellings at 1 m above the ground. About 25 readings were taken at different points in each location, and the GM was considered as the representative value of the gamma dose rate for the location [Figure 1].
Figure 1: Map shows the study area along the coastal region of Kollam district

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Calculation of indoor and outdoor annual effective dose

Absorbed gamma dose rates were used to calculate the AED received people living in the surveyed houses and its environment. The AED was computed using the indoor occupancy factor of 0.8 and the outdoor occupancy factor of 0.2. [12] The AED was determined using the following equations: [6]

AED Outdoor (mSvy−1 ) = Absorbed dose rate (nGy/h)

× T (h) × 0.7 Sv/Gy × 0.0.2 (1)

AED Indoor (mSvy−1 ) = Absorbed dose rate (nGy/h)

× T (h) × 0.7 Sv/Gy × 0.0.8 (2)

where T is time in i for 1 year (8760 h). The dose conversion factor used is 0.7 Sv/Gy as suggested by UNSCEAR, 1993.

Excess lifetime cancer risk

ELCR was calculated using the equation: ELCR = AED × DL × RF

where DL is the average lifespan (70 years) and RF is risk factor (Sv−1 ) which is the fatal cancer risk per sievert. For stochastic effects from low-dose background radiation, ICRP 103 suggested the value of 0.057 for the public exposure (ICRP, 2007).

Reduction coefficient

The reduction coefficients were obtained by taking the ratio of the indoor and outdoor dose rates (the concept of reduction coefficient will be more meaningful if the outdoor background radiation is normal and indoor unusually high. This would normally show the radon buildup in the dwelling units. In the present case, outdoor background levels are high enough. Added to it, the shielding effect due to pucca floor, walls and ceiling is rather questionable. One should not, under such circumstances comment about it. If at all, the shielding effect needs to be discussed).

Estimation of absorbed dose from soil radioactivity measurements

A total of 20 surface soil samples were collected from Neendakara where gamma dose was measured. The activity concentrations of 238 U, 232 Th, and 40 K in the samples were determined by gamma spectroscopy using a NaI (Tl) detector.

From the values of activity concentrations of 226 Ra, 232 Th, and 40 K in soil, the absorbed dose rates were computed using the dose coefficients given in UNSCEAR [17] total gamma dose (D) due to the presence of 226 Ra, 232 Th and their daughter products and 40 K was computed using the equation: [17]

D (nGyh−1 ) = 0.462 C U + 0.604 C Th + 0.0417 C K where, C U , C Th , and C K are the activity concentrations of 226 Ra, 232 Th, and 40 K in soil (Bq/kg).


  Results and discussion Top


Gamma absorbed dose rate

The range, median, GM, and geometric standard deviation of indoor and outdoor gamma absorbed dose rates along the coastal region of Kollam district, measured using portable survey dosimeter, are presented in [Table 2]. The measurements show that the maximum indoor dose rate was observed in Kovilthottam and the minimum in Thangassery with respective values of 1.102 μSv/h and 0.216 μSv/h. Outdoor dose rate consists of terrestrial and cosmic radiations together. Wide variation was observed in gamma dose rates in different locations ranging from 0.21 to 1.34 μSv/h. The variations observed in gamma dose rates might be attributed due to the differences in soil composition. The result showed higher outdoor dose rates in Kovilthottam, Karithura, Neendakara and Azheekal, in comparison with the values reported by UNSCEAR 2000 from different countries as global average of 59 nGyh−1 in the range of 18-93 nGyh−1 . [2] Average indoor gamma dose rates for Kovilthottam, Karithura, Neendakara, and Azheekal were found in the range 0.89-1.34 μSv/h. The values are higher than the mean absorbed dose rate that has been reported by UNSCEAR 2000 with mean of 84 nSv/h in the range of 20-200 nSv/h. [13] Wide variation was observed in indoor gamma dose rates quantified in different buildings ranging from 0.21 to 1.17 μSv/h and these differences might imply that the material used in construction of building differs in origin and consequently in strength of the radiation sources. Other speculation would be the differences in natural ventilation rate of buildings that would alter the concentrations of gamma emitting radionuclides inside dwellings. [14]
Table 2: Absorbed dose rate (μ Sv/h), annual effective dose (mSv/year), and excess lifetime cancer risk at the locations

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Indoor and outdoor annual effective dose

From the absorbed dose rate, the AED was computed using the indoor occupancy factor of 0.8 and the outdoor occupancy factor of 0.2 [15] and these are presented in [Table 2]. Average AED along the coastal regions of Kovilthottam, Karithura, Neendakara, and Azheekal ranges from 2.12 to 8.12 mSv/year. Based on the report of UNSCEAR, population-weighed average of effective environmental gamma dose rates due to indoor and outdoor along Kovilthottam, Karithura, Neendakara, and Azheekal are appreciably higher than the values estimated for world average and the people living along Kovilthottam, Karithura, Neendakara, and Azheekal regions receive an average two times (ranging from 1.3 to 2.7) higher environmental gamma radiation than the world population weighted average. The calculated AED values were found somewhat higher than the world average of 0.07 mSv/year (UNSCEAR, 2000) [Figure 2]. To estimate the AED, the conversion coefficient must be taken into account from the absorbed dose in air to the effective dose. Gamma radiation is less absorbed in children and infants resulting in a higher dose conversion coefficient (adults: 0.7, children: 0.8, and infants: 0.9). [2]
Figure 2: Annual effective dose (mSv/year) at different locations in comparison to world average annual effective dose

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Excess lifetime cancer risk

To assess the radiological risk, lifetime cancer risks were calculated from the AED values [Table 2]. The lifetime cancer risk calculated from indoor effective doses in all the residential houses ranges from 5.61 to 20.61 × 10−3 to while that from outdoor effective dose ranges from 1.18 to 14.12 × 10−3 . Notably, these values are higher than the world average of ELCR of 0.29 × 10−3[16] which require further studies for confirmation.

Indoor to outdoor dose ratio (reduction coefficient)

Reduction coefficient of 1.16, 1.016, 0.658, 0.839, and 0.745 corresponding to Thangassery, Sakthikulangara, Karithura, Kovilthottam, and Panmana, respectively, were obtained as listed in [Table 2]. These coefficients are slightly higher than 0.4 provided by the IAEA although the dose rate measurement was performed in European houses. The reduction coefficients in areas other than the centers of houses have never been provided by the IAEA. These reduction coefficients obtained at the several locations in each room are larger than those at the centers of the houses. The dose ratio depends on the window sizes. [17] In living rooms with large windows, the dose ratios are large because of the small shielding against photons entering the rooms. Walls are effective photon shields and in rooms with small windows. The ratio of indoor to outdoor gamma ray doses, in normal radiation background areas in India, is found to be approximately 1.2, particularly in houses which have tiled/cemented floors and concrete walls and ceilings. [18] In another study, Chougaonkar et al. [19] reported that the mean value of ratio was 0.8 for those dwellings which used construction materials such as bricks, cement, and masonry tiles, which were procured from a different region and these construction materials had less radioactivity content than the surrounding soil.

Estimation of dose from the radioactivity content of U, Th and 40 k in soil

The concentration of 226 Ra and 232 Th in the soil varied in the range of 171.2-445.8 Bq/kg and 306.2-2022 Bq/kg, respectively, with the corresponding GM values of 296.5 Bq/kg, and 1087 Bq/kg. From the results of 226 Ra and 232 Th, the total gamma dose rates in air were calculated. The absorbed dose from 238 U varied from 110.1 to 1051.2 nGy h−1 with a mean value of 750.1 nGy h−1 . The dose rate due to 232 Th varied from 306.1 to 1750.3 nGy h−1 with a mean value of 840.2 nGy h−1 .


  Conclusions Top


The present study has measured the indoor and outdoor gamma dose rates along the coastal regions of Kollam district using portable gamma ray survey dosimeter. From these values, AED was calculated by standard procedure for assessing ELCR of the population living along these regions. The indoor AED ranges from 1.32 to 7.12 mSv/year along the coastal regions of Kovilthottam, Karithura, Neendakara, and Azheekal, which is large as compared to the world wide average of the AED 0.48 mSv. Estimated ELCR from indoor AED ranges from 10.29 to 20.14 × 10−3 along Kovilthottam, Karithura, Neendakara, and Azheekal, respectively, and ELCR from outdoor ranges from 4.45 to 14.15 × 10−3 recorded in the above regions. Resulting average of the ELCR is 12.25 × 10−3 , which is large as compared to the resulting worldwide average of 0.29 × 10−3 .

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.[20]

 
  References Top

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    Figures

  [Figure 1], [Figure 2]
 
 
    Tables

  [Table 1], [Table 2]


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