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ORIGINAL ARTICLE
Year : 2018  |  Volume : 41  |  Issue : 3  |  Page : 136-142  

Studies on ambient gamma dose rate and enrichment of radon, thoron, and progeny concentration in various types of dwellings and outdoor environments of Kalliasseri, Kannur district, Kerala


Department of Studies and Research in Physics, Payyanur College, Kannur University, Kannur, Kerala, India

Date of Submission24-Apr-2018
Date of Decision03-Aug-2018
Date of Acceptance24-Sep-2018
Date of Web Publication19-Nov-2018

Correspondence Address:
Dr. V Prakash
Department of Studies and Research in Physics, Payyanur College, Kannur - 670 327, Kerala
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/rpe.RPE_29_18

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  Abstract 


The major contribution of natural background radiation exposure comes from radon, thoron, and their progeny. The activity concentration of these radionuclides depends on various factors, and the concentration level varies from place to place. The indoor concentration in dwellings depends mainly on the materials used for building construction and ventilation patterns. In the present study, an attempt is made to estimate the indoor and outdoor radon, thoron, and their progeny concentration in various types of dwellings and outdoor environments of Kalliasseri Panchayat, Kannur district, Kerala. 222Rn and 220Rn measurements were carried out using LR-115 type II-based pinhole cup dosimeters. Indoor and outdoor gamma exposure rate measurements were also carried out in all the dwellings using scintillation-based microradiation survey meter. The average concentrations of radon and thoron were estimated in about 40 dwellings categorized according to the construction type. The seasonal variation in the enrichment of radionuclides concentration has also been studied. The respective radon and thoron progeny levels were also estimated. The average radon concentration from the present study was well within the action level (200 Bq/m3) recommended by the International Commission of Radiological Protection (ICRP). The annual effective doses due to indoor radon and thoron were within the action level 3–10 mSv/y as suggested by the ICRP.

Keywords: Annual effective doses, gamma dose rate, pinhole cup dosimeter, radon, thoron


How to cite this article:
Mahamood K N, Prakash V. Studies on ambient gamma dose rate and enrichment of radon, thoron, and progeny concentration in various types of dwellings and outdoor environments of Kalliasseri, Kannur district, Kerala. Radiat Prot Environ 2018;41:136-42

How to cite this URL:
Mahamood K N, Prakash V. Studies on ambient gamma dose rate and enrichment of radon, thoron, and progeny concentration in various types of dwellings and outdoor environments of Kalliasseri, Kannur district, Kerala. Radiat Prot Environ [serial online] 2018 [cited 2018 Dec 10];41:136-42. Available from: http://www.rpe.org.in/text.asp?2018/41/3/136/245793




  Introduction Top


Indoor air quality is the most important issue nowadays because most individuals spend 90% of their time indoors. There are many pollutants that can deteriorate indoor air quality; however, radon and its progeny are a major pollutant for this and are an important global problem of radiation hygiene.[1] The indoor radon and thoron concentration varies with the types of construction material used in the buildings, ventilation patterns, lithology, and altitude in addition to the seasonal and diurnal variations.[2],[3],[4] The UNSCEAR (2000) estimates that 50% of the total dose received by the population is due to airborne radon and its daughter products and is considered the second leading cause of lung cancer after tobacco.[5]

Due to these reported adverse health effects of inhaled radon, thoron, and its progeny, many researchers have reported on the values of indoor radon in various parts of the world and India, as well.[6] On the basis of geological and geophysical characteristics, the coastal region of Kerala is expected to have higher concentration of radon. Kalliasseri Panchayat of Kannur district in the northern region of Kerala was selected for the present investigation as many of the occupants in this region are suffering from various types of cancers including lung cancer. Hence, the studies regarding concentration of radon, thoron, and their progeny in the area assume great significance. The significant health risks/adverse health effects due to the enrichment of these radionuclides concentration are a matter of serious concern nowadays. Given the above, estimation of indoor and outdoor radon, thoron, and their progeny concentration was carried out in the study area using newly developed pinhole-based radon thoron dosimeter with single entry face. The seasonal variation of enrichment of radionuclides concentration in different types of dwellings was also studied. The gamma absorbed dose (AD) rate and annual effective dose (AED) due to radon/thoron were also estimated to understand the amount of radiation exposure received by the population.

Geography of the study area

The selected area Kalliasseri is one of the Panchayats in Kannur district of Kerala which is situated in the southwest coast region of India. The exact coordinates of Kalliasseri are 11.9655°N and 75.3254°E covers an area of 15.37 km2. The Indian census data for the year 2011 reveal that the selected region has total population of about 32000. The temperature variation of this area is from 20°C (during December and January) to 35°C (during April and May).


  Materials and Methods Top


Micro radiation survey meter

In the selected areas, indoor and outdoor gamma radiation level measurements were made using Micro-R Survey Meter UR-705. It is designed around integrally coupled 1” × 1” NaI (Tl) Scintillator to a 1 1/2” photomultiplier tube and can measure and display dose rates in the range of 0–10000 μR/h on a dot-matrix LCD. The measurements were made using standard protocol, i.e., in the air at a distance of 1 m above the ground. For every location, about 15 readings were taken for indoor and outdoor separately. The gamma exposure rate measured in μR/h was converted into AD rate nGy/h using the conversion factor of 1 μR/h = 8.7 nGy/h, which can be derived from the definition of Roentgen.[7]

Pinhole-based 222Rn- 220Rn dosimeter

LR-115 type II detector-based pinhole dosimeter [Figure 1] technique was used for the measurement of radon, thoron, and their progenies. Dosimeter is a cylindrical plastic chamber consists of two equal compartments separated by a central disc, each compartment having a length of 4.1 cm and radius 3.1 cm. Four pinholes, each having a length of 2-mm and 1-mm diameter, are made on this circular disk to discriminate 220Rn. The dosimeter has only one entrance through which the gas enters the first chamber, namely, “radon + thoron” compartment through a 0.56-mm glass fiber filter paper and subsequently diffuses to the second chamber called “radon” chamber cutting off the entry of thoron into this chamber because of its very short half-life of 55.6 s compared to that of radon (3.825 days). The LR-115 solid state nuclear track detector films are fixed at the end of each compartment. The dosimeters were suspended indoor overhead on the ceiling at the minimum height of 1.5 m from the ground and at least 10 cm away from any wall surface for about 4 months.
Figure 1: The schematic diagram of pinhole-based twin cup dosimeter representing radon and radon + thoron chambers

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After exposure, LR-115 films have been retrieved from the dosimeters and were chemically etched using 2.5 N NaOH solution at a temperature of 60°C for 90 min without stirring. After etching, films were washed in distilled water and dried in open air. The sensitive layer of the films was carefully peeled off from their plastic bases and has been pre-sparked at 900 V before counting and counted using spark counter (model PCI-SC1) at operating voltage 450V. During counting, the films were placed on the spark head in such a way that the position of the film does not change after presparking. The track density obtained was converted into gas concentration (Bq/m3) using calibration factor.[2]

Analysis

Calculation of 222Rn and 220Rn gas concentrations

The radon (Cr) and thoron (Ct) gas concentrations in Bq/m3 can be determined using the relations as follows:



Where, T1 and T2 are the track densities observed in radon and radon + thoron chambers, respectively. Kr is the calibration factor of radon in radon chamber. For radon, Kr = 0.0172 tr. cm−2 d−1 per Bq m−3, d is the number of days of exposure time, Kt is the calibration factor of thoron in radon + thoron chamber. For thoron, Kt = 0.010 tr. cm−2 d−1 per Bq m−3.[2]

Calculation of potential alpha energy concentration values of 222Rn/220Rn

The radon progeny level or potential alpha energy concentration due to radon (PAECr) in mWL was calculated following the equation:



Where, Fr is equilibrium factor for radon having the value 0.4.[8]

The thoron progeny level or PAECt value (in mWL) was calculated following the equation:



Where, Ft is equilibrium factor for thoron having the value 0.1[9]

Calculation of annual effective dose due to 222Rn and 220Rn

The AEDs (in mSv) due to the exposure to radon, thoron, and their progeny in the dwellings of the study area were calculated using the following relations:[10],[11]

AED from radon and its progeny

= Cr (Bq m−3) × 0.46 × 7000 h × 9 nSv (Bq h m−3)−1

AED from thoron and its progeny

= Ct (Bq m−3) × 0.09 × 7000 h × 40 nSv (Bq h m−3)−1

Calculation of annual effective dose rate

The AED resulting from AD attributed to gamma-ray emission from the radionuclides (226Ra, 232Th, and 40K) is obtained using the formula:[12]

AEDIn = ADIn × DCF × OFIn × T

AEDOut = ADOu × DCF × OFOu × T

Where AED is the annual effective dose equivalent (mSv/y). DCF is the dose conversion factor for both indoor and outdoor and is same and is 0.7 Sv/Gy,[13] T (hr) is time to convert from year to hour (8760 h), and OFIn and OFOu are the occupancy factor for indoor (0.8) and outdoor (0.2) effective dose, respectively.


  Results and Discussions Top


Variation of gamma dose rates

The range of indoor and outdoor gamma dose rate is 6–14 μR/h with an average value of 8.86 ± 2.73 μR/h and 4–7 μR/h with an average value of 5.41 ± 1.16 μR/h, respectively. The maximum value of indoor gamma exposure rate was found in granite houses with concrete ceiling. The outdoor gamma exposure rates were almost the same in all the locations under the study area. The variation of indoor and outdoor gamma exposure rates in different types of dwellings is shown in [Figure 2].
Figure 2: Indoor and outdoor gamma exposure rates in different types of dwellings

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Variation of radon, thoron, and their progeny concentrations

The measurements of indoor and outdoor radon, thoron, and their progeny concentrations were carried out in about 40 dwellings for 1 year from June 2016 to May 2017. The whole year had been divided into three seasons (rainy, winter, and summer), and measurements were done to understand the seasonal variation of radionuclides concentration. Newly built as well as old houses with good and poor ventilation rates have been selected for the present investigation. It is obvious that the gas concentration inside the houses is governed mainly by their exhalation and ventilation parameters. Keeping this in mind, the dwellings selected were categorized based on construction materials that have been used to build the dwellings giving emphasis on flooring, which is one of the major sources of the content of radioactivity. The categories of dwellings are concrete floor with concrete roof, red oxide floor with wood and rooftile roof, ceramic tile floor with concrete roof, cemented floor with asbestos roof, and granite floor with concrete roof. Pinhole-based dosimeters have been placed in indoor overhead and their corresponding outdoor environment. Radon and thoron levels in each category of houses were compared, and the seasonal variations have been studied in each selected house.

The mean values of indoor and outdoor radon and thoron concentrations in the dwellings in three different seasons of a calendar year from the study area are given in [Table 1]. It can be seen that indoor radon concentration varies from 49.38 Bq/m3 to 157.43 Bq/m3 with a mean value of 105.23 ± 35.45 Bq/m3 and indoor thoron concentration varies from 40.56 Bq/m3 to 224.28 Bq/m3 with a mean value of 127.73 ± 64.44 Bq/m3. The outdoor radon concentration varies from 41.43 Bq/m3 to 115.89 Bq/m3 with a mean value of 77.54 ± 22.28 Bq/m3, and outdoor thoron concentration varies from 4.44 Bq/m3 to 149.89 Bq/m3 with a mean value of 56.63 ± 45.59 Bq/m3. The measured values of radon concentrations were within the reference level recommended by the International Commission of Radiological Protection (ICRP) (100–300 Bq m−3).[14] The level of thoron concentration in indoor air is higher than the world average value (10 Bq m−3).[10]
Table 1: Seasonal variation of indoor and outdoor radon and thoron concentrations in different types of dwellings

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The observed annual average progeny concentrations of indoor and outdoor radon and thoron in different types of dwellings are tabulated in [Table 2]. The average indoor radon and thoron progeny concentrations lie in the range of 9.10–14.67 mWL with a mean value of 11.78 ± 2.52 mWL and 22.76–80.51 mWL with a mean value of 46.98 ± 2.84 mWL, respectively. The average outdoor radon and thoron progeny concentrations lie in the range of 7.70–11.08 mWL with a mean value of 8.65 ± 1.40 mWL and 9.19–51.53 mWL with a mean value of 23.07 ± 17.17 mWL, respectively. The annual average value of the radon progeny level in the indoor and outdoor was below the action limit of 21.50 mWL for Indian dwellings.[8]
Table 2: Average indoor and outdoor radon and thoron progeny concentrations and annual effective dose due to indoor radon and thoron

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Seasonal variation

The seasonal variation of indoor and outdoor radon and thoron concentration is shown in [Figure 3] and [Figure 4], respectively. It can be observed that the average value of radon is found to be higher in the winter season and lower in the summer season. This may be due to poor ventilation conditions inside the houses that build more radon gas inside the dwellings during winter. The indoor gas concentration builds up due to low pressure inside the houses than outside, which tends to suck in radioactive isotopes from the building materials, soil, and floor through cracks or holes in walls and floor.[15] However, lower level of radon concentration in summer may be due to increased ventilation conditions inside the houses which leads high air exchange between indoor and outdoor environment.
Figure 3: Seasonal variation of indoor and outdoor radon concentration in different types of dwellings

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Figure 4: Seasonal variation of indoor and outdoor thoron concentration in different types of dwellings

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On the other hand, the average value of thoron concentration was found higher in the winter season and lower in the rainy season. Indoor thoron concentration may not influence by ventilation conditions due to its very short half-life. Thoron has very short diffusion length, and it cannot travel longer distances before it decays. Irrespective of this, the average thoron concentration has been found to be higher in the winter season like radon concentration. The lower average thoron concentration in rainy season may be because during the rainy season, the soil capillaries are mostly filled by water so that thoron cannot escape easily from these capillaries due to its very short half-life.[16]

The outdoor radon concentration was found to be higher in the winter and rainy seasons and lower in the summer season. However, the level of average outdoor concentration of radon was found almost the same in differently selected environs of each season. On the other hand, the average outdoor thoron concentration was found to be higher in the winter season and lower in the rainy season. It can be seen that the level of outdoor thoron concentration was almost equal in all selected different environs of each season.

Dependence of radon, thoron, and progeny on different types of dwellings

The concentrations of radon, thoron, and their progenies in the selected five different categories of houses were compared for all the seasons. The results show that comparatively higher indoor radon and concentration was shown by houses with granite floor and concrete roof. This may be due to higher concentration of 226Ra in granite.[17] Lower average radon concentration was shown by houses of cemented floor with asbestos roof, and lower average thoron concentration was shown by houses with red oxide floor and wood and rooftile-mixed houses. This may be because the cemented and red oxide coating on the floor reduces the entry of radon and thoron. Moreover, wooden roof and wall are not the sources of radon and thoron comparatively reduces the concentration of radon and thoron in such type of houses.

Comparison of annual effective doses due to radon, thoron, and gamma absorbed dose rates

The estimated values of annual average concentrations of radon and thoron in the selected houses were used to determine the AEDs due to radon and thoron. The indoor AED due to radon and its progeny received by the residents in dwellings found to vary from 2.43 to 3.93 mSv with a mean value of 3.16 ± 0.68 mSv and AED due to thoron and its progeny found to vary from 1.58 to 5.58 mSv with a mean value of 3.26 ± 1.72 mSv [Table 2]. These are within the action level limit (3–10 mSv/y) recommended by the ICRP.[18]

The average values of measured gamma exposure rates in indoor and outdoor have been calculated to estimate gamma AD rates as well as AED rates and are presented in [Table 3]. The average AD rate of indoor and outdoor environment found to vary from 56.55 to 116.58 nGy/h with a mean value of 74.66 ± 23.89 nGy/h and from 34.8 to 57.86 nGy/h with a mean value of 46.76 ± 9.77 nGy/h, respectively. The average values of AD for indoor are slightly higher, and due to outdoor gamma, it is within the recommended level (60 nGy/h) by the UNSCEAR.[10] The average values of indoor and outdoor AED rate (AEDIn and AEDout) were also calculated to compare with AED due to radon and thoron. AEDIn observed to vary from 0.28 to 0.57 mSv/y with a mean value of 0.37 ± 0.12 mSv/y and AEDout varied from 0.04 to 0.07 mSv/y with a mean value of 0.057 ± 0.01 mSv/y are shown in [Table 3].
Table 3: Average indoor and outdoor gamma exposure rate, absorbed dose and annual effective dose rate

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[Figure 5] shows comparison of AED due to indoor radon, thoron, and gamma exposure levels. It can be seen that indoor AED due to indoor radon and AED due to indoor thoron did not always correspond to indoor background radiation levels. It can be suggested that radon and thoron concentration contributes little to the gamma radiation level since radon and thoron concentrations are mainly due to alpha emitters. The higher values due to radon and thoron may be the use of uranium and radium-rich building materials for construction and poor ventilation conditions. The low background radiation level may be attributed to the lithology and altitude of the selected region.
Figure 5: Comparison of annual effective dose due to indoor radon, thoron, and gamma exposure. AEDR: Annual effective dose due to indoor radon, AEDT: Annual effective dose due to indoor thoron, AEDIn: Indoor annual effective dose rate

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[Table 4] shows comparison of the present study with reported results in other parts of India. It can be seen that the concentration of radon and thoron in the present study was comparable with the values reported for other regions of India. It is clear that the concentrations of radon and thoron from the present study were slightly higher than those values reported in Palakkad,[22] Kerala.
Table 4: Comparison of indoor radon and thoron concentrations with other regions of the country (India)

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  Conclusion Top


The annual average values of indoor and outdoor radon/thoron concentrations observed in the present study were below the reference level recommended by the ICRP. The radon and thoron concentrations have been found to vary according to different seasons of a year. It is also observed that radon levels in the dwellings of Kannur district are comparable with radon levels of different parts of India. The estimated AED due to radon and thoron was compared with AED rate due to gamma radiation level and it can be seen that gamma exposure level does not contribute the concentration of radon/thoron. Results show that the radon/thoron levels in dwellings with granite flooring and concrete roofing are found to be higher than that of other types of houses. It may be due to higher concentration of 226Ra in granite. The present study indicates that contribution of radon and thoron to the indoor exposure may be significant in certain occasion. The study concludes that enrichment of radionuclides concentration has no contribution to the adverse health effects noticed among the occupants of the study area as the levels of radiation exposure are well within the ICRP suggested reference levels.

Acknowledgments

The first author wishes to acknowledge the University Grants Commission for providing Maulana Azad National Fellowship to carry out the research work. The authors are thankful to the residents of the dwellings for their cooperation during the field work.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4]



 

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