Environ Monit Assess (2014) 186:3581–3594 DOI 10.1007/s10661-014-3640-x

Simultaneous measurements of radon and thoron, and their progeny levels in dwellings on anticlinal structures of Assam, India Debajyoti Barooah & Simi Barman & Sarat Phukan

Received: 13 September 2013 / Accepted: 13 January 2014 / Published online: 28 January 2014 # Springer International Publishing Switzerland 2014

Abstract Radon and thoron, and their progeny concentrations along with equilibrium factors for gas progeny and radiological risks to the residents have been measured in dwellings of Digboi and Mashimpur areas located on anticlines during the winter season. In this present investigation, twin-cup dosemeters fitted with LR-115 (II) nuclear detectors have been employed. The present work has shown that there exist considerable house-to-house variations in values with maximum values in mud houses and minimum values in assam type (AT) houses. It has been found that mean (and geometric standard deviations (GSD)) radon concentrations are 83.8 (1.3), 113.5 (1.1) and 157.2 (1.2) Bq m−3 in AT, reinforced cement concrete (RCC) and mud houses in Digboi area and 63.0 (1.1), 87.1 (1.4) and 182.1 (1.2) Bq m−3 in AT, RCC and mud houses in Mashimpur area, respectively. The overall mean radon concentrations in Digboi and Mashimpur are estimated to be 114.4 (1.4) and 100.0 (1.7) Bq m−3. The mean radon concentrations are found to be less than the lower reference level of 200 Bq m−3 of the International Commission on Radiological Protection (ICRP 2007). The thoron concentrations in Digboi area are estimated

D. Barooah (*) : S. Barman Department of Physics, Cotton College, Guwahati 781001 Assam, India e-mail: [email protected] S. Phukan Department of Geological Sciences, Gauhati University, Guwahati 781014 Assam, India

to be 31.1 (1.3), 50.8 (1.4) and 67.0 (1.6) Bq m−3 in AT, RCC and mud houses, respectively, whereas in Mashimpur area, the thoron concentrations are estimated to be 26.4 (1.3), 44.4 (1.3) and 77.7 (1.3) Bq m−3 in AT, RCC and mud houses, respectively. The mean annual effective doses in Digboi area are found to be 1.9 (1.3), 2.7 (1.2) and 4.1 (1.4) mSv y−1 in AT, RCC and mud houses, respectively, while in the case of Mashimpur area, the mean annual effective doses are found to be 1.5 (1.4), 2.2 (1.2) and 4.9 (1.3) mSv y−1 in AT, RCC and mud houses, respectively. Nevertheless, the obtained results are much lower than the upper reference level of 10 mSv (ICRP 2007). Keywords Radon . Thoron . Effective dose . LR-115 (II) detectors

Introduction Human beings have always been exposed to ionizing radiations from natural sources since the dawn of time. The major contribution of dose from natural radiation in normal background regions arises due to inhalation of alpha-emitting radon and thoron, and their progenies, which are ubiquitous in both indoor and outdoor environs (Porstendorfer 1994; UNSCEAR 2000). Worldwide average annual effective dose from the ionizing radiation sources is estimated to be 2.4 mSv of which about 1.275 mSv is due to radon exposure alone (UNSCEAR 2000; Rahman et al. 2008)

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Radon (222Rn) is a radioactive, colourless, odorless, tasteless, and almost chemically inert gas produced in the natural radioactive uranium (238U; half-life 4.5× 109 years) decay series. Compared to other noble gases, radon is the heaviest and has the highest melting and boiling points, critical temperature and pressure (Espinosa et al. 1991). It is found as the immediate decay product of radium (226Ra; half-life 1600 years) in the decay series. 222Rn has a half-life of 3.82 days and a mean life of 5.51 days and decays to its solid progeny 218 Po (RaA; half-life 3.1 min) by alpha emission of energy of 5.49 MeV (Fleisher 1997). On the other hand, the radon progenies 218Po, 214Po and 210Po decay by alpha emissions of energy of 6.00 MeV, 7.69 MeV and 5.30 MeV, respectively (Durrani 1993). Thoron gas (220Rn), the most important radioactive isotope of 222 Rn, is the immediate decay product of 224Ra (halflife 3.64 days) produced in the decay series of thorium (232Th; half-life 1.39×1010 years). 220Rn has a half-life of 55.6 s and a mean life of 80.2 s, and decays to its solid progeny 216 Po by alpha emissions of energy of 6.29 MeV (Fleisher 1997). The thoron progenies 216 Po, 212Po and 212Bi decay by alpha emissions of energy of 6.78 MeV, 8.78 MeV and 6.05 MeV, respectively (Fleisher 1997). As 238U and 232Th are very widely distributed in the Earth’s crust, radon and thoron are found in all types of rocks, soils, water, underground fluids and hydrocarbons (Abd-Elzaher 2013; Fleisher 1997; Barooah et al. 2011; Barooah and Phukan 2012; Segovia 1991; Sannappa et al. 2003). The main sources of indoor radon and thoron are building materials, soil and rocks underneath the building structures, water and energy sources (Abd-Elzaher 2013; Ielsch et al. 2001). Radon and thoron enter the atmosphere mainly by crossing the soil-air or building material–air interface. Radon usually enters into the built-up area from ground and under the structure through cracks in the basement walls, defective floor-wall joints, loosely fitted pipes constituting drainage and sewage systems (Khan 1991; Ramachandran et al. 1990; ICRP 1993). Local geology, meteorological and climatic parameters, building materials, ventilation rate (air change per hour), exhalation from ground and building materials, living style of the inhabitants and human activities influence the indoor radon levels causing a very wide range of concentrations (Ielsch et al. 2001; Akerblom and Mellander 1997; Jonsson 1997; Ramachandran and Subba Ramu 1990; Sannappa et al. 2003; Rydock et al. 2001; Singh et al. 2001; Singh et al.

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2008; Ramachandran et al. 1990; Subba Ramu et al. 1992). After inhalation, radon may cause significant damage to the delicate inner cells of the bronchioles, which may lead to the occurrence of lung cancer (Sevc et al. 1998). Radon has been established as a lung carcinogen (Lubin 2003; ICRP 1993; ICRP 2012; Field and Withers 2012; Field et al. 2000; Sevc et al. 1976; Stranden et al. 1988). Moreover, there is a correlation between the radon exposure and incidences of skin cancer, myeloid leukemia, chronic nephritis and renal sclerosis (Dwivedi et al. 1995). Since the early 1980s, many countries including India started working on measurements of indoor radon levels (Abu-Jarad and Fremlin 1981; Ramachandran et al. 2003; Carneiro et al. 2013). Unlike radon, there is a scarcity of data for indoor thoron and its progeny due to the general perception that its levels are negligible owing to short half-life and their insignificance in terms of inhalation dose. However, many studies in different countries and different international pooling studies show that there is risk of lung cancer and other health diseases from residential exposure to thoron and its progeny (WHO 2009; Eappen et al. 2008; Mayya et al. 1998; Steinhausler et al. 1994). Many researchers have already reported about the seasonal variations of indoor radon and thoron, and their progenies with the highest values during winter season of a year (Bhramanandhan et al. 2008; Ramola et al. 1998; Subba Ramu et al. 1988; Ramachandran et al. 1990; Sannappa et al. 2003; Virk et al. 1999; Singh et al. 2001; Sivakumar 2010; Rani et al. 2013). Moreover, the oil and gas structures affect the radon distribution, and an enhanced radon in soil is found to exist at the edges of the anticlinal structures (Barooah and Phukan 2012). As such keeping in view of health risks during winter season (December–February), we have measured indoor radon and thoron, and their progeny concentrations, equilibrium factors for radon and thoron progenies, inhalation dose rates, lifetime fatality risk factors and annual effective doses in 30 dwellings belonging to Digboi and Mashimpur areas of Assam, India, which are located on anticlinal structures by employing passive time-integrated technique and also studied their variations with different types of houses. This study aims to determine the dwellings in which exposure levels might be unduly high, so that the need for remedial actions could be assessed as per the standard guidelines (ICRP 2007). Moreover, this study would provide additional useful data on indoor radon concentrations in India.

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Geology of the study area Geology of Digboi The studied Digboi town is an oil-producing area. The Digboi oilfield is located in the Tinsukia district of Assam and bounded by 95°35′ to 95°43′E longitudes and 27°20′ to 27°25′N latitudes. The area forms the northeasternmost extension of the Tipam Hill, a foothill range of the Patkai Hills. This is one of the oldest oilfields in the world, and production started in the year 1889. The oilfield lies on an anticline immediately south of the Naga thrust, with a steeper northern flank with dips up to 90° and a gentler southern flank with variable dips reaching up to or even exceeded 60° at the western end of the anticline, but about 30°–40° in the eastern part (Mathur and Evans 1964). The Naga thrust is concealed by the alluvium in the area. Geologically, the anticlinal area exposes the Tipam Group (Miocene) of rocks. This is underlain by the Barail Group (Oligocene) of rocks in the southern side of the Naga thrust. On the immediate south of the Naga thrust, the sub-horizontal sedimentary formations of the Assam shelf are concealed under a thick sequence of alluvium, and the Tipam Group of rocks is situated at a depth of about 4 km from the surface. The Tipam Group of rocks, which is exposed in the Digboi anticline, is classified into the lower Tipam Sandstone Formation and the overlying Girujan Clay Formation with a transitional boundary. The Tipam Sandstone formation is composed of moderately coarse sandstone. The Girujan Clay Formation contains brightly coloured mottled clays, light coloured sandstones and mottled sandstones (Mathur and Evans 1964). Geology of the Mashimpur The studied Mashimpur area is located on Mashimpur anticline, which is a tightly folded doubly plunging anticline and is a part of the Cachar-Tripura-Mizoram fold belt of the Assam-Arakan basin. It is represented by a hill surrounded by valleys which represent synclines. The area is bounded by 92°36′–92°47′E longitudes and 24°40′–24°53′N latitudes. Geologically, the Mashimpur anticline exposes rocks of Neocene stratigraphic succession. The oldest rocks exposed around the anticline belong to the Surma Group of rocks (Miocene), which are exposed in the core part of the anticline. The rocks of the Tipam Group (Mio-Pliocene)

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flank the anticline on both sides in the central and southern part of the study area. The rocks of Dupitila Group (Plio-Pleistocene) are exposed in the southeastern corner of the study area, which is part of the associated syncline in the eastern flank of the Mashimpur anticline. In general, the valley areas surrounding the anticline, which represent synclines, are covered by alluviums with occasional patches of Older Alluvium (Pleistocene). The sedimentary succession of the study area is composed of sandstone, siltstone, mudstone, shale, mottled clay, laterite, unconsolidated boulder beds and alluvium (Fig. 1). Type of dwellings and materials used for construction For the present investigation, we selected living rooms of dwellings on ground floors only. The selected dwellings under study are divided into three types, viz. assam type (AT), reinforced cement concrete (RCC), and mud house. All the mud houses had earthen floors and bamboo walls with plaster and asbestos roofs in Mashimpur. But in the case of Digboi, the mud houses had thatched roofs. All the RCC houses had cemented floors and cemented walls in both study areas. On the other hand, the selected AT houses had cemented floors, bamboo walls with plaster and tin roofs in both the studied areas.

Experimental methods The long-term measurements of the indoor radon levels, by using solid-state nuclear track detectors (SSNTDs), are of clear relevance for quantifying human exposure in dwellings. In this research work, the assessment on the inhalation dose due to the indoor radon and thoron, and their daughter products was made by employing plastic twin-cup dosemeters, fitted with LR-115 (II) nuclear track detectors under natural conditions on a timeintegrated scale (Mayya et al. 1998). Each chamber of the plastic cup has a length of 4.5 cm and a radius of 3.1 cm (Fig. 2). The SSNTD films were cut into sizes of 2.5 cm×2.5 cm and were exposed in the bare mode, filter mode and membrane mode of the dosemeter cup. The bare detector (placed outside on the outer surface of the dosemeter) records the tracks formed by the alpha particles emitted from radon and thoron, and their progenies. In the bare mode, the detector film views a hemisphere of air of radius, at least 9.1 cm, the range of 212Po alpha, or 6.9 cm, the range of 214Po alpha in air (Subba

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Fig. 1 Map showing the study area

Ramu et al. 1992). The detector film placed in the compartment, covered with a semi-permeable membrane (latex) of 25-μm thickness, records tracks formed due to radon only. The membrane has a permeability constant of 10−8–10−7 cm2 s−1 and allows more than 95 % of the radon gas for diffusion and suppresses thoron gas to less than 1 % (Ramachandran et al. 1987). The glass fibre filter paper of thickness 0.56 mm in the other compartment of the dosemeter allows diffusion of both radon and thoron. No solid daughter product can get diffused into either of the two chambers.

Fig. 2 Schematic diagram of the twin-cup dosemeter

The dosemeter cups loaded with SSNTDs were suspended from the midpoint of the ceiling of the houses at a height of about 7 ft from the ground, and care was taken such that no surface (i.e. wall, ceiling or roof) was closer than 10 cm from the detectors. This avoided the errors due to tracks from the deposited activity on the nearby surfaces. The SSNTD used in this study was a 12-μm-thick alpha sensitive layer of red dyed strippable LR-115 (II) (Kodak cellulose nitrate), deposited on a 100-μm-thick non-etchable polyester base (cellulose acetate). The LR-115 (II) detector is free from ‘self plate-out effect’; i.e. tracks due to the alpha particles emitted from the daughter products deposited on the films are not registered. An LR-115 (II) detector is only sensitive to alpha particles having energies in the range of 1.9–4.2 MeV for revealing tracks as ‘through etched holes’. To enlarge the tracks, the retrieved detectors were chemically etched in 2.5 N NaOH solutions at 60 °C for 110 min under a mild stirring condition for uniform etching. After etching, the films were washed in distilled water properly, and then they were dried up in

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the laboratory conditions. Then, tracks were counted using a Leitz Optical Microscope of suitable magnification (×100) for at least 100 fields of view to obtain a representative value of track density of the through etched holes in terms of track per square centimetre, and counting tracks for large number of fields of view minimises the uncertainty due to counting errors (AbdElzaher 2013). The gas concentrations (CR for radon and CT for thoron) in the ambient atmosphere were determined from the observed track densities, using the following relations (Mayya et al. 1998): T1 CR ¼ d:K R

ð1Þ 0

CT ¼

T 2− d C R K R d KT

ð2Þ

where T1 and T2 are the track densities recorded in membrane and filter modes of the dosemeter, respectively. KR =K′R =0.019 track cm−2 Bq−1 m3 d−1, where KR is the calibration factor for radon in the compartment covered with membrane and K′R is the calibration factor for radon in the compartment covered with the filter paper (Barooah et al. 2011), and KT = 0.016 track cm−2 Bq−1 m3 d−1 is the calibration factor for the thoron in the compartment covered with the filter paper (Mayya et al. 1998). The duration of exposure (winter season) is d=90 days. Since the radon decay constant is far smaller than the usually encountered air change rates, radon may be assumed to be spatially uniform (Ramachandran et al. 2003). The activity fractions of the progeny (FR−A = CCR−A , etc.) are R governed by their wall loss rates (λfw =20 h−1 and λcw =0.2 h−1 for fine and coarse fractions, respectively) and ventilation rates (λV) through the following equations (Mayya et al. 1998): F R−A ¼

λR−A λR−A þ f A λwf þ ð1− f A ÞλCW þ λV

ð3Þ

F R−B ¼

F R−A λR−B λR−B þ f B λwf þ ð1− f B ÞλCW þ λV

ð4Þ

F R−C ¼

F R−B λR−C λR−C þ f C λwf þ ð1−f C ÞλCW þ λV

ð5Þ

In the above relations, fA =0.21, fB =0.025 and fC = 0.001 are the unattached fractions for the respective species, and λR−A, λR−B and λR−C are the decay constants for the respective radon progeny. Similarly, the equilibrium factors for thoron progeny can be obtained from the following relations: F T −B ¼

λT −B λT−B þ λCW þ λV

ð6Þ

F T −C ¼

F T−B λT −C λT −C þ λCW þ λV

ð7Þ

where λT−B and λT−C are the decay constants for the respective thoron daughter products. The bare track density T3 can be related to the concentrations of both radon and thoron gases as well as their progeny concentrations by the following equation: T 3 ¼ K B d ½ðC R þ C R−A þ C R−C Þ þ ð2C T þ C T −C ފ

ð8Þ

where KB =0.021 track cm−2 Bq−1 m3 d−1 is the bare mode calibration factor for all the alpha emitters (Mayya et al. 1998). CR−A and CR−C are the concentrations of alpha-emitting radon progeny (218Po and 214Po), and CT−C is the concentration of alpha-emitting thoron progeny (212Po) present around the dosemeter. The factor 2 that multiplies CT accounts for the short-lived 216Po. As such, the bare track densities are dependent on the ventilation rates, which represent the progeny fractions of both gases. Since the half-life of 220Rn is very short, it shows a non-uniform distribution inside the rooms. But the thoron decay products 212Pb and 212Bi, having longer half-lives, are distributed more or less uniformly inside the rooms. So, their activity fractions are related to a representative average thoron concentration (C T), instead of the measured CT by the following relations (Mayya et al. 1998): C T −B ¼ C T −B and C T−C ¼ C T F T −C

ð9Þ

where CT−B and CT−C, and FT−B and FT−C are the concentrations and activity fractions (with respect to average thoron) of 212Pb and 212Po, respectively. The progeny working levels were determined using the following

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relations (Ramachandran et al. 2003; Mayya et al. 1998): W LR ¼ ¼

W LT ¼

CR F R 3700

ð10Þ

C R ð0:104 F R−A þ 0:518 F R−B þ 0:37 F R−C Þ 3700

CT F T C T ð0:908 F T −B þ 0:092 F T −C Þ ¼ ð11Þ 275 275

where FR and FT indicate the equilibrium factors for the radon and thoron progenies respectively, which determine the degree of equilibrium between the gas and its short-lived progeny. Then, the inhalation dose rates in mSv y−1 assuming 7,000 h in indoors was found out using the following formula (UNSCEAR 2000; Mayya et al. 1998): h i E ¼ 7000  10−6 ð0:17 þ 9 F R ÞC R þ ð0:11 þ 32 F T ÞC T

ð12Þ Also, dose in terms of working level month (WLM) was calculated. The annual exposures have been obtained through radon progeny by using WLM=36×WL (Zubair et al. 2011). Lifetime fatality risk has been estimated by using a conversion factor of 3 × 10−4 WLM−1 (ICRP 1993). Finally, the annual effective dose in mSv y−1 was obtained by the conversion of potential alpha energy concentration (PAEC) using a conversion factor of 3.9 mSv WLM−1 for radon and 3.4 mSv WLM−1 for thoron daughters, respectively. A computer program in FORTRAN was developed in order to carry out these computations.

Results and discussion The measured values for the indoor radon and thoron, and their progenies, equilibrium factors, lifetime fatality risks and radiological doses in 30 dwellings of Digboi and Mashimpur areas during winter season (3 months, i.e. December–February) with their geometric means (GM), geometric standard deviations (GSD) and standard deviations (SD) are presented in Tables 1, 2, 3, and 4. Variations of gas concentrations with different house types in Digboi and Mashimpur are represented in Figs. 3 and 4, respectively. Figures 5 and 6 represent variations of progeny levels with different house types

in Digboi and Mashimpur, respectively. The results show that there were distinctive variations of values in different dwellings. It has been observed that the estimated values also varied with different construction types. It is evident from the obtained results that the radon concentrations, radon progeny levels and equilibrium factors for radon were higher than the corresponding thoron concentrations, thoron progeny levels and equilibrium factors for thoron in all dwellings of the study areas. It has also been observed that the estimated values are maximum in mud houses and minimum in AT houses (Figs. 3, 4, 5, and 6). Tables 1 and 2 show radon and thoron concentrations, their progenies, equilibrium factors for radon and thoron progenies and inhalation dose rates in AT, RCC and mud houses in Digboi and Mashimpur areas during winter season, respectively. It has been observed that in Digboi area, the radon concentrations vary from 63.7± 30.7 (AT house) to 219.5±36.9 Bq m−3 (mud house) with mean values of 83.8 (1.3), 113.5 (1.1) and 157.2 (1.2) Bq m−3 in AT, RCC and mud houses, respectively, while in Mashimpur area, the radon concentrations range from 47.9 ± 13.5 (AT house) to 244.7 ± 37.2 Bq m−3 (mud house) with geometric mean values of 63.0 (1.1), 87.1 (1.4) and 182.1 (1.2) Bq m−3 in AT, RCC and mud houses, respectively. On the other hand, the corresponding thoron concentrations in Digboi area vary from 18.1±8.1 (AT house) to 95.6±27.4 Bq m−3 (mud house) with mean values of 31.1 (1.3), 50.8 (1.4) and 67.0 (1.6) Bq m−3 in AT, RCC and mud houses, respectively. In the case of Mashimpur area, the estimated thoron concentrations vary 20.6±7.1 (AT house) to 103.8±19.8 Bq m−3 with mean values of 26.4 (1.3), 44.4 (1.3) and 77.7 (1.3) Bq m−3 in AT, RCC and mud houses, respectively. It has been estimated that there exist substantial gas and progeny concentrations in the dwellings, which vary from house-to-house and also vary considerably with the different construction types. The considerable gas and progeny levels may be attributed to poor ventilations during the winter season. During the winter season, the doors, windows and ventilators were kept closed to conserve heat that helped in building up of radon and thoron concentrations. Moreover, the substantial gas and progeny levels during winter could also be explained on the basis of combination of causative factors, such as local geology (e.g. anticlinal structure of oilfield and uranium and radium content in soil and rock, especially in sandstone and shale), radioactive contents of household articles,

Environ Monit Assess (2014) 186:3581–3594 Table 1 222Rn and 220Rn concentrations (CR and CT), 222Rn and 220Rn progeny levels (WLR and WLT), equilibrium factors for 222Rn and 220Rn progeny (FR and FT) and inhalation dose rates (E) in Digboi

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House type

House no.

AT

1

WLR (mWL)

FT

E (mSv y−1)

0.40

0.05

2.1 2.6

91.1

33.8

9.3

5.4

0.38

0.04

3

137.4

37.5

14.6

6.5

0.39

0.05

4.0

4

79.0

38.1

8.0

5.9

0.37

0.04

2.3

5

65.8

18.1

6.2

2.5

0.35

0.04

1.7

GM

83.8

31.1

8.6

5.0

0.38

0.04

2.4

SD

30.1

8.1

3.3

1.6

0.02

0.01

0.9

1.3

1.3

1.4

1.4

1.05

1.12

1.3

6

127.9

28.1

12.4

4.1

0.36

0.04

3.3

7

111.6

50.0

12.2

9.1

0.40

0.05

3.6

8

103.2

66.3

9.6

8.9

0.34

0.04

3.0

9

105.3

72.5

11.1

12.3

0.39

0.05

3.5

10

121.6

50.0

11.1

6.5

0.34

0.04

3.2

GM

113.5

50.8

11.2

7.7

0.37

0.04

3.3

SD

10.6

17.3

1.1

3.1

0.03

0.01

0.2

1.1

1.4

1.1

1.5

1.07

1.12

1.1

11

173.7

91.9

19.3

17.5

0.41

0.05

5.9

12

134.2

27.5

13.3

4.2

0.37

0.04

3.5

13

140.0

95.6

16.0

19.3

0.42

0.06

5.2

14

219.5

80.0

23.4

14.0

0.40

0.05

6.6

15

134.2

70.0

14.2

12.0

0.40

0.05

4.3

GM

157.2

67.0

16.9

11.9

0.40

0.05

5.0

SD

36.9

27.4

4.1

5.9

0.02

0.01

1.2

1.2

1.6

1.2

1.7

1.04

1.14

1.3

human habits, unscientific construction pattern of the houses, increased exhalations from ground and building materials, porosity and permeability of the underlying soil/rocks, pressure gradient between interface, soil moisture and water saturation grade of the medium, and faulted and folded bed rocks (Schery and Gaeddert 1984). During winter, some dwellers burnt coal to keep the rooms warm, paying least attention to the ventilation conditions. Due to the high radioactive contents, the fly ashes thus produced were another important cause of concern for the elevated indoor radon levels inside the houses of the studied areas. On the other hand, the structures of the houses of the area revealed that there were some cracks and other openings between the indoor space and the soil and at the floor-wall interface. These openings might have provided potential paths for the subsoil radon and thoron to escape into the rooms resulting in high radon and thoron levels (Vaupotie et al.

6.0

FR

2

GSD

7.0

WLT (mWL)

33.1

GSD Mud

CT (Bq m−3)

63.7

GSD RCC

CR (Bq m−3)

1999). Moreover, during the winter time, the amount of water in air is high. As such as radon and thoron, and their decay products are moderately soluble in water, they remain trapped by water vapor and are more difficult to remove from indoor environ than the free gases (Brahmanandhan et al. 2008). All the rooms where measurements were carried out were living rooms on ground floors, which might have also allowed more gases to diffuse inside the houses because of high porosity of materials used (Ramola et al. 1995). Thus, it has been observed that the increased radon and thoron gases inside these selected dwellings were a result of several inevitable factors. As such, it was observed that there had been always some sources of radon and thoron as well as traps for confinement of the gases inside the dwellings located in the Digboi and Mashimpur areas. There is no trend of distinctive and abrupt increase in radon concentrations in the selected houses. As such, the

3588 Table 2 222Rn and 220Rn concentrations (CR and CT), 222 Rn and 220Rn progeny levels (WLR and WLT), equilibrium factors for 222Rn and 220Rn progeny (FR and FT) and inhalation dose rates (E) in Mashimpur

Environ Monit Assess (2014) 186:3581–3594

FR

FT

E (mSv y−1)

3.0

0.36

0.04

1.3

4.9

2.8

0.33

0.04

1.4

7.6

5.2

0.35

0.04

2.2

31.9

8.3

6.3

0.42

0.05

2.4

67.9

25.0

8.2

5.6

0.45

0.06

2.4

GM

63.0

26.4

6.5

4.3

0.38

0.05

1.9

SD

WLR (mWL)

13.5

7.1

1.8

1.6

0.05

0.01

0.6

1.1

1.3

1.3

1.4

1.12

1.18

1.3

6

87.4

46.9

8.7

7.2

0.37

0.04

2.6

7

135.8

56.3

13.2

8.2

0.36

0.04

3.8

8

55.6

33.1

6.4

6.9

0.43

0.06

2.0

GM

87.1

44.4

9.0

7.4

0.39

0.05

2.7

SD

40.4

11.7

3.5

0.7

0.04

0.01

0.9

House no.

AT

1

47.9

20.6

4.6

2

52.6

21.3

3

79.5

36.9

4

73.2

5

GSD RCC

GSD Mud

CR (Bq m−3)

CT (Bq m−3)

House type

WLT (mWL)

1.4

1.3

1.4

1.1

1.08

1.21

1.3

9

155.8

71.9

16.7

12.6

0.40

0.05

4.9

10

145.3

44.4

20.1

14.3

0.51

0.09

5.8

11

176.3

73.1

16.0

9.4

0.34

0.04

4.6

12

244.7

103.8

22.7

13.9

0.34

0.04

6.5

13

169.0

91.3

23.5

29.9

0.52

0.09

7.6

14

228.4

96.9

23.2

15.3

0.38

0.04

6.7

15

176.3

79.4

22.8

21.2

0.48

0.08

6.9

GM

182.1

77.7

20.5

15.6

0.42

0.06

6.1

SD

37.2

19.8

3.2

6.8

0.08

0.02

1.1

1.2

1.3

1.2

1.4

1.19

1.44

1.2

GSD

houses may not belong to the edges of anticlines of Digboi and Mashimpur, and hence did not exhibit any kind of halo of high-radon concentrations (Barooah and Phukan 2012). It has been observed that indoor gas levels show wide variations in different dwellings in both the study areas with maximum values in mud houses and minimum values in AT houses (Figs. 3, 4, 5, 6). Because mud houses had earthen floors, the radioactive gases might have diffused much easily through the earthen floors of the houses from the underneath. They built up inside the mud houses, aided by the prevailing poor ventilations (having no window). As such, the mud houses are characterized by the highest radon and thoron concentrations. It was observed that the concrete surfaces of AT and RCC houses had no covering materials, which might have also helped high exhalation of radon and thoron from underneath and helped in the building up of considerable radon and

thoron concentrations in AT and RCC houses as well. Except in the case of mud houses, both the radon and thoron concentrations are found to be higher in Digboi than in the Mashimpur area. As the numbers of selected mud houses in Mashimpur area are greater than in Digboi area, the mean gas level also gets increased. Nevertheless, the mean radon values of different construction types in this present investigation are lower than the recommended lower reference level, i.e. 200 Bq m−3 (ICRP 2007). During the winter season, only one house (mud house no. 14) in Digboi and two houses (mud house nos. 12 and 14) in Mashimpur show indoor radon levels in the action level, i.e. 200– 300 Bq m−3 (ICRP 2007). The inhabitants of these three dwellings were advised to enhance ventilations even during the winter season. Although the obtained values are quite comparable with that reported at some other places in India (Chauhan 2010; Kumar and Prasad 2007;

Environ Monit Assess (2014) 186:3581–3594 Table 3 Annual exposure (222Rn and 222Rn), life time fatality risk factor and annual effective dose in Digboi

3589

House type

House no.

Annual exposure 222 Rn (WLM)

Annual exposure 220 Rn (WLM)

Annual exposure 222 Rn+ 220Rn (WLM)

Life time fatality risk factor (×10–4)

Annual effective dose (mSv y−1)

AT

1

0.25

0.22

0.47

1.41

1.8

2

0.34

0.19

0.53

1.59

2.1

3

0.53

0.23

0.76

2.28

3.0

4

0.29

0.21

0.50

1.50

1.9

5

0.22

0.09

0.31

0.91

1.2

GM

0.31

0.18

0.49

1.48

1.9

RCC

Mud

SD

0.12

0.06

0.16

0.49

0.7

GSD

1.36

1.42

1.33

1.34

1.3

6

0.45

0.15

0.60

1.80

2.3

7

0.44

0.33

0.77

2.31

3.0

8

0.35

0.32

0.67

2.01

2.6

9

0.40

0.44

0.84

2.52

3.3

10

0.40

0.23

0.63

1.89

2.4

GM

0.41

0.28

0.70

2.10

2.7

SD

0.04

0.12

0.10

0.30

0.4

GSD

1.09

1.44

1.33

1.34

1.2

11

0.70

0.63

1.33

3.99

5.2

12

0.48

0.15

0.63

1.89

2.4

13

0.58

0.70

1.28

3.84

5.0

14

0.84

0.50

1.34

4.12

5.2

15

0.51

0.43

0.94

2.82

3.7

GM

0.61

0.43

1.06

3.20

4.1

SD

0.15

0.21

0.31

0.96

1.2

GSD

1.23

1.73

1.34

1.35

1.4

Zubair et al. 2011; Ramola et al. 1997; Ramola et al. 1998; Virk et al. 1999), the values are less than that as reported earlier by some researchers in India (Singh et al. 2001; Kumar et al. 2007; Kumar et al. 1994; Gupta et al. 2012; Khan et al. 2008). However, it has been found that the average radon concentrations, i.e. 114.4 (1.4) and 100.0 (1.7) Bq m−3, in Digboi and Mashimpur are higher than the average value reported for the dwellings worldwide of 40 Bq m−3 (UNSCEAR 2000). Nevertheless, it has been found that the overall mean radon concentrations in Digboi and Mashimpur, i.e. 114.4 (1.4) and 100.0 (1.7) Bq m−3, are well below the action level (ICRP 2007). Radon progeny levels in Digboi area are found to vary from 6.2±3.3 (AT house) to 23.4±4.1 mWL (mud house) with mean values of 8.6 (1.4), 11.2 (1.1) and 16.9

(1.2) mWL in AT, RCC and mud houses, respectively, while in the case of Mashimpur area, the radon progeny levels are found to range from 4.6±1.8 (AT house) to 23.5±3.2 mWL (mud house) with mean values of 6.5 (1.3), 9.0 (1.4) and 20.5 (1.2) mWL in AT, RCC and mud houses, respectively. It has been obtained from Table 1 that the thoron progeny levels in Digboi area range from 2.5±1.6 (AT house) to 19.3±5.9 mWL (mud house) with mean values of 5.0 (1.4), 7.7 (1.5) and 11.9 (1.7) mWL in AT, RCC and mud houses, respectively, while the corresponding values in Mashimpur area vary from 2.8±1.6 (AT house) to 29.9±6.8 mWL (mud house) with mean values of 4.3 (1.4), 7.4 (1.1) and 15.6 (1.4) mWL in AT, RCC and mud houses, respectively (Table 2). The results show that the radon and thoron progeny levels vary from house to house and also

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Environ Monit Assess (2014) 186:3581–3594

Table 4 Annual exposure (222Rn and 222Rn), life time fatality risk factor and annual effective dose in Mashimpur

House type

House no.

Annual exposure 222 Rn (WLM)

Annual exposure 220 Rn (WLM))

Annual exposure 222 Rn+ 220Rn (WLM)

Life time fatality risk factor (×10–4)

Annual effective dose (mSv y−1)

AT

1

0.17

0.11

0.28

0.84

1.0

2

0.18

0.10

0.28

0.84

1.0

3

0.27

0.19

0.46

1.38

1.7

4

0.30

0.23

0.53

1.59

2.0

5

0.30

0.20

0.50

1.50

1.9

GM

0.24

0.16

0.39

1.18

1.5

RCC

Mud

SD

0.06

0.06

0.12

0.36

0.5

GSD

1.29

1.40

1.33

1.33

1.4

6

0.31

0.26

0.57

1.71

2.1

7

0.48

0.30

0.78

2.34

2.9

8

0.23

0.25

0.48

1.44

1.8

GM

0.32

0.27

0.60

1.79

2.2

SD

0.13

0.03

0.15

0.46

0.6

GSD

1.35

1.08

1.22

1.22

1.2

9

0.60

0.45

1.05

3.15

3.9

10

0.72

0.52

1.24

3.72

4.6

11

0.58

0.34

0.92

2.76

3.4

12

0.82

0.50

1.32

3.96

4.9

13

0.85

1.08

1.93

5.79

7.0

14

0.84

0.55

1.39

4.17

5.2

15

0.82

0.76

1.58

4.74

5.8

GM

0.75

0.56

1.31

3.94

4.9

SD

0.12

0.25

0.34

1.01

1.2

GSD

1.16

1.41

1.26

1.26

1.3

180

140

200 120 100 Radon 80

Thoron

60 40 20

Gas concentrations (Bq m-3 )

Gas concentrations (Bq m-3 )

160

180 160 140 120

Radon

100

Thoron

80 60 40 20

0 AT

RCC

Mud

Fig. 3 Variations of gas concentrations with different house types in Digboi

0

AT

RCC

Mud

Fig. 4 Variations of gas concentrations with different house types in Mashimpur

Environ Monit Assess (2014) 186:3581–3594

3591

25

Progeny levels (mWL)

20

15 Radon progeny Thoron progeny

10

5

0 AT

RCC

Mud

Fig. 5 Variations of radon progeny levels with different house types in Digboi

vary with different construction types. It is found that the estimated progeny levels have the highest values in mud houses and minimum values in AT houses (Figs. 3, 4, 5, 6). As such, the results show that the decay products exhibit the same trend of variations as that of their parent gases in all types of dwellings. It has been observed that in the dwellings of Digboi area, the estimated equilibrium factors for radon progeny are 0.38 (1.05) for AT house, 0.37 (1.07) for RCC house and 0.40 (1.04) for mud house, and the equilibrium factors for thoron progeny are 0.04 (1.12) for AT house, 0.04 (1.12) for RCC house and 0.05 (1.14) for mud house (Table 1). It is evident from Table 2 that in the dwellings of Mashimpur area, the estimated equilibrium factors for radon progeny are 0.38 (1.12) for AT house, 0.39 (1.08) for RCC house and 0.42 (1.19) for 25

Progeny levels (mWL)

20

15 Radon progeny Thoron progeny

10

5

0 AT

RCC

Mud

Fig. 6 Variations of thoron progeny levels with different house types in Mashimpur

mud house, and the equilibrium factors for thoron progeny are 0.05 (1.18) for AT house, 0.05 (1.21) for RCC house and 0.06 (1.44) for mud house, respectively (Table 2). It has been evident that although there is no wide variation, the equilibrium factors between the gas and its progeny show maximum values in mud houses and are in tandem with gas and progeny concentrations. The estimated equilibrium factors (for both radon and thoron progenies) are quite comparable with the values reported earlier in Indian dwellings (Mayya et al. 1998; Ramola et al. 2003). Moreover, the results as obtained in this present work show that the estimated equilibrium factors are comparable with the global equilibrium factors 0.38 and 0.09 for radon and thoron progenies, respectively (Mayya et al. 1998). From Tables 3 and 4, it is evident that the total annual exposures due to radon and thoron collectively in the AT, RCC and mud houses are 0.49 (1.33), 0.70 (1.33) and 1.06 (1.34) WLM in the Digboi area and 0.39 (1.33), 0.60 (1.22) and 1.31 (1.26) WLM in the Mashimpur area, respectively. The results show that the average lifetime fatality risks of residents of the Digboi area are 1.48 (1.34)×10−4 (AT houses), 2.10 (1.34)×10−4 (RCC house) and 3.20 (1.35)×10−4 (mud houses), while the corresponding values in Mashimpur area are 1.18 (1.33)×10−4 (AT houses), 1.79 (1.22)× 10−4 (RCC houses) and 3.94 (1.26)×10−4 (mud houses). Annual exposure and lifetime fatality risk of the residents follow the same trend as that of gas and progeny concentrations having maximum values in mud houses and minimum values in AT houses. It has been observed from Table 3 that the annual effective doses in the Digboi area range from 1.2±0.7 to 3.0 ± 0.7 mSv y−1 in AT houses, 2.3 ± 0.4 to 3.3 ± 0.4 mSv y−1 in RCC houses and 2.4±1.2 to 5.2± 1.2 mSv y−1 in mud houses. It has also been observed that the annual effective doses in the Mashimpur area vary from 1.0±0.5 to 2.0±0.5 mSv y−1 in AT houses, 1.8±0.6 to 2.9±0.6 mSv y−1 in RCC houses and 3.4± 1.2 to 7.0±1.2 mSv y−1 in mud houses. The annual effective doses vary from house to house with maximum values in mud houses and minimum values in AT house in both Digboi and Mashimpur areas. The considerable variations of annual effective doses with different construction types may be attributed to the trend of variations of radon and thoron, and their decay products in different types of houses. The variations in values among dwellings may be due to the difference in the concentrations of radioactive elements, viz. uranium

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Environ Monit Assess (2014) 186:3581–3594

and thorium in the soil, bedrock (especially sandstone, shale) and building materials and their exhalations. It has been found that radon and thoron daughters contribute a significant part of the effective dose from natural sources. These estimated values are higher than the world average of annual effective dose (AED) of 1.275 mSV (Rahman et al. 2008). Nevertheless, the obtained highest mean values for mud houses in both the study areas are below the upper reference level of 10 mSv (ICRP 2007). As such, the inhabitants in Digboi and Mashimpur are free from health risks due to inhalation of radon and thoron, and their progenies.

minimizing indoor radon and thoron levels. The gas progeny levels and equilibrium factors also show maximum values in mud houses and minimum values in AT houses. The obtained results show that the mean values of annual effective doses in AT and RCC houses lie below the lower limit of the recommended action level (3–10 mSv). Although the mean annual effective doses in the case of mud houses in both the study areas are in the range of the action level, they are much below than the upper reference level of 10 mSv (ICRP 2007). As such, the residents of the Digboi and Mashimpur areas located on anticlines are free from health risks due to inhalation of radon and thoron, and their progenies.

Conclusions

Acknowledgement The authors are thankful to the dwellers of the studied area for their cooperation during the field work. We are thankful to Dr. R. William Field of the University of Iowa, USA for his valuable suggestions.

Indoor radon and thoron, and their progeny concentrations along with their radiological exposures to the residents have been carried out in 30 dwellings in residential areas of Digboi and Mashimpur. The measurements have been made in living rooms on ground floors in AT, RCC and mud houses during the winter season spanning over 3 months. The present work has shown that there exists a considerable variation in values among the dwellings. It has been found that mean radon concentrations in Digboi area are 83.8 (1.3), 113.5 (1.1) and 157.2 (1.2) Bq m−3 in AT, RCC and mud houses, respectively. On the other hand, the mean radon concentrations in Mashimpur area are 63.0 (1.1), 87.1 (1.4) and 182.1 (1.2) Bq m−3 in AT, RCC and mud houses, respectively. The thoron concentrations in Digboi area are found to be 31.1 (1.3), 50.8 (1.4) and 67.0 (1.6) Bq m−3 in AT, RCC and mud houses, respectively. In the case of Mashimpur area, the thoron concentrations are found to be 26.4 (1.3), 44.4 (1.3) and 77.7 (1.3) Bq m−3 in AT, RCC and mud houses, respectively. It has been found that the overall mean radon concentrations in Digboi and Mashimpur are 114.4 (1.4) and 100.0 (1.7) Bq m−3. The present work has shown that the indoor gas and progeny levels are dependent on various factors, especially the construction of houses, poor ventilation and underlying rock bed (viz. sandstone) with maximum values in mud houses and minimum values in AT houses. The mean radon levels are found to be higher than the world average values of indoor radon levels (40 Bq m−3). Nevertheless, the present values are lower than the upper reference level of recommended action level (200–300 Bq m−3) (ICRP 2007). However, the residents are advised to increase ventilation for

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