Indoor radon concentration in hydrocarbon and non-hydrocarbon pertaining areas across the Main Boundary Thrust in Attock district of Pakistan

Abstract

The areas pertaining the hydrocarbon and non-hydrocarbon in the Attock district of Pakistan were selected for the measurement of indoor radon concentration. This area was not previously surveyed for such kind of study. CR-39-based NRPB radon dosimeters were utilized for this study. Attock city, Hassan Abdal and Jand belong to the non-hydrocarbon areas, where radon concentration was measured to be 57 ± 11, 58 ± 11 and 56 ± 11 Bq m⁻³, respectively. The hydrocarbon pertaining areas consist of Toot and Meyal villages, the concentration of radon was found to be 48 ± 10 and 37 ± 8 Bq m⁻³, respectively. The cap rock in the geological structures of the hydrocarbon areas may have restricted the migration of underneath radon. The annual effective dose in the studied locations was estimated to be 1.28 ± 0.25 mSv y⁻¹, which is less than the action level recommended by the ICRP. Estimated excess lung cancer risks per million persons per year in the surveyed area show higher values 96, 69, 32–110 and 41–124 as compared to the world average reported by the BEIR-IV (1988), ICRP (1987), US-EPA (1986) and UNSCEAR (1993) models, respectively. Present results could be used as a reference for any future studies in Pakistan for the problems of radon emissions with respect to the types of rocks.

Keywords

Hydrocarbon and non-hydrocarbon areas CR-39-based dosimeter Main boundary thrust Indoor radon concentration Annual effective dose Excess lung cancer risks

Introduction

Radon (²²²Rn) is a long-lived (T_1/2 = 3.82 d) radioactive, inert, odourless and tasteless gas. It is a decay product of ²²⁶Ra, which is a progeny of the natural decay chain headed by ²³⁸U. Uranium is ubiquitous in nature since from the formation of the planet, therefore the underground soil, borehole water supply and construction materials are the main sources of indoor radon.^1,2 The higher concentrations of radon can be a health hazard for the humans.³ Moreover, long-term exposure to low-intensity radiation, particularly due to ²²²Rn and its short-lived alpha-emitting decay products ²¹⁸Po and ²¹⁴Po, may cause the lung cancer.⁴ Furthermore, the measurement of radon concentration is useful for the prediction of earthquakes and the exploration of hydrocarbons, uranium and the sites of geothermal energy.⁵⁻⁸ In several studies, radon gas was found trapped in oil and gas-bearing reservoirs and unable to migrate to the earth’s surface unless under certain favourable circumstances, radon could find a way to come out of the trapping rock.^9–12 Investigations, based on radon measurements carried out over a period of two months or so in Oklahoma (USA), indicated a strong correlation between the oil reserves and the intensity of radon emission signals.¹² In general, anticlinal traps give halo patterns with low radon values at the centre and high values outside of the boundary.^9,13−15 Zhongjun et al.¹⁶ and Zuhui et al.¹⁷ carried out a radon survey with CR-39 detectors in Saihantala Hollow, Inner Mongolia and Weibel Hollow, Shandong and discovered few oil and gas deposits trapped along with their anomalies.

The aim of the present study was to measure and compare the indoor radon concentrations in the dwellings of oil producing and non-oil producing regions in Attock district of Pakistan. The measurements were carried out with CR-39 etched track detector placed in the National Radiological Protection Board (NRPB) radon dosimeters.¹⁸

Study area

The present study focussed on the district Attock in the Punjab province of Pakistan. The area is presented in a geological map shown in Figure 1. The maximum terrain elevation in this region is 389 m above the sea level. The region has latitude 33°30′29.24″ and the longitude 72°20′2.98″. Attock district is spread over an area of 6857 km².¹⁹ The district comprises of the following six tehsils: Attock, Fateh Jang, Jand, Hassan Abdal, Hazro and Pindi Gheb. Four tehsils (Attock, Hassan Abdal, Jand and Pindi Gheb) were selected for the monitoring of radon concentrations in the houses of these units. The Attock and Hassan Abdal are lying in the north of main boundary thrust (MBT) whereas Jand and Pindi Gheb are located towards its south.^20,21

The Attock district lies within the Kohat–Potwar sedimentary basin.²² The cities of Attock district are located in the Kurram–Cherat–Margalla fold and thrust belt.^20,23 The major geology of the region comprises of the sedimentary rocks exposed in various sections on the northern and southern portions of the area with the alluvium laid down in the Quaternary age in between the highs and lows of the region.²⁴ The northern side of the region consists of the Gandhgar range and the Attock–Cherat range that consist of the exposures of the oldest formation of Precambrian age.²⁵ The northern part is the part of the Northern Potwar Deformed Zone in this basin.^19,26,27

The south of the district Attock consists of another prominent feature in the form of Kalachitta hills. The Kalachitta hills consist of the Mesozoic rocks adjacent to the Rawalpindi group due to a major thrust called the MBT.²⁸ The south of Kalachitta hills is the Khair-i-Murat range exposed along the Khair-i-Murat fault.²⁰ The Palaeocene and Miocene rocks have come close due to this major fault. Further south towards the Jand area, the Siwaliks group of Pliocene age is exposed.^13,27 The hydrocarbons are produced from the Toot and Meyal oil fields in the tehsil Pindi Gheb. The hydrocarbon reserves in the earth are often sheltered or preserved by seal or cap rocks, which are impermeable to the gases.²⁹ The seal rocks in the region of Pindi Gheb were investigated in the three major formations: Kuldana Formation, Nammal Formation and Datta Formation.³⁰

Materials and methods

For the measurement of indoor radon in the district Attock, the study area was divided into two sections: the hydrocarbon pertaining area (Toot and Meyal villages in tehsil Pindi Gheb) and the non-hydrocarbon area (Attock city, Hassan Abdal, Jand). Toot and Meyal are among the number of declared oil fields. Forty houses were selected for the monitoring of radon. Selected dwellings were the old houses in every cities/villages of the study area. Three types of rooms were selected in the study area, i.e. bed room, living room, and kitchen. Therefore, 120 NRPB dosimeters containing CR-39 etched track detector were prepared. The dosimeters were installed at a height around 2 m at specific locations in a bed room, living room and kitchen of the houses. As these were old houses occupied by the low-income community, in most of the houses, the floors were made of mud and in some houses, baked bricks were laid to make floor. The baked bricks were interconnected by the mud in the floor. In most of the houses, the dimension of the window used for the ventilation of the bedroom varied from 0.92 × 0.92 to 1.22 × 1.22 m², and in the living room from 1.22 × 1.22 to 1.52 × 1.52 m². The kitchens of most of the houses were forced ventilated (exhaust fans were installed there). As the measurement period of radon was in winter, thus most of the time windows would remain closed in those houses. Therefore, a greater chance of radon accumulation in this period in the dwellings would be from the underground. The study area and the scheme of dosimeters installation are presented in Table 1. Some random duplicate samples were also installed in each part of the district for the validation of our experimental results.

Table 1.

Information on the study area, number of installed dosimeters, indoor radon concentration and derived annual effective dose, values in parentheses represent the range.

		Radon concentration (Bq m⁻³)			Weighted average radon concentration
Study areas	Dosimeters installed	Bedroom	Living room	Kitchen	WA_Rn	Annual effective dose (mSv)
Non-hydrocarbon areas
Attock city	30	59 ± 11 (30−68)	56 ± 10 (28−64)	51 ± 10 (26−60)	57 ± 11 (29−66)	1.43 ± 0.27 (0.73–1.6)
Hassan Abdal	30	60 ± 12 (51−77)	57 ± 11 (48−74)	52 ± 10 (43−65)	58 ± 11 (49−75)	1.46 ± 0.27 (1.23–1.89)
Jand	30	57 ± 11 (45−76)	55 ± 10 (43−74)	50 ± 9 (38−68)	56 ± 11 (44−75)	1.41 ± 0.27 (1.11–1.89)
Average		58.66 ± 11.33	56 ± 10.33	51 ± 9.66	57 ± 11	1.28 ± 0.25
Hydrocarbon pertaining areas
Toot village	15	50 ± 10 (41−58)	47 ± 10 (38−55)	41 ± 9 (33−51)	48 ± 10 (39−56)	1.21 ± 0.25 (0.98–1.41)
Meyal village	15	38 ± 8 (24 −58)	36 ± 8 (20−56)	31 ± 7 (16−52)	37 ± 8 (22−57)	0.93 ± 0.20 (0.55–1.43)
Average		44 ± 9	41.5 ± 9	36 ± 8	42.5 ± 9	1.07 ± 0.227
Whole study area		53 ± 10	50 ± 10	45 ± 9	51 ± 10	1.28 ± 0.25

After an exposure period of 90 days (1 January to 31 March 2014), the dosimeters were collected from all the houses. The weather conditions in the study area during this period are almost alike except little rains at the end of January and the start of February, no snowfall is ever reported in this area. In March, the weather started to become pleasant until it was fully calm at the end of the month. But overall the weather was cold during the whole period, therefore no seasonal variations are expected for this period. CR-39 detectors were detached from the dosimeters and etched for 14 h in 6 M NaOH at 80℃. The detectors were analysed through the optical microscope to determine track densities. After applying the background correction, the track densities were converted to radon concentration using a calibration factor of 2.7 tracks cm⁻² h⁻¹/kBq m⁻³.³¹ The concentration of radon was then calculated using equation (1)

C_{Rn} = (\frac{T_{p}}{{Cal}_{f} \times T_{exp}}) \times 1000

(1)

where C_Rn is the radon concentration in Bq m⁻³, Tρ is the track density in tracks cm⁻², Cal_f is the calibration factor for the dosimeter used in the survey in units of tracks cm⁻²kB q⁻¹ m³ h⁻¹ and T_exp is the exposure time in hours. The deviation in the measured concentration of the duplicate samples was found to vary from 4% to 12% with respect to the original samples. The results of duplicate samples were also incorporated in the original set of results to get improved statistical data of radon concentration in the relevant part. The weighted average radon concentration (WA_Rn) was calculated using equation (2)

{WA}_{Rn} = 0.55 BedRoom + 0.40 LivingRoom + 0.05 Kitchen

(2)

The coefficients in equation (2) are based on the interviews of the house occupants. The occupants spend about 55% of their indoor time in a bedroom and 40% in a living room and 5% in a kitchen. Similar residence information was also reported by Faheem et al.³²

For the estimation of average annual effective dose (H) (mSv y⁻¹) received by the inhabitants of the non-hydrocarbon areas, hydrocarbon pertaining areas and the whole study area due to the indoor radon and its progeny, equation (3) which is a model of UNSCEAR² was adopted

H = {WA}_{Rn} \times {EEC}_{f} \times O_{f} \times T_{y} \times D_{cf}

(3)

where WA_Rn stands for weighted average indoor radon concentration in Bq m⁻³, EEC_f (0.4, taken for indoor inhabitants) for equilibrium equivalent concentration (EEC) factor, O_f for occupancy factor (0.8 as taken from UNSCEAR² report), T_y for time (8760 h y⁻¹) and D_cf for dose conversion factor (9 nSv h⁻¹ per Bq m⁻³).

An exact amount of actual effective dose received from α-emitters by lung tissue is difficult to calculate, because the coefficients suggested by different authors^33,34 vary by a factor of 3, 0.05 mSv per year per Bq m⁻³ being in the middle of the range. Therefore, effective doses are not commonly used in studies of radon exposure. Instead, the product of two known variables was used: the airborne concentration of radon progeny (Bq m⁻³) and the length of exposure to that concentration (h), and expressed as working level month (WLM). Specifically, concentration of radon progeny is expressed as working level (WL). One WLM is the human exposure to radon progenies for 170 h (i.e. an occupational month). In the environmental situations, exposure to 1 WL for a year (8760 h) would correspond to an integrated exposure of 51 WLM. However, a conversion factor of 73.9 Bq m⁻³= 1 WLM y⁻¹ can be used to convert radon concentration level into WL. The excess lung cancer risk was estimated using equation (4) as given by Matiullah et al.³⁵

Excess Lung Cancer Risk (ELCR) = 0.5 \times 0.8 \times WLM \times Riskfactor

(4)

where 0.5 is the equilibrium factor and 0.8 is the occupancy factor (i.e. the fraction of time spent). The excess lung cancer risk is defined as the occurrence of excess deaths per million persons per year due to the lung cancer as a result of exposure to radon and its progenies. On the other hand, the risk coefficient is defined as the number of lung cancer cases per million persons (PMP) per WLM. This can generally be determined from the epidemiological data for occupationally exposed mine workers. This is because limited data are available for lung cancer cases due to indoor radon exposure. Therefore, to assess the population lung cancer risk due to indoor radon exposure, the data for mine workers are usually considered. A number of investigators and expert groups have summarized the epidemiological findings and estimated the most probable risk. The lifetime probability of getting excess lung cancer due to the life time exposure to radon and its progenies reported by several international agencies like UNSCEAR,³⁶ BEIR-IV,³⁷ ICRP,³⁸ US-EPA,³⁹ NCRP,⁴⁰ etc. is given in Table 2. As can be seen, there is a wide variation amongst the estimated values reported by different agencies. This variation is mainly due to the use of different assumed parameters in the models proposed by the above-mentioned agencies whose validity is not certain because of the non-availability of the required information regarding the deaths due to lung cancer caused by indoor exposure to radon and its daughters. As an example, in BEIR-IV³⁷ report, the age-specific excess relative risk is higher for younger persons and declines for elder persons. In BEIR-IV³⁷ and ICRP³⁸ models, the primary risk data are given for miners, i.e. only data for males have been used instead of both males and females. This shows that interpretation of lung cancer risk due to radon is not available with high accuracy. Therefore, only approximate estimates of lung cancer risks are possible with the use of these models. The excess lung cancer risk for the study areas was estimated using the relevant factors and the risk coefficients given in Table 2 and presented in Table 3.

Table 2.

Lifetime probability of lung cancer due to the lifetime exposure to radon progeny.

International agencies	Cancer death per million persons (PMP) per working level month (WLM)	References
UNSCEAR (1988, 1993)	150–450	36
BEIR-IV (1988)	350	37
ICRP (1987)	250	38
US-EPA (1986)	115–400	39
NCRP (1984)	130	40

Table 3.

Estimated excess of lung cancer per million person per year (MPY) using the risk factors recommended by international agencies like BEIR-IV,³⁷ ICRP,³⁸ EPA³⁹ and UNSCEAR.³⁶

City	WA_Rn (Bq m⁻³)	WLM/Y (Bq m⁻³ h)	Estimated excess lung cancer risk per MPY using the recommended risk factors (per WLM) of the following international agencies
City	WA_Rn (Bq m⁻³)	WLM/Y (Bq m⁻³ h)	BEIR-IV	ICRP	EPA	UNSCEAR
Attock City	57	0.77	108	77	35–123	46–139
Hassan Abdal	58	0.78	110	78	36–125	47–141
Jand	56	0.75	106	76	35–121	45–136
Toot village	48	0.64	91	65	30–104	39–117
Meyal village	37	0.50	70	50	23–80	30–90
Whole study area	51	0.69	96	69	32–110	41–124

Results and discussion

The measured indoor radon concentration along with the WA_Rn, the derived annual effective dose and excess lung cancer risk in the area under study is given in Tables 1 and 3, respectively. In the hydrocarbon enriched area, the radon concentration lies between 16 and 58 Bq m⁻³, while that in the non-hydrocarbon area it was found between 26 and 77 Bq m⁻³. Indoor radon concentration in bedrooms, living rooms and kitchens of the houses in the hydrocarbon enriched and non-hydrocarbon areas of district Attock is presented in Figure 2. The main contribution of indoor radon is the underneath soil of a house. Radon concentration in the bed rooms is higher than that in the living rooms, which is more than that in the kitchens. In a same house, this type of variation reflects the radon mitigation due to the more ventilation rate in a kitchen than that in a bedroom. The frequency distribution of radon concentration in various types of rooms is shown in Figure 3.

The concentrations of radon in all types of houses in the non-hydrocarbon area are almost the same. In the hydrocarbon area, radon concentration is smaller in the rooms of Meyal village than that in Toot village. In fact, radon concentration is smaller in the hydrocarbon pertaining area than that in the non-hydrocarbon area. The possible reason for the lower concentration in the hydrocarbon area than that in the non-hydrocarbon area may be the differences in the geological structures of both the areas. The hydrocarbon deposits are normally trapped under a cap rock which is impermeable for the fluids, which confines the exhalation and emanation of radon.^9,11 The Kuldana, Nammal and Datta formations form the cap rock for the hydrocarbon reservoirs.¹³ Radon trapped in the cap rock is restricted to come out of the cap rock; therefore its contribution is small in the hydrocarbon areas.

The study area can also be classified with respect to the major geologic features and the rock types. The rocks are different across MBT. The geometry of deformation is also changed from the north to south. The location of Attock and Hassan Abdal is in the north of the MBT along another feature that is Khairabad fault.⁴¹ As can be seen from Figure 1, the rocks exposed in and around this area are older rocks that are thrusting over the younger rocks lying on the southern side of the MBT in the areas of Pindi Gheb and Jand. The rocks exposed in and around the non-hydrocarbon area are older mostly of Precambrian and Mesozoic ages. The rocks exposed in the hydrocarbon-bearing areas are of younger ages that are the Siwaliks and Rawalpindi groups of Miocene to Pliocene age.⁴² The mechanism of cap rock shielding is probably responsible for the lower emissions of radon in the southern areas. The higher concentration of radon was found in the northern part of the study area. The reason for this enhanced concentration is due to the geology of the area. The styles of the deformation, ages of the rocks and the presence of the hydrocarbon seal rocks in one area and absence from the other could be the causes of radon variation in the area under study.

Indoor radon concentration in the hydrocarbon areas is presented in Table 4 together with other published data of Al-Asha and Qatif cities of Saudi Arabia⁴³ and the hydrocarbon area of Kuwait.⁴⁴ The Al-Asha city contains the world’s first largest oil field (Ghawar) and Kuwait contains the world’s second largest oil field (Burgan). The measured radon concentration in the hydrocarbon pertaining areas of Toot and Meyal villages of Pakistan shows higher values than the Saudi Arabian⁴³ and Kuwaiti⁴⁴ oil field areas. The probable reason for this low radon concentration in Saudi Arabian and Kuwaiti oil field areas is that it was measured in houses which lie at the centre of anticlinal traps. Furthermore, the present data also compared with some other reported data such as the Magurchara gas explosion area of Bangladesh⁴⁵ and the Rawalpindi oil refinery vicinity of Pakistan.⁴⁶ As expected, the reported values in these areas^45,46 show much higher radon concentration compared to our measured data. This is because due to the extraction of oil and gas, the underground radionuclides were brought to the surface and therefore raising a higher radon concentrations near and in the vicinity of the oil and gas field. Beside this, the Magurchara gas field area can be defined as an anticline area (a natural shield preserving the hydrocarbon deposits as well as the radon). Any such area having the gas explosion history may contain high radon concentration because radon is an inert gas and has less flammability as compared to natural gas. On the other hand, oil refinery can be installed in any place of a country based on the infrastructural condition. That means the refinery installation area may not intrinsically contain high radon concentration. But as the crude oil is extracted from the well, such a process stream may result in the high radon concentration.

Table 4.

Comparison of indoor radon concentration in the hydrocarbon pertaining areas with the literature.

Hydrocarbon pertaining areas	Oil fields	Indoor radon concentration (Bq m⁻³)	References
Al-Asha	Ghawar (world’s first largest oil field)	8	Al-Jarallah and Rehman,⁴³ Saudi Arabia
Qatif	Qatif	22	Al-Jarallah and Rehman,⁴³ Saudi Arabia
Kuwait	Burgan (world’s second largest oil field)	14	Bem et al.,⁴⁴ Kuwait
Sylhet	Magurchara (gas explosion area)	310–537	Khan and Chowdhury,⁴⁵ Bangladesh
Rawalpindi	Oil refinery vicinity	12–190	Rahman et al.,⁴⁶ Pakistan
Attock	Meyal	37	Present study, Pakistan
Attock	Toot	48	Present study, Pakistan

The derived average annual effective dose of the whole area is found to be 1.28 ± 0.25 mSv, which is slightly higher compared to the UNSCEAR² reported value of 1.15 mSv y⁻¹ (see in Figure 4). The excess lung cancer risk was estimated for all the study areas using the risk factors recommended by some international agencies like UNSCEAR³⁶ and BEIR-IV,³⁷ ICRP³⁸ and US-EPA.³⁹ On the other hand, for comparative study, the evaluated excess lung cancer risk with respect to radon concentration by some international agencies like UNSCEAR,³⁶ BEIR-IV,³⁷ ICRP,³⁸ US-EPA³⁹ and NCRP⁴⁰ is reproduced in Figure 5. The minimum value of excess lung cancer was found in Meyal village (hydrocarbon pertaining region) as 70, 50, 23–80 and 30–90 from the application of BEIR-IV,³⁷ ICRP,³⁸ US-EPA³⁹ and UNSCEAR,³⁶ respectively. These values are lower than the world average: 76, 54, 25–87 and 32–97 reported by BEIR-IV,³⁷ ICRP,³⁸ US-EPA³⁹ and UNSCEAR,³⁶ respectively. The maximum values of excess lung cancer were found in Hassan Abdal which lies in non-hydrocarbon-bearing region of the study area as 110, 78, 36–125 and 47–141 from the application of BEIR-IV,³⁷ ICRP,³⁸ US-EPA³⁹ and UNSCEAR,³⁶ respectively, which are higher than the world average. In the whole study area, the excess lung cancer risks show overall higher values as 96, 69, 32–110 and 41–124 compared to the world average reported by the BEIR-IV,³⁷ ICRP,³⁸ US-EPA³⁹ and UNSCEAR,³⁶ respectively. These differences amongst our estimated values and international agencies evaluated ones are due to the variations of local occupancy factor and measured radon concentrations. Besides this, the WLM used in equation (4) was calculated using a conversion factor 73.9 Bq m⁻³= 1 WLM y⁻¹, which also may vary with studies. Moreover, the risk factors (i.e. cancer deaths PMP/WLM) published by UNSCEAR and US-EPA pertain to Western populations and for other agencies as global average. To arrive at proper risk projections, one must take the mathematical models derived from epidemiological studies of miners and apply them to the case of indoor exposures for a population with defined vital statistics. Because both tobacco usage and radon inhalation are considered as responsible for lung cancer, the risk per WLM varies from population to population. Also other factors such as age, sex, occupation, economic conditions, geographical locations, etc. have direct impact on the risk per WLM. The use of an occupancy factor of 0.8 overestimates the excess lung cancer risk in the rural areas but may be valid for the inhabitants of the urban regions of Pakistan.

Conclusions

This study concludes that houses in the non-hydrocarbon areas have high indoor radon concentration as compared to that in the hydrocarbon enriched areas of Attock district of Pakistan. In the areas under study, the bed rooms of the houses show the high and the kitchens show the low indoor radon concentration. The maximum value (77 ± 15 Bq m⁻³) of indoor radon concentration was found in the bedroom of the Hassan Abdal region, and the lowest value (16 ± 4 Bq m⁻³) was found in the kitchens of Meyal village. Radon concentrations in all parts of the study area were above the world average of 40 Bq m⁻³ (UNSCEAR)² except in the Meyal village of the hydrocarbon areas where lower than the world average was measured. A detailed statistical analysis shows that 18% of the samples in the whole area lie below the world average and the remaining 82% are above the world average. Weighted average radon concentration in the study area was less than the US-EPA action level of 148 Bq m⁻³ (US-EPA).⁴⁷ The doses received by the people of the whole area were within the ICRP-65 recommended limiting range (3–10 mSv). Excess lung cancer risks in the whole area are higher than the world average mentioned earlier. Therefore, it is necessary to educate the public about possible sources of radon, their harmful effects and remedial actions. Houses should be well ventilated and if possible forced ventilation like exhaust fans should be installed.

Footnotes

Authors’ contribution

All authors contributed equally in the preparation of this manuscript (i.e. from experiments to the publication).

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The work was partially supported by the University of Malaya Research Grant (RP006D-13AFR).

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