INTRODUCTION

More than 65% of Indian population lives in rural areas [1]. Natural radiation is present everywhere on the earth and is not escapable. The natural and anthropogenic radiation sources always affect the mankind [2]. The natural sources release 87% of the radiation dose that was received by the mankind [2]. Naturally occurring primordial radionuclides mainly include 238U, 235U, and 232Th series, 40K, and 226Ra [3, 4]. The health effect due to exposure to radionuclides and inhalation by human beings within the indoor environment is a major public concern worldwide [4]. The background radiation comes from cosmic and terrestrial sources [5]. The United Nations Scientific Commission on Effects of Atomic Radiation (UNSCEAR) reported cosmic radiation dose at the sea level of 32 nSv/h and external terrestrial radiation dose of 59 nSv/h with the average annual dose rate of 0.07 mSv/year [6, 7]. According to the UNSCEAR report, the population weighted average AEDE (annual effective dose equivalent) due to cosmic rays and terrestrial gamma radiation is 0.87 mSv [6, 7]. The rural and urban natural background radiation comes from cosmos, food, air, building materials, industrial activities, and the human body itself [8]. Terrestrial radiation makes a higher contribution to the total background radiation as compared to cosmic radiation [9]. The risk arising due to gamma dose was studied for the indoor and outdoor environment [10–17]. The main aim of this study is to compare the gamma dose rates in rural and urban areas of Durg district and assess the cancer risk in Durg areas. Here we make the first estimation of the equivalent dose for different organs of the human body for this region. This study will serve as baseline data for future gamma radiation effect in rural and urban areas of the Durg district, Chhattisgarh region.

EXPERIMENTAL

Selection of the measurement sites. The study location is a famous district of Chhattisgarh as the world largest steel plant is situated here. The Durg district of Chhattisgarh is one of the most densely populated districts of India. It is located in the southern part of the prosperous Chhattisgarh plain between 21°04' and 21°37'N and 81°16' and 81°40'E [18] with the average elevation of 317 m (1040 ft) above sea level. The total area of Durg district is 223836 ha; it is located in the central part of Chhattisgarh. It is situated on the east bank of Shivnath River and is endowed with industrial and cultural competence, social harmony, and resources [18]. This district does not involve the forest part. The gamma dose rates were measured in 42 locations of rural and 10 locations of urban areas of Durg district, Chhattisgarh, India, where the total population is 184931 (rural population: 70378; urban population: 114553). The male population is about 51%, and the female population is 49% [1]. All the study areas are shown in Fig. 1. The study location has been selected by 6 km2 grids as per guidelines of the Board of Research in Nuclear Sciences, Department of Atomic Energy.

Fig. 1.
figure 1

Grid map of study areas of Durg district, Chhattisgarh, India.

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Geology of Durg district. The study area is a part of Chhattisgarh basin that consists of argillaceous and calcareous sediments represented by mainly shale and limestone [19, 20]. The Durg district is covered by four different lithological units, namely, Chandi limestone, Tarenga formation, Gunderdehi formation, and Chandi sandstone [19, 20]. About 80% of the study area is covered by Chandi limestone, and the remaining part of the study area is covered by Tarenga formation, Gunderdehi formation, and Chandi sandstone. The soil of the study area consists of clay loam, gravelly clay loam, gravelly sandy clay loam, gravelly sandy loam, and sandy clay loam [19, 20].

Measurement of the gamma dose rate. A Polimaster PM-1405 survey meter based on Geiger–Muller technique was used for measuring the indoor and outdoor gamma dose rate levels for rural and urban areas of Durg district. All the readings were recorded in nSv/h. This survey meter records the terrestrial as well as the cosmic radiation. This apparatus was placed at a height of 1 m above the ground surface. The energy range for gamma radiation measurement of this device is 0.05–3 MeV, and the detection range for dose rate is from 0.01 µSv/h to 100 mSv/h [16, 21]. The GPS (GARMIN OREGON-650) coordination device was used for determining the latitude (N) and longitude (E) of all the study areas. The radiation map of indoor and outdoor gamma dose rate was prepared using Arc GIS 9.3 software.

Statistical analysis of the relationship between outdoor and indoor gamma dose rates. The Statistical Package for the Social Sciences (SPSS), version 21, was used to facilitate the analysis. The paired sample t-test was conducted to determine the relationship between the outdoor and indoor gamma dose rates for the rural and urban areas of Durg district based on the hypotheses formulated below.

Hypothesis for the correlation study of an urban area: Ho(cu) = there is no relationship between the outdoor and indoor gamma dose rates for an urban area; Ha(cu)= there is a relationship between the outdoor and indoor gamma dose rates for an urban area

Hypothesis for the correlation study of a rural area: Ho(cr) = there is no relationship between the outdoor and indoor gamma dose rates for a rural area; Ha(cr) = there is a relationship between the outdoor and indoor gamma dose rates for a rural area.

Paired difference study for the urban area: Ho(pdu) = there is no significant difference between the outdoor and indoor gamma dose rates for an urban area; Ha(pdu) = there is a significant difference between the outdoor and indoor gamma dose rates for an urban area.

Paired difference study for the rural area: Ho(pdr) = there is no significant difference between the outdoor and indoor gamma dose rates for a rural area; Ha(pdr) = there is a significant difference between the outdoor and indoor gamma dose rates for a rural area.

Calculation of the annual effective dose equivalent (AEDE). Indoor and outdoor gamma dose rates were used for calculating the indoor and outdoor annual effective dose equivalent (AEDE). The ionizing radiations affecting human beings are evaluated based on AEDE [22]. The annual effective dose equivalent was calculated using the following equation:

$$ \text{AEDE}\left( \text{indoor/outdoor} \right)\text{ }=D\left( \text{indoor/outdoor} \right)\text{ }\times T\times \text{ conversion coefficient }\!\!\times\!\!\text{ occupancy factor}, $$
(1)

where D(indoor) is the indoor absorbed gamma dose rate (nGy/h), D(outdoor) is the outdoor absorbed gamma dose rate (nGy/h), and T is the time conversion factor (hour into a year, 1 year = 8760 h).

The dose conversion coefficient reported by UNSCEAR for an adult is 0.7, and the occupancy factor for indoor and outdoor is 0.8 and 0.2, respectively [6]. The conversion coefficient is higher for children and infants by approximately 10 and 30%, respectively [6]. Thus, Eq. (1) for an adult becomes

$$ P=\text{ AEDE}\left( \text{indoor} \right)\text{ }\!\!~\!\!\text{ }=D\left( \text{indoor} \right)\text{ }\times \text{ }8760\text{ }\times \text{ }0.7\text{ }\times \text{ }0.8, $$
(3)
$$ Q=\text{ AEDE}\left( \text{outdoor} \right)~=D\left( \text{outdoor} \right)\text{ }\times \text{ }8760\text{ }\times \text{ }0.7\text{ }\times \text{ }0.2. $$
(3)

For children:

$$ R~=\text{ AEDE}\left( \text{indoor} \right)~=\text{D}\left( \text{indoor} \right)\text{ }\times \text{ }8760\text{ }\times \text{ }0.8\text{ }\times \text{ }0.8, $$
(4)
$$ S~=\text{ AEDE}\left( \text{outdoor} \right)~=\text{D}\left( \text{outdoor} \right)\text{ }\times \text{ }8760\text{ }\times \text{ }0.8\text{ }\times \text{ }0.2. $$
(6)

For infants:

$$ T~=\text{ AEDE}\left( \text{indoor} \right)\text{ }\!\!~\!\!\text{ }=D\left( \text{indoor} \right)\text{ }\times \text{ }8760\text{ }\times \text{ }0.9\text{ }\times \text{ }0.8, $$
(6)
$$ U~=\text{ AEDE}\left( \text{outdoor} \right)~=D\left( \text{outdoor} \right)\text{ }\times \text{ }8760\text{ }\times \text{ }0.9\text{ }\times \text{ }0.2. $$
(7)

The AEDE indoor and outdoor values for rural and urban areas were calculated using the Eqs. (2)–(7). The total annual effective dose equivalent was calculated by summation of the indoor and outdoor AEDE values:

$$ \text{Total AEDE }\!\!~\!\!\text{ = AEDE}\left( \text{indoor} \right)\text{ + AEDE}\left( \text{outdoor} \right). $$
(8)

Calculation of cosmic radiation. The cosmic radiation was calculated using the elevation above the sea level. The measured outdoor gamma dose rate involved the terrestrial radiation and cosmic radiation. The terrestrial radiation level was estimated by subtracting the cosmic radiation level from the outdoor gamma dose rate. Equation (9) was used for calculating the cosmic radiation level [6, 23]:

$$ E\left( a \right)~=E\left( 0 \right)\left[ 0.21{{e}^{1.649a}}+0.79{{e}^{0.4528a}} \right], $$
(9)

where E(a) is the effective dose rate, a is the altitude above sea level (km), and E(0) is the cosmic radiation at the sea level.

Risk Assessment

Lifetime effective dose. The total annual effective dose equivalent and duration of life were used for calculating the lifetime effective dose:

$$ \text{Lifetime Effective Dose }=\text{ }[52\left( P~+Q \right)\text{ }+\text{ }16\left( R\text{ }+\text{ }S \right)\text{ }+\text{ }2\left( T\text{ }+\text{ }U \right), $$
(10)

where the quantities P, Q, R, S, T, and U were calculated by Eqs. (2)–(7), and the duration of life was taken as 70 years (adult: 52 years, children: 16 years; infants: 2 years) [10, 16].

Calculation of excess lifetime cancer risk (ELCR). Low levels of gamma rays cause a stochastic health risk, which can be probably used for the assessment of cancer induction and genetic damage. Cancer risk estimates the potential carcinogenic effects involving the probability of cancer incidence in population for a specific lifetime. The value of lifetime effective dose and risk factor was used for calculating the excess lifetime cancer risk. The value of the risk factor used by ICRP 60 for the public is 0.05 [10, 16]. The cancer risk was calculated only for those areas where the lifetime effective dose exceeded 100 mSv [21, 24]:

$$ \text{ELCR }\!\!~\!\!\text{ = Lifetime Effective Dose }\!\!~\!\!\text{ }\times \text{ Risk factor}, $$
(11)

Calculation of the effective dose for different organs and tissues of human body (Dorgan). The value of AEDE was used for calculating the effective dose for different organs and tissues of the human body. The value of conversion factor of the organ dose from the air dose for bone marrow, kidneys, liver, lungs, ovaries, testes, and whole body are 0.69, 0.62, 0.46, 0.64, 0.58, 0.82, and 0.68 respectively [25, 26].

$$ {{D}_{\text{organ}}}=\text{ AEDE }\!\!~\!\!\text{ }\!\!\times\!\!\text{ Conversion factor}, $$
(12)

RESULTS AND DISCUSSION

Data on the radiation level (indoor and outdoor gamma dose rate, AEDE, and lifetime effective dose values) for 42 rural and 10 urban areas of Durg district of Chhattisgarh are presented in Table 1. The radiation maps for the indoor and outdoor gamma dose rate are shown in Fig. 2. In this study, the indoor and outdoor gamma dose rate levels at rural areas range from 145 ± 6 to 253 ± 10 and from 117 ± 5 to 185 ± 7 nSv/h, respectively. For urban areas, the indoor and outdoor gamma dose rates levels range from 191 ± 8 to 224 ± 9 and from 115 ± 5 to 189 ± 8 nSv/h, respectively. Histograms for the gamma dose rate for urban and rural areas are shown in Fig. 3. The arithmetic and geometric mean values of the indoor gamma dose rate for rural areas (200 ± 8 and 198 ±8 nSv/h) are insignificantly lower compared to urban areas (203 ± 6 and 202 ± 6 nSv/h). The arithmetic and geometric mean values of the outdoor gamma dose rate for rural areas (154 ± 6 and 153 ± 6 nSv/h) are insignificantly higher than those for the urban areas (152 ± 5 and 151 ± 5 nSv/h). The maximum value of the indoor gamma dose rate, 253 ± 10 nSv/h, was found in Anjora (Dhaba Anjora) rural area; UNSCEAR reported gamma dose rates varied from 20 to 200 nSv/h. Our results show that the indoor gamma dose rates for the rural area points DV-1, DV-5, DV-7–DV-10, DV-12, DV-13, DV-16–DV-19, DV-21, DV-23, DV-29, DV-31, DV-33, DV-35, DV-38, DV-39, and DV-41 and from urban area points DU-1, DU-2, DU-7, DU-8, DU-10 exceed 200 nSv/h. The elevation in the study area varies from 270 to 325 m above MSL (mean sea level). The variation of the cosmic radiation with the elevation in this range is shown in Fig. 4. The cosmic radiation level throughout the study area varies from 32.87 to 33.32 nSv/h.

Table 1. Indoor and outdoor gamma dose rates and AEDE (annual effective dose equivalent) values for rural (DV points) and urban (DU points) areas of Durg district of Chhattisgarh
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Fig. 2.
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Radiation map for outdoor and indoor gamma dose rate of Durg district, Chhattisgarh, India.

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Fig. 3.
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Gamma dose rate histograms for (a) urban and (b) rural area. (f) Frequency.

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Fig. 4.
figure 4

Plot of the dose rate from cosmic rays vs. elevation above sea level.

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Tarenga formations are dominantly constituted by silt and very fine mudstone with minor calcareous, cherty, and dolomitic beds [19]. The terrestrial gamma dose rate in the area of Tarenga formation varies from 108.1 to 138.1 nSv/h. Gunderdehi formation consists mainly of calcareous purple to grey shale including sandstone and siltstone [19]. The radiation level in this lithology varied from 100.8 to 111.9 nSv/h. Chandi formation contains dolomite and stromatolitic limestone. It generally exists in two forms: Chandi limestone and Chandi sandstone [19, 20]. The gamma radiation level for Chandi limestone varies from 81.9 to 152.1 nSv/h, and for Chandi sandstone, from 99 to 156 nSv/h.

Annual effective dose equivalent (AEDE). The indoor and outdoor annual effective dose equivalents for an adult with standard error bars for rural and urban areas are shown in Fig. 5. The indoor and outdoor AEDE values for rural areas range from 0.71 ± 0.03 to 1.24 ± 0.05 and from 0.14 ± 0.01 to 0.23 ± 0.01 mSv/year, respectively, and for urban areas, from 0.94 ± 0.04 to 1.10 ± 0.04 and from 0.14 ± 0.01 to 0.23 ± 0.01 mSv/year, respectively. The indoor AEDE values was found to be higher than the outdoor AEDE values for all the study areas, both rural and urban, due to higher occupancy factor in the inside (people spend more time inside). The arithmetic mean value of total AEDE for rural, 1.17 ± 0.05 mSv/year, is virtually equal to that for urban, 1.18 ± 0.05 mSv/year. The world population weighted average reported for AEDE is 0.87 mSv/year (UNSCEAR 2000).

Fig. 5.
figure 5

Outdoor and (2) indoor annual effective dose equivalents with standard error bars for (a) rural and (b) urban areas of Durg district, Chhattisgarh, India.

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Figure 6 shows the total annual effective dose equivalents with standard error bars for the rural and urban areas of Durg district in comparison with the world population weighted average value. As can be seen, the AEDE values of for rural and urban from Durg district are slightly higher than the world average value. The data presented in this study can serve as useful baseline data for Durg region.

Fig. 6.
figure 6

(1) Total annual effective dose equivalents with standard error bars for (a) rural and (b) urban area of Durg district and their comparison with (2) world population weighted average value.

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Hypothesis test. As from Table 2, the mean outdoor and indoor gamma dose rate is 152.3 and 202.5 nSv/h, respectively. The standard deviation is 24.7 and 11.6 for ten urban area observations. The significance value for the paired sample correlation between the outdoor and indoor gamma dose rates for the urban area is 0.092, which is greater than 0.05. Thus, the null hypothesis should be selected, namely, the hypothesis that there is no relationship between the outdoor and indoor gamma dose rates for the urban area. The correlation between the outdoor and indoor gamma dose rates for the urban area is negative. The significance value of the paired sample t-test between the outdoor and indoor gamma dose rates for the urban area is 0.001, which is less than 0.05 and allows the null hypothesis for this case to be rejected. Namely, there is a significant difference between the outdoor and indoor gamma dose rates for the urban area. The mean for the outdoor and indoor gamma dose rates is 153.8 and 200 nSv/h, with the standard deviation of 14.0 and 25.7, respectively, for 42 rural area observations. The significance value for the paired sample correlation for the rural area is 0.394, which is also greater than 0.05. Thus, in this case also the null hypothesis should be selected, i.e., the hypothesis that there is no relationship between the outdoor and indoor gamma dose rates for the rural area. The correlation between the outdoor and indoor gamma dose rates for the rural area is very weak (0.135). The significance value of the paired sample t-test for the rural area is 0.000, which is less than 0.05 and allows the null hypothesis for this case to be rejected. Namely, the difference between outdoor and indoor gamma dose rates is significant for the rural area also.

Table 2. Statistical data of pair t-test for the gamma dose rate from urban and rural areas of Durg district, Chhattisgarh
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Indoor and outdoor gamma dose rate ratio. The indoor and outdoor gamma dose rate ratio for the rural and urban areas ranges from 0.89 to 1.66 and from 1.01 to 1.95, respectively, with an arithmetical mean of 1.31 and 1.41. The arithmetic mean value of the indoor to outdoor ratio for the rural area is slightly lower than the world population weighted average of 1.4 [6], and for the urban area this ratio is slightly higher. Only one rural study area (DV-06) of Durg district has the indoor to outdoor ratio lower than 1.00 (the indoor gamma dose rate is lower than the outdoor gamma dose rate).

The overall statistical data for the gamma dose rates for the rural and urban areas from the Durg district of Chhattisgarh are given in Table 3 in comparison with the world population weighted average.

Table 3. Statistical data on gamma dose rate for Durg district of Chhattisgarh and comparison between rural and urban values
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Excess lifetime cancer risk (ELCR). The possibility of the incidence of cancer and the potentially carcinogenic effects from gamma dose rates during a particular lifetime are evaluated by excess lifetime cancer risk (ELCR) [14, 21]. The ELCR values from the gamma dose rate for different areas (country/city) of the world are given in Table 4. The lifetime effective dose values exceed 100 in a few study areas. The maximum value of ELCR is 5.27 × 10–3 in Pitaura area. In this study, ELCR ranges from 5.17 × 10–3 to 5.27 × 10–3, exceeding the reported values for Turkey; Jhelum valley Northwest Himalayas, Pakistan; Alappuzha Kerala Ondo State, Nigeria; and Ebonyi State [10–17]. High uranium content of a water sample from Auri area (DV-17) was reported [32]; however, in this place the lifetime effective dose due to gamma radiation was found to be 85.2 ± 3.4 mSv, which is less than 100 mSv.

Table 4. Reported values of the excess lifetime cancer risk from gamma dose rate in different areas (country/city) of the world
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Effective dose for different body organs (Dorgan). The calculated average values of Dorgan for bone marrow, kidneys, liver, lungs, ovaries, testes, and whole body were 0.81, 0.81, 0.68, 0.54, 0.73, 0.75, and 0.96 mSv/y, respectively. The variation of Dorgan for different body organs and tissues and their Box plot are shown in Fig. 7. The effective dose for testes due to radiation was found to be higher than the tolerable limit of 1.00 mSv/years [15] in areas DV-8, DV-10, DV-12, DV-18, DV-19, DV-21, DV-31, DV-33, DV-38, DV-39, DV-41, DU-8, and DU-10. Only one area, DV-41, is characterized by a high effective organ dose for bone marrow.

Fig. 7.
figure 7

(a) Comparison of effective dose values for different organ of the body for rural (DV points) and urban (DU points) areas of Durg district and (b) Box plot for organ dose values.

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The reported values of the outdoor terrestrial gamma dose rate (cosmic ray dose rate subtracted from the outdoor gamma dose rate) near uranium mines in India and other countries are compared with the urban and rural areas of Durg district in Table 5. As can be seen, the mean value of terrestrial radiation for the rural and urban areas of Durg district is higher than the values for the Gobi village, Yadgiri district, Karnataka; Bagjata, Singhbhum district, Jharkhand; Turamdih, Jharkhand; few areas of Jaduguda, Jharkhand; Sikar district, Rajasthan, and lower than the values for Narwapahar, Jharkhand; Mailuu Suu, Kyrgyzstan, and few villages around uranium mine, Jaduguda [9, 27–31].

Table 5. Reported values of the outdoor gamma dose rate near uranium mines area (country/city) of the world in comparison with the data for Durg district
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CONCLUSIONS

The arithmetic and geometric mean values of the annual effective dose equivalent for rural and urban areas of Durg district are slightly higher than the world population weighted average. The maximum AEDE value and lifetime effective dose were found to be 1.45 ± 0.06 mSv/year and 105 ± 4 mSv in Pitaura (rural area). In 7% of rural areas from Durg district, the lifetime effective dose exceeds 100 mSv, whereas for the urban areas this value is not exceeded. The contribution of cosmic radiation (32.87 to 33.22 nSv/h) is lower than that of the terrestrial radiation (81.9 to 156.0 nSv/h). The arithmetic mean value of indoor gamma dose rate for the urban areas is slightly higher than that for the rural areas. In Anjora (Dhaba Anjora), Acchoti, and Pitaura areas, ELCR values were found to be 5.24 × 10–3, 5.17 × 10–3, and 5.27 × 10-3, respectively. The rural population of these three areas of Durg district is 2287, 2596 and 1605, respectively. This study will be helpful for making preventive measures toward cancer risk in the region. Rural and urban houses in Durg district are made of concrete, and almost everywhere the indoor gamma dose rate is higher than the outdoor value, except one location. The strongest radiation effect is exerted on testes, whereas the other parts are not affected. The paired sample t-test shows that there is no relationship between the outdoor and indoor gamma dose rates for rural and urban areas. The difference between the outdoor and indoor gamma dose rates is significant. The radiation level obtained in this study is higher than in some areas near uranium mines. The radiation map of the gamma dose rate is reported for this region the first time.