Health risk assessment of natural background radiation in the soil of Eastern province, Saudi Arabia

ABSTRACT The activity concentrations of natural radionuclides 238U, 232Th, and 40K in surface soil samples from Eastern Province, Dammam city, Saudi Arabia were measured by gamma spectrometry using (HPGe) detector. The activity concentration average values of the radionuclides 226Ra, 232Th, and 40K in the soil samples ranged from 11.85–30.43, 5.74–15.97, and 362–469 Bqkg−1, respectively. With overall mean values of 16.73, 10.40, and 419.99 Bqkg−1 respectively. These values are within the world average. The mean value radium-equivalent activity value was 63.93Bqkg−1, lower the international limit 370Bqkg−1. The Representative level index (Iɣ), internal and external hazard index (Hin, Hex) of all samples under study were less than the limit of unity. The absorbed dose rate due to three primordial radionuclides lies in the range of 24.23–39.48 nGyh−1 with a mean of 31.68 nGyh−1, which yields an annual effective dose of 0.039 mSvy−1.The mean value of Excess lifetime cancer risk was calculated as 0.14 × 10−3 which is below the World average (0.29 x 10−3). The results obtained in this study are lower than the world recommended values and are found to be safe for the public and environment. Baseline data of this type will almost certainly be important in making estimates of exposure to populations.


Introduction
Gamma radiations emitted from naturally occurring radioisotopes, such as 40 K and 238 U, 232 Th and their decay products that are present in environmental materials such as soil, rock, water, granite, building materials, etc (El-Taher, 2012). Radioactivity distribution in soil results from the rock from which it derives and is based on the nature of its geological formation (UNSCEAR, 2000). Soil not only acts as a source of continuous radiation exposure to humans but also as a medium of migration for the transfer of radionuclides to biological systems (Senthilkumar & Narayanaswamy, 2016). The soil is an important environmental material which is used for many purposes such as building raw materials and products, for landfilling in playgrounds, for streets, etc., containing natural radionuclides that contribute to indoor and outdoor exposure. Therefore, measurement of natural radioactivity in soil is very important to determine the amount of change of the natural background activity with time as a result of any radioactive release. Various studies over the world in the last decades have been done to determine health hazards due to background radioactivity in the soil (Bolat, Öner, & Çetin, 2017;Ismail, Abdullahi, Samat, & Yasir, 2018;Rajesh, Kerur, & Anilkumar, 2017;Shilpa, Anandaram, & Mohankumari, 2018). Such data also enrich the global data bank on radioactivity that will allow a more accurate estimation of global average values of dosimetric quantities. In Saudi Arabia, there are few surveys of radioactivity in soils have been carried for different regions (Alashrah, 2016;Alashrah & El-Taher, 2016;Al-Ghamdi, 2014;El-Taher & Al-Zahrani, 2014). The objective of this study is to measure the activity concentrations of 226 Ra, 232 Th, and 40 K in the surface soil samples from Dammam city, Eastern province, Saudi Arabia, to estimate the potential health impact to the human in the area under investigation. The outcomes of this work will form a baseline data set, which will enable the estimations of population exposure. The data will also contribute to the radioactivity mapping of Saudi Arabia surface soils levels.

Location of study area
The Eastern Province is the largest province by area and is the third most populous province in Saudi Arabia. The Eastern Province is home to most of Saudi Arabia's oil production. The province's capital is the city of Dammam, which hosts the majority of the region's population. Dammam city has an area of 800 Km 2 and it is bound between latitudes 26°24′and longitudes 50°08′. The temperature in Dammam city varies from 10°C to 29°C with an average of 15°C. Figure 1 shows the geographical location of this area. This study was carried out to determine, the concentrations of the natural radionuclides in surface soil from different sampling stations in Dammam city. This area is an industrial region as there are large industrial companies such as for petroleum and chemical factories. This feature makes this region an interesting candidate for radiological studies, although, there is no study on environmental doses from external exposures before.

Samples collection and preparation
Surface soil samples (about 0-5 cm) were collected from 23 different locations of Dammam city. Each sample was taken with a coring tool within an area of 1 m 2 , five cores were taken for each sample, one in the middle and four cores from the corner, these samples were then mixed to make a single sample after removing a top layer of vegetation and roots. The samples were dried at 105°C for 24 h to remove moisture, then subsequently crushed, ground to fine powder, homogenized by passing through a 2 mm test sieve (IAEA, 1989). About 500 g of the homogenized soil samples was transferred into cylindrical containers. They were carefully sealed and stored for at least 30 days before gamma-ray analysis was performed to allow 226 Ra and its short-lived progenies to reach secular equilibrium.

Measurements
Gamma spectroscopy measurements were carried by using High Purity Germanium (HPGe) detector with coaxial-type vertical dipstick cryostat (Canberra Model number CPVD 530-15,200) P-type with diameter 61 mm، height 54.6 mm. The system has a resolution (FWHM) of (3.0-3.5 keV) for 1332.5 keV gamma-ray peak of 60Co coupled to a computer-based MCA. Spectrum analyzer carried out by using Gamma Vision V6.0 software. The detector was housed inside a massive cylindrical lead shield to reduce the background radiation. Qualitative and quantitative analysis of gamma spectra were processed using the GENIE 2000 program. The spectrometer was tested for its linearity and then calibrated for energy using gamma sources supplied by the International Atomic Energy Agency, Vienna. Counting of samples and background was done for 36000 s duration.
The activity concentrations of the natural radionuclides in the measured samples (A S ) were computed using the following relation (El-Taher & Al-Zahrani, 2014): where C a is the net gamma counting rate (counts per second), ε the detector efficiency of the specific γ-ray, P r the absolute transition probability of Gamma-decay and M s the mass of the sample (kg).

Calculation of the radiological hazard indices
2.4.1. Radium equivalent activity Radium equivalent activity gives a single index, which expresses the gamma yield from a various mixtures of 226 Ra, 232 Th, and 40 K in the sample. The radiumequivalent activity index was given as (Beretka & Matthew, 1985;UNSCEAR, 2000): where A Ra , A Th , and A K were the radioactivity concentration in Bqkg −1 of 238 U, 232 Th, and 40 K. The maximum value of Ra eq must be less than 370 Bqkg −1 .

The external and internal hazard index
To estimate the level of gamma-ray radiation associated with natural radionuclides in specific construction materials is defined by the terms external hazard index (Hex) and internal hazard index (Hin) as the following Equations (3) and (4) (Beretka & Matthew, 1985): where A Ra , A Th , and A K are the activity concentrations of 226 Ra, 232 Th, and 40 K, respectively. The values of H ex and H in indices must be less than 1 mSvy −1 in order to cause any harmful effect to population (ICRP-65, 1993).

Representative level index
The representative level index, Iγ is used to estimate gamma radiation associated with the natural radionuclides in soil and calculated by the following equation (NEA-OECD, 1979): where A Ra , A Th , and A K are the activity concentrations of 226 Ra, 232 Th, and 40 K, respectively, in Bqkg −1 .

Absorbed gamma dose rate (D)
According to UNSCEAR, 2000, the absorbed gamma dose rate in air 1 m above the ground surface for uniform distribution of natural radionuclide's can be calculated using the activity concentration (A Ra , A Th , and A K ) in (Bqkg −1 ) and dose conversion factors 0.462, 0.604, and 0.0417 nGyh −1 /Bqkg −1 for 238 U, 232 Th, and 40 K substituted into the formula. D nGhy À1 À Á ¼ 0:462 A Ra þ 0:604 A Th þ0:0417A K (6)

Annual effective dose (D eff )
The annual effective dose equivalent (D eff ) was calculated from the absorbed dose (D) by applying the dose conversion factor of 0.7 Sv Gy −1 with an outdoor occupancy factor of 0.2 and 0.8 for indoor (UNSCEAR., 2000): h=year ð Þ Â 0:7 Â ð10 3 mSv=nGy 10 9 Þ Â 0:2 (7) 2.4.6. Excess lifetime cancer risk (ELCR) Based upon calculated values of annual effective dose excess lifetime cancer risk (ELCR) was calculated using the following equation; where (D eff ) is the annual effective dose, DL life expectancy (70 years) and RF (Sv −1 ) are a fatal risk factor per Sievert, which is 0.05for the public (ICRP-60, 1990).  Ra,232 Th, in all samples were measured to be lower than the world's average values. The high activity of 40 K in Some samples may be due to the fact that these samples were collected from areas near the industrial or agriculture lands. The mean activity of 232 Th was lower than that of 226 Ra, whereas the 40 K activity was the highest one and represents the largest contribution about 93% of the total activity ( 226 Ra + 232 Th + 40 K) in soil, which is in agreement with the well-known fact that potassium in the earth's crust is of the order of percentage while U and Th are in ppm level (Ramasamy, Suresh, Meenakshisundaram, & Ponnusamy, 2011) . The activity concentrations values 226 Ra, 232 Th, and 40 K in the soil samples from the study region were shown in Figure 2. A comparison of the mean values obtained in the present work with the same investigations soil of Saudi Arabia and other countries were presented in Table 2. The comparison shows that the values of soils under consideration were comparable with others and were within the world average values as reported by UNSCEAR, 2000. The variations in the concentrations of the radioactivity in the soil of the various locations of the world, depending upon the geological and geographical conditions of the area (UNSCEAR, 2000).

Radiological hazard indices
The computed values of (Ra eq ) were tabulated in Table 1, where the other indices average values were summarized in Table 3. The following observations can be detected from these Tables 1 and 3:    Therefore, based on these results of the radiological hazard, one can conclude that there is no health hazard of the study area and it is still in the zones of normal radiation level, which leaves the soil radioactivity there less a threat to the environment as well as the human health. Comparison of the values of Radium equivalent (Ra eq ), absorbed dose (D), the annual effective dose (D eff ) for the soil samples were plotted in Figure 3. The results of external (Hex), internal hazard index (H in ), level index (Iɣ) were presented in Figure 4. The relative contribution to dose due to 40 K was 56.99%, followed by the contribution due to 226 R and 232 Th as 22.55%, 20.46%, respectively, which were represented in Figure 5.

Conclusion
The average activity concentrations of 226 Ra, 232 Th, and 40 K were 16.73,10.40, and 419.93 Bq/kg, Table 3. Mean values of external (H ex ) and internal (H in ) hazard indices, Representative level (I γ ), absorbed dose rates (D _ ), annual effective doses (D eff ) and excess lifetime cancer risk (ELCR) for soil samples under investigation.  respectively. The study showed that the measured values are lower than the values in the worldwide soil. The values of the Radium equivalent and absorbed dose rate were 63.93 Bq/kg and 31.68 nGy/h, respectively. These values were lower than the acceptable limits 370 Bq/kg for Radium equivalent and 59 nGy/h, for absorbed dose rate. The external hazard index (H ex ), internal hazard index (H in ) and representative level index (I gr ) were found to be less than the acceptable limit of unity. The results obtained in this study fall within the range of values reported in similar studies conducted worldwide and are below the values that cancause the significant radiation hazard to both the environment and human health and can be used as a building raw materials or other human activities without any radiological risk. However, this data may provide a general background level for the area being studied and may also serve as a guideline for future measurement and assessment of possible radiological risks to human health in this region.

Disclosure statement
No potential conflict of interest was reported by the author.