Assessment of natural radioactivity in urban soil samples from Dhaka city and its associated health hazard

TL;DR

This study measured natural radioactivity in Dhaka city soil and found no immediate health risks.

Contribution

This is the first comprehensive dataset of urban soil radioactivity in Dhaka city.

Findings

Mean activity concentrations of 226Ra, 232Th, and 40K were 24.2, 52.0, and 352 Bqkg-1 respectively.

Radium equivalent activity was 125.7 Bqkg-1, below the global safety threshold.

Estimated effective dose rates and cancer risk were below international limits.

Abstract

Humans are constantly exposed to radiation from their natural environment including soil and gamma radiation has harmful effects on them, so determination of radioactivity concentration in soil are very important. The present study aims to measure the activity concentrations of naturally occurring radionuclides 226Ra, 232Th, and 40K in urban soil samples collected from thirty different areas of Dhaka city. The analyzed was performed using a High-Purity Germanium (HPGe) gamma-ray spectrometer. The results showed that the mean activity concentrations of 226Ra, 232Th, and 40K were found 24.2 ± 1.0 Bqkg-1, 52.0 ± 2.0 Bqkg-1, and 352 ± 11 Bqkg-1, respectively. The average concentrations of 226Ra and 40K fall below the internationally recommended safety limits of 35 Bqkg-1 and 400 Bqkg-1, respectively, while 232Th value exceeded the recommended limit of 30 Bqkg-1 by approximately 1.7 times. The mean value of radium equivalent activity (Raeq) was calculated as 125.7 Bqkg-1, which was far below the global safety threshold of 370 Bqkg-1. The estimated outdoor effective dose rates were 0.070 mSvy-1 and below the worldwide recommended limit of 1 mSvy-1. Additionally, excess life time cancer risk (ELCR) was below than the internationally accepted limit of 0.29 × 10−3. All things considered, the study is the first comprehensive dataset of urban soil in the area and revealed that there are no immediate health dangers due to the low radioactive hazard indices.

Figures

Location map of urban soil samples in Dhaka, Bangladesh.

Frequency distribution (Fig 2a–c) and normal Q-Q plots (Fig 2d–f) analysis of 226Ra, 232Th and 40K in urban soil samples from Dhaka city.

Equations

Full text

1. Introduction

Determining the concentrations of radionuclides in soil, both man-made and natural is essential for establishing baseline levels, which are critical for tracking changes over time, especially in the event of radioactive releases. This information is vital for both environmental monitoring and public health protection [1,2]. In 2011, the accident at the Fukushima Daiichi Nuclear Power Plant highlighted transboundary nature of nuclear accidents by releasing massive amounts of radioactive material into the surrounding environment. Approximately 80% of this material was eventually transported into the Pacific Ocean, contaminating terrestrial and other ecosystems [3–5]. Because radionuclides are found in a variety of geological elements, including rocks, soil, sand, water, and coal, plants, animals, and even human tissues [6–11]. As a result, both the environment and human beings are continuously exposed to radiation [6,12]. Understanding ionizing radiation is crucial for human radiation exposure, in contrast to nonionizing radiation. Electromagnetic radiation emitted by radioactive materials or occasionally by nuclear events is known as ionizing radiation [7,13]. Soil, in particular, is a major source of natural radioactivity and plays a key role in the migration and transfer of radionuclides into the broader environment [14,15]. The natural radioactivity in soil, primarily due to the decay of radionuclides from the uranium and thorium series, is widely recognized as a fundamental indicator of radiological contamination [16]. Because radionuclides from rocks can enter soil through rain and water flow and interact with soil particles, soil has a significant impact on natural background radiation. Soil radioactivity is a persistent source of radiation and radionuclide mobility in the environment due to human activities including the disposal of industrial waste and the heavy use of phosphate fertilizers [17,18]. The region has a high background radiation level, or the amount of background radiation brought on by human activity, as well as possible exposure impacts for the general public and geological and environmental factors related to soil radioactivity [19,20]. People receive maximum external gamma radiation from soil due to naturally occurring radionuclides such as ^238^U, ^232^Th and ^40^K [6,21]. Soil can also absorb and retain artificial radionuclides such as ^137^Cs, which is often unevenly distributed across different soil depths and terrestrial and artificial radionuclides can enter the human body through direct contact or through the food chain [22,23]. Long-term exposure to uranium and thorium, especially by inhalation, can result in a number of health issues, including oral tissue necrosis, chronic lung conditions, acute leukopenia, and anemia [12]. While thorium exposure has been connected to leukemia and malignancies of the bone, kidney, liver, lung, and pancreas, radium inhalation or ingestion can cause tumors in the bones, skull, and nasal tissues [18]. In recent times, there have been no detailed investigations on urban soils in Dhaka city and their associated health hazard assessments. Even though Dhaka city is growing quickly, there is still no complete study of the natural radionuclides and the radiological risks they pose in its urban soils. To fill this gap, the current study conducts a comprehensive investigation utilizing numerous soil samples, high-resolution gamma spectrometry measurements, a thorough radiological risk assessment, and rigorous statistical analyses. This method helps us better understand activity concentrations in a city that is both very crowded and hasn’t been studied much in terms of radiation. Therefore, the objectives of this study are to quantify the specific activities of the naturally occurring radionuclides ^226^Ra, ^232^Th, and ^40^K in Dhaka soils and calculate the associated radiological hazard indices. This study can contribute to urban planning, environmental and public safety policy making.

2. Materials and methods

2.1. Study area

Urban soil samples were gathered from thirty various areas in Bangladesh’s metropolis, Dhaka, for the study, and this city is the central zone of the country. The population of this city is approximately 1.25 million, according to a 2011 national survey [24]. This region was chosen because of its dense population, rapid urbanization, and risk of radioactive exposure from both natural and man-made sources. There are some nuclear medicine centers located in this city. Locations were chosen based on the accessibility of soil samples and to ensure a representative assessment of the area. Since the composition of soil and consequently, the concentration of radionuclides varies from one site to another, selecting multiple locations allowed for a more comprehensive understanding of how these variations influence radiological parameters such as effective dose rates and hazard indices. A location map of urban soil samples in Dhaka, Bangladesh, was created using ArcGIS Pro 2024 software and is shown in Fig 1.

Location map of urban soil samples in Dhaka, Bangladesh.

2.2. Sample collection and processing

From the preceding study area, 30 urban soil samples were collected from 30 different locations of Dhaka city for instances near hospitals, university campus, school, commercial area, park, residence area, etc. Because the study aims to assess the general urban background rather than point out potential contamination sources. No formal permits or approvals were required prior to sample collection because all soil samples were collected from publicly accessible locations. The sampling was conducted exclusively in open public areas, and no samples were taken from private property, restricted zones, or protected sites. Therefore, approval from any regulatory or local authority was not necessary for this study. Each of the locations was considered to prepare one representative soil sample. During sample soil sample collection from Dhaka city, a stratified sampling approach was employed [25]. The soil samples were taken between 0 and 5 cm below the surface. In order to improve reproducibility and dependability, three replicate samples were taken from each site throughout the winter months of December 2023 to February 2024. Nearly 1 kg soil samples were collected in polyethylene bags, then properly marked and identified by their locations using the Global Positioning System (GPS). Then all collected samples were transferred to the environmental radiation & radioactivity laboratory of the Atomic Energy Centre, Dhaka.

During sample preparation, International Atomic Energy Agency (IAEA) guidelines were following [26]. All irrelevant components of the soil samples were removed, such as portions of stone, pebbles, leaves and roots, and then the mass of the sample was assessed after it had dried for a few days at room temperature and then oven-dried and achieved a steady mass. The samples were then ground into a fine powder and homogenized using a 2 mm sieve. For further examination, around 500 g of soil sample was transferred to a cylindrical pot. All the sample beakers were sealed tightly, wrapped with thick vinyl tape around the screw necks and stored for around 40 days to attain secular equilibrium between parents and their progenies [27,28].

2.3. Gamma ray spectrometry system

The activity concentrations of ^226^Ra, ^232^Th, and ^40^K in the soil samples were determined using a p-type High-Purity Germanium (HPGe) detector. The model of the detector was NATS GCD-30185, Baltic Scientific Instruments, Riga, Latvia. The diameter of the detector was 59.4 mm and the thickness was 56.6 mm. The operating bias voltage of the detector was + 2700, with a relative efficiency of 30% and an energy resolution of 1.67 keV full width at half maximum (FWHM) at the 1332 keV peak of ^60^Co. A 16k multi-channel analyzer and associated electronics for data acquisition of photo-peak areas were connected to the system. Gamma-ray spectra were analyzed using Spectra Line GP© software. Energy calibration was performed using gamma-ray point source (^137^Cs and ^60^Co) and efficiency calibration was performed by using a standard reference material which code was 8501-EG-SVE, Eckert & Ziegler Analytics along with a multi-nuclide gamma-ray source ^252^Eu [29].

A background count was also taken using an empty container under identical conditions. The counting time for both background and sample were 20000 seconds. The counts of the background spectrum were subtracted from the counts of the sample spectrum to obtain final counts, from which the activities of each radionuclide were calculated.

For spectral analysis, the single gamma-ray line at 1460.822 keV was used to determine the activity concentration of ^40^K. The activity concentration of ^226^Ra was assessed using the photo-peaks at 295.221 keV and 351.922 keV from ^214^Pb, and 609.320 keV, 1120.310 keV, and 1764.551 keV from ^214^Bi. The activity of ^232^Th was derived from the photo-peaks at 238.630 keV and 300.087 keV from ^221^Pb, 911.205 keV and 968.970 keV from ^228^Ac, and 583.190 keV and 2614.533 keV from ^208^Tl [30]. However final activity was calculated using weighted mean approach considering all gamma lines [3]. From sample collection to counting precautions were taken to prevent any form of contamination.

2.4. Calculation of activity

The minimum detectable activity (MDA) represents the lowest activity level that the detection system can reliably measure. Any sample with a count rate below this threshold may yield a negative value, which is not accurate. The MDA for each radionuclide in the detector system can be calculated using the following formula [31,32]:

$$[eqn]$$

Where, 𝜎𝐵 denotes the standard deviation of the background count rate obtained from measurements conducted under identical conditions as the samples, but without any radioactive source or sample present, t is the counting time, $[eqn]$ is absolute efficiency at photon energy E, $[eqn]$ is the gamma photon emission probability at the gamma line corresponding to peak energy and w is the mass of the soil sample. For ^226^Ra, ^232^Th and ^40^K at 95% confidence for detector, the MDA was 0.33, 0.08 and 0.38 respectively for soil samples. By subtracting the required peak energy area from a linear background distribution of pulse spectra, the net sample amount was computed. It was possible to derive activity levels for each sample based on their net counts using the following formula [33]:

$$[eqn]$$

here, S represents the activity concentration of the soil sample in Bqkg^-1^, CPS is the count rate (count per seconds), $[eqn]$ represents the efficiency of the gamma energy, $[eqn]$ gives the absolute intensity of $[eqn]$ ray, and $[eqn]$ represents the sample mass in kilograms.

The uncertainty associated to the activity concentration of a radionuclide $[eqn]$ can be shown by following formula [3,34]:

$$[eqn]$$

Here, $[eqn]$ is the counting statistical uncertainty, $[eqn]$ is the uncertainty in gamma emission probability, $[eqn]$ is the uncertainty in detector efficiency and $[eqn]$ is the uncertainty in sample mass and $[eqn]$ is the uncertainty in time.

Next, weighted mean method was applied to obtain the final concentration of activity ( $[eqn]$ ± $[eqn]$ ) By integrating the different activity concentrations (S ± σS_m_) obtained from each distinct gamma-line. The weighted mean activity (Sm ± σSm) was calculated using the following formula [3]:

$$[eqn]$$

where $[eqn]$ is the weighted mean activity, and $[eqn]$ is the measured activity concentration for the i-the gamma line. $[eqn]$ is the weighted factor.

The weighted mean’s standard error is provided by [3]:

$$[eqn]$$

where $[eqn]$ is the appropriate uncertainty for the gamma line.

2.5. Radium equivalent activity (Raeq)

The radium equivalent activity is a widely used index for comparing the specific activities of materials containing ^226^Ra, ^232^Th and ^40^K. It is calculated using the following expression as [35]:

$$[eqn]$$

Where, Ra_eq_ is the radium equivalent activity in Bqkg^-1^. S_Ra_, S_Th_ and S_K_ are the activity concentrations (in Bqkg^-1^) of ^226^Ra, ^232^Th and ^40^K, respectively.

2.6. Absorbed dose rate

The concentration of radionuclides in the soil determines how much the natural radionuclides contribute to the absorbed dose rate in the air. Based on the radionuclides in the soil, absorbed dose rate conversion factors can be used to calculate the dose. The following formula is used to get the outdoor gamma absorbed dose rate using the conversion variables specified in UNSCEAR 2000 [6]:

$$[eqn]$$

Where, $[eqn]$ is the absorbed dose rate in air in nGyhr^-1^. S_Ra_, S_Th_ and S_K_ are the activity concentrations of ^226^Ra, ^232^Th and ^40^K in Bqkg^-1^.

2.7. Annual effective dose equivalent

Using the UNSCEAR (2000) conversion coefficient of 0.7 SvGy ⁻ ¹ and an outdoor occupancy factor of 0.2, the absorbed dose rate in air was converted to the annual effective dose equivalent (AEDE), assuming that people spend, on average, 20% of their time outdoors. Annual effective dose equivalent (AEDE) for outdoor was calculated using the following formula [6,12]:

$$[eqn]$$

where, $[eqn]$ is the annual effective dose equivalent (AEDE) for outdoor (mSvy^-1^), $[eqn]$ is the outdoor absorbed dose rate (nGyhr^-1^), 8760 is the number of hours in a year, 0.7 is the dose conversion coefficient for absorbed dose to effective dose (SvGy^-1^), 0.2 is the outdoor occupancy factor, 10^−6^ is the unit conversion factor from nGy to mSv.

2.9. Excess lifetime cancer risk

Examining the correlation between a populations lifetime cancer development likelihood and anticipated consumption of particular substances or exposure to particular circumstances can yield the excess lifetime cancer risk (ELCR) [36]:

$$[eqn]$$

Where, AEDE is the annual effective dose rate equivalent (mSvy^-1^), D_L_ is the duration of life (70 years) [18] and R_F_ is the risk factor (0.05 Sv^-1^ recommended by ICRP) associated with radiation exposure [37] and $[eqn]$ is used for convert mSv to Sv.

3. Results and discussion

3.1 Activity concentration

Urban soil samples were measured by using gamma ray spectrometry system for radioactivity analysis. Table 1 demonstrates activity concentrations of ^226^Ra, ^232^Th and ^40^K in thirty soil samples collected from different locations of Dhaka city.

Table 1: Activity concentrations of 226Ra, 232Th and 40K from the collected urban soil of Dhaka city.

From Table 1, it has been seen that the activity concentration in soil samples varied in the range of 13.8 ± 0.4 (N16 samples from Gulshan) to 35.0 ± 1.0 Bqkg^-1^ (N8 sample from Agargaon) for ^226^Ra, 32.0 ± 2.0 (N16 from Gulshan) to 66.0 ± 2.0 Bqkg^-1^ (N27 from Kuril Bishwa Road) for ^232^Th, and 253 ± 11 (N30 sample from Kamarpara) to 530 ± 12 Bqkg^-1^ (N8 sample from Agargaon) for ^40^K respectively. The studied average ^226^Ra activity concentration found 24.2 ± 0.1 Bqkg^-1^ which was less than the global average of 35 Bqkg^-1^, the average activity concentration of ^232^Th estimated in this study was 52.0 ± 3.0 Bqkg^-1^ and thus much higher than the global average value of 30 Bqkg^-1^ and for ^40^K the average activity concentration estimated in this study was 352 ± 11 Bqkg^-1^ which was also below the global average value of 400 Bqkg^-1^. From Table 1 clearly demonstrates that the order of radionuclide abundance in Dhaka city urban soil was ^40^K > ^232^Th > ^226^Ra. Some locations (N2, N5, N7, N8, N11, N18, N29), the activity concentration of ^40^K exceeded the recommended global limit, likely due to abundance of ^40^K in rock and minerals and weathering, anthropogenic activities, soil composition [32]. Higher ^232^Th activity concentration in the soil sample caused by a number of things, including urbanization, anthropogenic activities, presence of thorium bearing mineral geology and erosion and weathering process [23,38]. Additionally, radium can move easily in environment and also the abundance of thorium is higher than uranium in earth crust [6].

Table 1 also shows that the skewness values of +0.584 for ^226^Ra, − 0.997 for ^232^Th, and + 1.057 for ^40^K indicate that the activity concentration distributions deviate from perfect normality and exhibit different degrees of asymmetry. The positive skewness observed for ²²⁶Ra and ⁴⁰K indicates that their distributions are right-skewed, meaning that most samples have relatively low to moderate activity concentrations, with a few samples showing comparatively higher values. Whereas, the negative skewness of ^232^Th indicates a left-skewed distribution, indicating a limited number of samples with lower activity values relative to the mean. The Kurtosis values for ^226^Ra, ^232^Th and ^40^K + 0.397, + 0.632 and +0.156, all approximating zero, indicating that the distributions are nearly normal.

Table 2 demonstrates comparison of present work with previously done similar works of some countries all around the world.

Table 2: Comparison of present work with previous work around the world.

Table 2 presents a comparison of the activity concentrations of ^226^Ra, ^232^Th, and ^40^K in soil from different countries worldwide, together with the results of the present study conducted in Dhaka, Bangladesh. To ensure consistency and avoid selective bias, only one representative dataset per country is included, and all values refer to surface soil measured using gamma-ray spectrometry. The activity concentrations of ^226^Ra and ^40^K observed in Dhaka soil are similar to the reported values of Japan, Saudi Arabia, Ghana, and Tunisia, while lower than values reported from China, Nigeria, Malaysia, and Turkey; however, they are comparable to Turkey and Mexico. The ^232^Th concentration in Dhaka soil is slightly higher than in Japan, Saudi Arabia, India, Tunisia and the UNSCEAR world average value but lower than in China, Nigeria, Malaysia and Cameroon. The variations of radionuclides may be due to the geological circumstances, topographical characteristics, rock formation, mineralogical features, and soil conditions, weathering can all contribute to changes in radionuclides [5,14]. A number of variables, such as industrial processes, building materials, human activity, and the redistribution of naturally existing radionuclides, instructional waste practice, presence of thorium rich mineral may be responsible for the higher ^232^Th concentrations found in Dhaka soil [18,23,51]. The physical, chemical, and geological structure and location features of soil samples vary with human activities, making the results very location and area specific [29,52,53].

3.2 Radiological hazard indices

Table 3 demonstrates radium equivalent activity (Ra_eq_), outdoor absorbed dose, indoor absorbed dose, outdoor annual effective dose and indoor annual effective dose of all the soil samples.

Table 3: Radium equivalent activity, annual effective dose rate, excess lifetime cancer risk and hazard indices of soil samples.

From Table 3, it has been observed that radium equivalent activity of investigated urban soil samples ranged from 84.05 to 154.46 Bqkg^-1^ with the mean value of 125.74 Bqkg^-1^ which is less than the world average value of 370 Bqkg^-1^ [6]. The range of outdoor absorbed dose rates was found to be 38.96 to 71.49 nGyhr^-1^. The average value (57.30 nGyhr^-1^) of outdoor absorbed dose rates was found lower than the world average values of 59 nGyhr^-1^ [6]. The outdoor annual effective dose rate can be found in the range of 0.048 to 0.088 mSvy^-1^ with an average value of 0.070 mSvy^-1^, and in comparison, with the world average value, it is within the world recommended limit which indicates there is no significant radiological risks to the local community or ecosystem from the soil. The range of ELCR vale was found 1.67 $[eqn]$ to 3.07 $[eqn]$ The average value of excess lifetime cancer risk (ELCR) was 2.46 $[eqn]$ and below than the world recommended value of 0.29 $[eqn]$ indicating there is no matter of concern toward the overall public health but monitoring is important [6,54].

The descriptive statistics of ^226^Ra, ^232^Th and ^40^K and radium equivalent activity, absorbed dose, annual effective dose and ELCR for urban soil of Dhaka city presented in Table 4.

Table 4: Mean, median, SD, minimum and maximum of 226Ra, 232Th and 40K and radium equivalent activity, absorbed dose, annual effective dose and ELCR for urban soil.

Table 4 shows the minimum, maximum, median, mean, SD, skewness and Kurtosis of ^226^Ra, ^232^Th and ^40^K and Ra_eq_, absorbed dose, annual effective dose and ELCR for urban soil of Dhaka city. From the minimum and maximum values, it is seen that the radionuclide concentration exhibits a wide range of variations across the urban soil samples. The median is near the mean for all radionuclides, and the mean value is higher than the standard deviation, suggesting less variability.

3.3 Analysis of frequency distributions and Q-Q plots

The distributions of radionuclide activity concentrations in urban soil samples from Dhaka city were assessed using histograms and normal Q–Q plots generated with Python 3.13.3, employing its mathematical and visualization libraries (Fig 2). The histograms (Fig 2a–2c) indicate that the activity concentrations of ²³²Th, and ⁴⁰K were asymmetric, showing varying degrees of skewness and multimodality. Specifically, ²²⁶Ra and ⁴⁰K exhibited positive skewness, while ²³²Th showed slight negative skewness. These multimodal patterns likely reflect the heterogeneous mineralogical composition of the soils, which may influence gamma radiation emission and potentially affect localized radiological exposure.

Frequency distribution (Fig 2a–c) and normal Q-Q plots (Fig 2d–f) analysis of 226Ra, 232Th and 40K in urban soil samples from Dhaka city.

The normal Q–Q plots (Fig 2d–2f) corroborated these observations, as data points for all three radionuclides deviated from the 45° reference line, particularly in the tails, indicating deviations from normality. Despite these deviations, no pronounced extreme outliers were observed for ²²⁶Ra, ²³²Th, and ⁴⁰K, suggesting the absence of anomalous contamination within the study area.

Overall, the combined histogram and Q–Q plot analyses indicate that although the radionuclide distributions are not strictly normal, the activity concentrations show consistent patterns without extreme outliers. The observed skewness and multimodality likely arise from soil heterogeneity, which is a key factor in understanding the environmental distribution and radiological characteristics of ²²⁶Ra, ²³²Th, and ⁴⁰K in the study area. These findings align with previous studies [55,56], highlighting the importance of site-specific assessment in radiological risk evaluation.

The Shapiro–Wilk test for ²²⁶Ra, ²³²Th, and ⁴⁰K was performed in Table 1 to support the clarifications of frequency distributions and Q–Q plots of the obtained values. As it has been found that all p-values were greater than 0.05, the data did not significantly deviate from a normal distribution. This formally supports the findings from the Q–Q plots and frequency distributions.

4. Conclusion

Thirty urban soil samples from Bangladesh’s most polluted city were used in this study to determine the activity concentrations of ^226^Ra, ^232^Th, and ^40^K as well as the related radiological health risk. The results showed that ^232^Th and ^40^K are the primary determinants affecting radiological exposure and risk in urban soils, with ^226^Ra having a negligible effect. The variations of radionuclides activity concentration may be the industrial activities and characteristics of the soil environment of Dhaka city. While the activity levels of ^226^Ra and ^40^K fall within the internationally recommended safety limits, the concentration of ^232^Th exceeds limit. However, However, the radium equivalent activity remained below the safe limit. Also, the study calculated an annual effective dose rate (0.070 mSvy^-1^) was found below the UNSCEAR limits of 1mSvy^-1^. The excess lifetime cancer risk (ELCR) was also within the permissible global threshold. Notably, no artificial radionuclide such as ^137^Cs was detected, suggesting the absence of nuclear contamination or anthropogenic radioactive sources near the study area. The findings of this study suggest continuous monitoring of soil radioactivity in Greater Dhaka to ensure the radiological safety of the residents of this city. Additionally, this study presents the first systematic baseline data, incorporating radiological health risk assessment and statistical analysis, to inform policy making and future radiological mapping and risk management as Bangladesh prepares to operate the Rooppur Nuclear Power Plant.