Introduction

Soil, a significant reservoir for environmental contaminants, consists of various organic and mineral components. As a result, it naturally contains certain levels of radioactive elements, primarily influenced by the parent rock type from which the soil originated. The physicochemical properties of soils further play a crucial role in determining the behavior, concentration, and distribution of radioactive materials within them [1]. Upon inhalation and/or ingestion, these radionuclides emit gamma rays, beta particles, and alpha particles, thereby irradiating the host organism [2]. The presence of natural radionuclides in dust depends on its amount in the origin soil. Also, the origin of dust is mainly related to atmospheric dust, agricultural activities, plant types of the area, soil characteristics, and environmental pollution.

The radiological risk associated is important from the point of view of radiation protection, and some research was reported recently [3,4,5,6]. Naturally occurring radioactive materials (NORM) such as 40K and 238U, 232Th, and their decay products that are present in environmental materials such as soil [7, 8], rock [5, 9], water [10,11,12], and building materials [13,14,15,16,17], can be harmful to human health. Based on the geological formation of the soil, the distribution of radioactivity in soil depends on the type of rock from which it is derived, as well as the nature of its geological composition [18]. Soil not only acts as a source of continuous radiation exposure for humans, but also acts as a means of transporting radioactive materials in the form of dust into the respiratory system [19].

A number of factors influence the distribution of NORM in different geoenvironmental components (e.g., soil, sediment, water, dust), including weathering processes, local geology, and climate conditions [20]. The presence of NORM in sediments or soil is typically related to external radiation exposures if gaseous radon inhalation is not considered. Since exposure to NORM in water involves multiple pathways, these effects are negligible due to the low levels of NORM in natural water sources [21]. Unlike the water levels of NORM, the dust levels of NORM cannot be ignored. Additionally, radiation exposures through NORM in dust samples are not limited to external routes (ignoring radon inhalation). The radionuclides contained in dust can enter the lungs through inhalation.

This research was carried out to evaluate the concern of NORM in aerosol dust of the capital city, Nicosia, and assessed the radiological risk indexes in the study area. For this purpose, the NORM concentration, Radium equivalent activity index (Raeq), external hazard indices(Hex), internal hazard indices (Hin), and gamma activity concentration index (Iγ) were calculated in the study area.

Methodology

Study site

Cyprus, latitude and longitude are 35° 22′ 11.368″ N, 32° 56′ 17.808″ E, and 35° 40′ 11.104″ N, 34° 34′ 40.762″ E, the third-largest island in the Mediterranean Sea. The North section of Cyprus is neighbor of Syria in the East and with Turkey in the North in the Mediterranean Sea. This island has 220 km of length and 90 km of breadth. Cyprus Island is 9251 km2. The capital city of Cyprus, Nicosia, was selected as the study area. A copper mine is located 30 km away from the study area. An ancient Roman slag pile containing copper was found in the western coastal region of the state in 1914, and the company was founded in 1916. When the mining operation was abandoned in 1974, the tailing deposits were exposed to the environment [22].

Sample collection and preparation

A total of 26 urban dust samples (each weighing over 250 g) have been collected from different locations in the most densely populated district of North Nicosia in order to determine the amount of pollution (Fig. 1). The dustpan and brush used at each sample site were clean, and sampling was conducted with care so that small particles were not disrupted during the process. This study uses a similar method of sampling preparation to those documented in previous studies reported in the literature with the same results [22]. In order to facilitate sample handling, self-sealed polyethylene containers were used to transport samples to the laboratory. Following the drying process, the samples were mechanically sieved and mixed after drying at 80 °C for 48 h. Subsamples were weighed and stored in polyethylene flasks in a cold, dry area until analysis. Because these particles can remain suspended for extended periods of time, they were selected for the study. Another concern associated with fine particles is the increased health risks compared to coarser particles [23]. A sieve of 100 mesh size was used to separate fine particles. The size of those fine particles is between a few microns to 100 microns. The samples were put inside a cylindrical container.

Fig. 1
figure 1

Geographical location of 26 sampling points in the study area

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Measurement and analysis process

In the current study, the levels of NORM in dust samples were determined using the gamma-spectroscopy protocol reported in previous studies [7, 16, 22, 24,25,26,27,28]. Measurements were determined using a High Purity Germanum (HPGe) well detector with 80% efficiency related to the NaI detector, and the sample counting time was 80,000 s per sample. There were three radionuclides identified at the following energies: 226Ra (351.9 keV for 214Pb, 609.2 keV for 214Bi), 232Th (238.6 keV for 212Pb, 583.1 keV for 208Tl, 911 keV for 228Ac) and 1460.83 keV for 40K. The energy range of approximately 60–1500 keV was calibrated using four standard point sources containing 241Am, 133Ba, 137Cs, and 60Co. Based on IAEA reference materials RGU-1 (U-ore), RGTh-1 (Th-ore), and RGK-1 (K 2SO4) packed in the same manner as the samples in the same geometry, the spectrometer was calibrated for efficiency over the photon energy range of 186–2700 keV. The quality assurance of measurements was assessed through the analysis of Standard Reference Material IAEA Soil-375 [29]. Data acquisition and analysis were carried out using the GENIE 2000 and Gamma Analysis Software V.3.3, respectively. The following equation was used to determine the minimum detectable activity (MDA) [30] for the detector at a 95% confidence level:

$$MDL\left(\frac{Bq}{kg}\right)=\frac{{K}_{\alpha }.\sqrt{{N}_{B}}}{\eta (E){P}_{\gamma }{T}_{C}M}$$
(1)

where \({P}_{\gamma }\) is the probability of gamma emission, \({K}_{\alpha }\) is the statistical coverage factor equal to 1.645, NB is the background count (cps), \(\eta (E)\) is the photo-peak efficiency (dimensionless),\({T}_{C}\) is the counting time(s), and M is the sample mass (kg). The Minimum Detectable Activity (MDA) for each of the radionuclides of interest was computed using Eq. (1), resulting in values of 0.50 Bq kg–1 for 226Ra, 0.70 Bq kg–1 for 232Th, and 2.2 Bq kg–1 for 40K.

Radiological health risks assessment

Radium equivalent activity index (Ra eq )

Naturally, NORM radiation concentrations in surrounding environmental components such as soil, sediments, or dust are not uniform. The "Radium equivalent activity (Raeq)" index is used to remove radionuclide non-uniform activity. This may be calculated using the following Eq. (2):

$${Ra}_{eq}=\left(\frac{{A}_{Ra}}{370}+\frac{{A}_{Th}}{259}+\frac{{A}_{K}}{4810}\right)\times 370$$
(2)

where ARa, ATh, and AK indicate the radioactive concentrations of 226Ra, 232Th, and 40K, respectively. The maximum allowed value of Raeq was set at 370 Bq kg−1 for prospective radiological safety assessment [31].

External & internal hazard indices (H ex & H in )

The external hazard index (Hex) can be calculated using Eq. (3) to quantify the externally exposed radiation (ionizing) doses to individual people from dusts. Furthermore, the internal hazard index (Hin) is used to measure the radiological dangers caused by radon and its products, which can be computed using Eq. (4) [32].

$${H}_{ex}=\frac{{A}_{Ra}}{370}+\frac{{A}_{Th}}{259}+\frac{{A}_{K}}{4810} \le 1$$
(3)
$${H}_{in}=\frac{{A}_{Ra}}{185}+\frac{{A}_{Th}}{259}+\frac{{A}_{K}}{4810} \le 1$$
(4)

where ATh, ARa, and AK represent the radioactivity abundances of radionuclides 232Th, 226Ra, and 40K, respectively. According to UNSCEAR (2000), Hex and Hin values should be less than unity in order to minimize the radiation hazard [18].

Gamma activity concentration index ( \(I\gamma \) )

Gamma activity concentration index (Iγ) can be used to evaluate the risk levels of natural radiation from dusts associated with gamma-emitters. Due to the excessive radiation emitted by surface materials, I was connected with the yearly dose criterion and used as a screening tool to identify substances that could harm human health. Iγ can be estimated by Eq. (5) [33, 34]

$${I}_{\gamma }=\frac{{A}_{Ra}}{150}+\frac{{A}_{Th}}{100}+\frac{{A}_{K}}{1500}$$
(5)

where ARa, ATh, and AK are the activity concentrations of 226Ra, 232Th, and 40K (Bq kg−1), respectively. The dose criterion of 1 mSv y−1 is met for I ≤ 6 [35].

Statistical analysis

Statistical analysis parameters (Min, Max, Mean, Kurtosis, Skewness) of the radioactivity concentration data were analyzed using Minitab (ver 19) software. Pearson's correlation and principal component analysis (PCA) were applied to investigate the sources of radioactivity concentration in the dust. Also, Cluster analyses were performed to show the similarity of radionuclides and correlation parameters.

Results and discussion

Radioactivity concentrations

The measured radioactivity concentrations of NORM in the Nicosia metropolis aerosol dust were presented in Table 1. The activity concentrations of 226Ra, 232Th, and 40K renged from 25.9 ± 3.1 to 52.4 ± 5.3 Bq kg−1, 21.7 ± 1.5 to 46.3 ± 3.7 Bq kg−1, 471 ± 7 to 1302 ± 22 Bq kg−1, respectively. This table also shows the average Earth crust values for the 226Ra, 232Th, and 40K radionuclide concentrations.

Table 1 The measured radioactivity concentration of NORM (Bq kg−1) and radiological health risks in aerosols dust collected from the study area
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The highest mean value was 40K (787 Bq kg−1), and the lowest mean value was 232Th (31.8 Bq kg−1). The mean concentrations of 226Ra and 232Th were slightly lower than and higher than the Earth's crust's average background value for soils, respectively. At the same time, the mean concentrations of 40K exceeded the corresponding background values for soils in Earth's crust [36]. The mean concentrations of 226Ra, 232Th, and 40K measured in dust samples in this study were compared with central Bangladesh dust [37], whereas 226Ra (86.0 Bq kg−1), 232Th (43.4 Bq kg−1) concentrations were higher than and 40K (448 Bq kg−1) concentration lower than the mean concentration of this study. The 226Ra, 232Th, and 40K concentration in dust samples of near gold mining Nyanza, Kenya [38], was reported 27 Bq kg−1, 60 Bq kg−1, and 112 Bq kg−1, respectively. Where 232Th concentration was reported higher than the 226Ra and 40K concentration values were less than the results of this research.

Abbasi et al. [39] measured the concentration of 226Ra, 232Th, and 40K in surface soil samples of the North Cyprus area. The measured concentrations were 83.7 Bq kg−1 for 226Ra, 53.6 Bq kg−1 for 232Th, and 593.9 Bq kg−1 for 40K. The comparison shows that the concentrations of 226Ra and 232Th in surface soil samples are higher than those of 226Ra and 232Th in dust samples in the same area. On the other hand, the concentration of 40K in surface soil samples is lower than 40K concentratiomn in dust samples in the same area. This comparison indicates that Cyprus’s source of airborne particles and dust can be an external origin.

The activity concentrations of 226Ra, 232Th, and 40K observed in this study were compared with the surface soil measurements conducted in different parts of the world (Table 2). As shown in Table 2, our average 226Ra concentrations in dust samples were higher than those measured in China (Baoji) [40], Egypt [41], Greece (Agios Dimitrios) [42], but lower than those in China (Xitulvye) [43], Malaysia (Kedah) [44], Turkey (Kangal) [45], Serbia [46], Nigeria [47], Portugal (Douro) [48], Spain (Velilla) [49], North Cyprus [39], and equal, Bangladesh (Rampal) [50]. Similarly, our average 232Th activity concentration levels in dust samples were higher than those measured in Turkey (Kangal) [45], Egypt [41], but lower than those in China (Baoji) [40], China (Xitulvye) [43], Malaysia (Kedah) [44], Serbia [46], Greece (Agios Dimitrios) [42], Portugal (Douro) [48], Spain (Velilla) [49], North Cyprus [39], Bangladesh (Rampal) [50], Nigeria [47]. On the other hand, our average 40K concentration levels in dust samples were lower than those measured in Portugal (Douro) [48], but higher than those in Turkey (Kangal) [45], Egypt [41], China (Baoji) [40], China (Xitulvye) [43], Malaysia (Kedah) [44], Serbia [46], Greece (Agios Dimitrios) [42], Spain (Velilla) [49], North Cyprus [39], Bangladesh (Rampal) [50], Nigeria [47].

Table 2 Comparison of the 226Ra, 232Th, and 40K concentration results of this study and other studies from different countries with analogical symbol
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The box plot of 226Ra, 232Th, and 40K activity concentration with mean, individual, and 95th percentile values was presented in Fig. 2. In this Figure, the Earth’s crust average values are shown as reference lines to the comparison of this research results.

Fig. 2
figure 2

Box-plot of 226Ra, 232Th, and 40K activity concentration in the studied area aerosols dust samples (grey point, circle points, and yellow box mark are represents mean, individual values, and 95th percentile values, respectively). Reference lines of Earth’s crust average value shown by dashed lines

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Fig. 3
figure 3

Hot plot of radiological health risks indexes (Raeq, Hex, Hin, and Iγ) in study area

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Risk assessment

The Raeq index, external hazard index (Hex), internal hazard index (Hin), and gamma activity concentration index (Iγ) of all examined samples were presented in Table 1. The Raeq index, Hex, Hin, and Iγ ranged from 93.6 to 209.9 with a mean of 141.0, 0.3–0.6 with a mean of 0.4, 0.3–0.7 with a mean of 0.5, 0.7–1.6 with a mean of 1.1, respectively. The maximum value of all four risk indexes was observed in the DS-4 sample. In the DS-4 sample, the Raeq, Hex, Hin, and Iγ index values were 1.49, 1.50, 1.40, and 1.45 times higher than the mean value of the risk indexes in the study area. The DS-4 sampling site is a high-traffic square, which typically refers to a busy urban intersection with a significant vehicular and pedestrian traffic flow. Environmental studies are particularly interested in such areas due to their potential to influence public health and environmental quality. It can be the effect of exhaust pollution and wear and tear of moving car parts. A more detailed analysis would be helpful at the DS-4 site to understand the high concentration of NORM at the DS-4 site.

The minimum value of all four risk indexes was calculated in the DS-16 sample, where this sampling point is a closed area. All the values of Raeq in the studied samples are found to be lower than the criterion limit of 370 Bq kg−1 [51]. The values of the indices (Hex and Hin) should be < 1. Table 1 shows that the mean values of Hex (0.4) and Hin (0.5) are below the criterion value (< 1). The Iγ mean value was calculated under 2, while the dose criterion of 0.3 mSv y−1 is met for I γ ≤ 2. This indicated that the annual effective dose due to aerosol dust in the study area was under 0.3 mSv y−1. The hot plot of radiological health risk indexes (Raeq, Hex, Hin, and Iγ) were presented in Fig. 3.

Statistical assessments

Pearson correlation coefficients were used for 226Ra, 232Th, and 40K activity concentration to create relations in the aerosol dust samples and the resulting correlation matrix is shown in Table 3. There was a positive correlation among all parameters. Significant correlations (p< 0.05) were observed for the pairs 226Ra and 232Th (r = 0.854). The Ra and all radiological health risks indexes Raeq (r = 0.691), Hex (r = 0.691), Hin (r = 0.806), and Iγ (r = 0.622) show significant correlations. The correlations between 40K and other two radionuclides (226Ra, 232Th) activity concentrations show no significant correlations.

Cluster analysis was applied to the aerosol dust data to examine the classification of radionuclide groups and radiological health risk indexes to recognize relationships among them. The analysis results are presented as a dendrogram in Fig. 4. The vertical axis represents the similarty percentage of association between the variables, while the greater similarty shows the more significant association. The cluster analysis presents two distinct larger subgroups: the first contains 226Ra and 232Th radionuclides, and the second includes 40K and radiological health risks indexes. The strongest observed association (similarity > 82%) was between 40K and radiological health risks indexes.

Table 3 Pearson correlation coefficients between 226Ra, 232Th, and 40K activity concentration values and radiological health risks indexes
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Fig. 4
figure 4

Cluster analysis of variables of 226Ra, 232Th, and 40K concentration values with radiological health risks indexes

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Conclusion

Twenty-six sampling sites were selected to investigate natural radioactive materials activity concentration in the aerosol dust of Nicosia. The concentration of 226Ra, 232Th, and 40K in aerosol dust was measured. It was found that the average level of 226Ra, 232Th in the study area was lower than the Earth’s crust average value, while the average value of 40K radionuclide in aerosols dust was higher than the Earth's crust average value. The radiological health risks indexs were calculated in the study area. The calculated results of radium equivalent activities (Raeq) were lower than the limit of 370 Bq kg−1 set by NEA-OECD (Nuclear Energy Agency). The external hazard index (Hex), internal hazard index (Hin), and gamma radiation hazard index (Iγ) were calculated in all samples. The internal and external hazard indexes were found well below the acceptable limit illustrated by UNSCEAR in all samples. Also, the gamma radiation hazard index (Iγ) was obtained at less than the met annual effective dose of 0.3 mSv y−1. Hence, the radiological risk to human ratio in Nicosia City aerosol dust looks to be negligible.