Introduction

Radioactivity is an integral process of the environment, and its radionuclide emits ionizing radiations, either from natural or artificial sources. Exposure from natural sources is inevitable and contributes about 85% of humans’ exposure to ionizing radiation1, which affects organisms on earth. Ionizing radiations consist of gamma radiations, which are hazardous to life. They are carcinogenic2,3 and could have health effects on the biological system of any organism it comes in contact with. It has been reported that high doses of gamma radiation could lead to chromosomal abnormalities, hematological depression, cataracts, erythremia, severe nausea, loss of appetite, serious risk of infections, skin peels, sterility4, leukemia, and thyroid, breast, lung, bladder, colon, liver, esophagus, ovarian, skin, and stomach cancers5. Reports by refs6,7,8 have shown links between gamma radiation and cancer.

The level of exposure to gamma radiation in any region primarily depends on the geology and geographical conditions of that region9,10,11. Also, human activities such as mining, oil and gas exploration, production and use of phosphate fertilizers, illegal disposal of radioactive waste, nuclear weapon testing, and nuclear reactor accidents enhance environmental gamma radiation in any region12,13.

Taraba State is endowed with abundant solid mineral resources, of which uranium ore is one. Radiometric surveys in the state estimated uranium concentration in the ores in the range of 0.03 − 0.25%14,15,16,17, with the peak count rate of 1200 counts per minute (cpm) against the overall average background count rate of about 30 cpm15. A laboratory gamma ray analysis of some samples from the Donga part of the state revealed the uranium grade in the region as 1.5% and 0.1% for primary and secondary uranium contents, respectively, while in-situ assessments of thorium concentration and potassium percentage in that region ranged from 47.3 to 3654 \(\:mg{kg}^{-1}\) and 4.26 to 6.26%, respectively17.

In Taraba, uranium deposits are formed by varieties of geological processes, and the most prominent ones are unconformity-related deposits, sandstone-type deposits, breccia-type deposits, paleo-placer deposits, vein-type deposits, intrusive hosted deposits, and phosphorite (uranium in phosphate rocks)17. The unconformity-related deposits account for about 45% of annual uranium production in the state, while sandstone deposits account for about 30%18. The state is underlain by basement complexes and sedimentary rocks19, in which the former formed part of the Pan-African mobile belt that lies within the West African and Congo cratons and the southern “Tuareg Shield.” The undifferentiated basement complex and granitoid (UBG) spread across about 70% of the land mass of Taraba (Fig. 1) and are intruded by igneous and sedimentary rocks such as gneisses, phyllites, schists, pegmatites, migmatites, granitoids, and granites20, as well as granodiorites, monzorites, and charnockites17, which are rocks of high naturally occurring radioactive materials (NORMs)11.

A few studies carried out on environmental assessments of gamma radiations in Taraba State by refs’21,22,23,24 did not take into account the state’s geology, which gamma radiations of any region depend on.

This study is aimed at assessing the TGRD rates in Taraba State and its relation to excess cancer risk in the state. The study measures TGRD rates in the ambient air of the state using a handheld gamma survey meter and Global Positioning System (GPS) and statistically examines the null hypothesis “there is no significant relation between TGRD rates and the excess cancer risk in the state.” Taraba State is one of the states with a high incidence of cancer risks in Nigeria25. Ref26 said, “Out of 3237 patients examined in the histopathology section of the state’s Teaching Hospital from 2011 to 2022, 488 of them had cancer of various types.”. To the best of our knowledge, no epidemiological studies in the state ascertain the cause of the high incidence of cancer in the state, but the authors believed that the activities of NORMs may be a contributing factor.

In lieu of the above, this study also intends to generate baseline data on the status of TGRD in the state, which is very important for health risk assessments and radiological impact assessments, and will serve as working documents and protection measures for government agencies, corporate bodies, and decision makers of the state. It will also contribute to the data bank of Nigeria, as the country is planning to construct two nuclear power plants (Geregu and Akwa Ibom power plants), each 1200 MW, by the year 2035, and uranium deposits in Taraba have been targeted as the source of uranium for the two nuclear power plants. In view of this, environmental assessments of TGRD and their associated hazards are very important for the nuclear power plant site assessments. This is also in line with the ongoing national programme to identify areas of need so as to fulfil the International Atomic Energy Agency (IAEA) nuclear power plant siting requirements.

Materials and methods

Study area

This study was carried out in Taraba State, which lies within the longitudes of 7o00′E and 12o00′E and latitudes of 5o00′N and 10o00′N (Fig. 1). Its total land area is 54,473 km2, with a population of around 3,066,800 and a population density of 52.3 people per kilometre. The state is intruded by gneisses, phyllite, schist, pegmatite, migmatite, and granitoid rocks17,20.

Fig. 1
figure 1

Geological map of Taraba State.

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Instrument

A compact Gamma Surveyor by GF Instruments, Czech Republic, was used for the TGRD rate measurements. The Gamma surveyor is a multi-channel gamma-ray spectrometer, a scintillation detector that offers complete spectral measurements, from which it evaluates the cps values in regions of interest (ROI) and determines the concentrations of K (%), U (\(\:mg{kg}^{-1}\)), and Th (\(\:mg{kg}^{-1}\)) elements, precise radiometric measurements, and a quick and selective search for gamma-ray sources. The concentration of K was directly determined by the meter, while that of eU and eTh was determined based on the detection of radioisotopes of 214Bi and 208Tl, which are daughter products of the U and Th series, respectively. The natural TGRD rate values in (\(\:nGy{h}^{-1}\)) were calculated from the measured concentrations of K, eU and eTh using the following IAEA standards: 13.078 \(\:\text{n}\text{G}\text{y}{h}^{-1}\) is contributed by 1% concentration of K, 5.675 \(\:\text{n}\text{G}\text{y}{h}^{-1}\) by \(\:1\:mg{kg}^{-1}\) concentration of eU and concentration of \(\:1\:mg{kg}^{-1}\) of eTh contributes 2.494 \(\:\text{n}\text{G}\text{y}{h}^{-1}\)27.

Before measurements, “assay” calibration was done on the four calibration pads – 40K, 238U, 232Th and the background of the meter. The probe was placed in the middle of the calibration pad, and the actual used type was selected from the menu. The real known concentrations of 40K, 238U, and 232Th were entered and corrected by the geometry of the calibration pad.

The 40K, 238U, and 232Th concentrations were corrected by multiplying with the geometrical factor value (G) of the calibration pad estimated using Eq. (1)28:

$$\:G=1-\frac{h}{r}$$
(1)

where h is the height of the middle of the detector above the pad and r is the diameter of the pad.

The measurement time was set to 30 min according to the manual, and after that peak stabilisation was done and calibration started. When the measurements finished, the calibration was saved in the probe, which marked the end of calibration, and the meter resumed its regular operation using the new settings as standard.

Terrestrial gamma radiation dose measurement

Prior to measurements, the Gamma surveyor was set to count for five minutes while held at 1 m above the ground surface. Five measurements were made at each point due to the random nature of radioactivity, and the average of the five readings was calculated and recorded. At each measuring point, the coordinates of the location were obtained using a Garmin GPS Map78, as demonstrated by29,30,31. The measurements were carried out randomly across the entire state as shown in (Fig. 2).

Fig. 2
figure 2

TGRD measuring points in Taraba State.

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Risk assessment

The outdoor effective dose per annum otherwise known as the outdoor annual effective dose (OAED), and excess lifetime cancer risk (ELCR) resulting from exposures due to the presence of natural TGRD rates in Taraba State were indirectly determined using Eq. (2)1 and (3)32,33 and adhering strictly to the procedures outlined in UNSCEAR 2000 report.

$${\text{OAED}}={\text{TGRD}} \times {\text{876}}0 \times {{\text{O}}_{{\text{out}}}} \times {\text{C}} \times {\text{1}}{0^{ - \,{\text{6}}}}$$
(2)
$${\text{ELCR}}={\text{OAED}} \times {\text{ALE}} \times {\text{RF}}$$
(3)

where, C is the conversion coefficient assumed to be 0.7 mSvnGy− 1 and Oout is the outdoor occupancy factor used, which was 0.2, assuming that Taraba people spend 20% of their daily time outdoors, RF is the risk factor – the fatal cancer risk per Sievert, assumed to be 0.05 Sv− 1 according to the Commission of International Radiological Protection 106 (ICRP 2008) recommendations34 and ALE is the world average human life expectancy assumed to be 70 years according to the UNSCEAR 2000 report1.

Results and discussions

Terrestrial gamma radiation dose (TGRD) rates

The measured TGRD rates in the entire Taraba State ranged from 2 to 8467 \(\:\text{n}\text{G}\text{y}{h}^{-1}\). Using descriptive statistics of IBM SPSS Statistics Software (version 27.0.1), the mean TGRD rates in the state were found to be \(\:108\:\pm\:13\:\text{n}\text{G}\text{y}{h}^{-1}\) (Table 1). This mean value is about 82.72% greater than the world average value \(\:59\:\text{n}\text{G}\text{y}{h}^{-1}\)1. The mean TGRD rates in Taraba State was compared with average TGRD rates in other states of Nigeria that have similar geology to Taraba in (Table 2), and the percentage of variations in each state is presented in (Fig. 3). The pie chart revealed the average TGRD in Taraba State is higher than the average TGRD in Zamfara and Yobe states by 12 and 8%, respectively, while it is less than the average TGRD in Katsina, Gombe and Bauchi states by 2, 7 and 9%, respectively.

Table 1 TGRD rates for each of the geologies of Taraba State.
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Table 2 Comparing the mean TGRD rates in Taraba State with other States in Nigeria.
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Fig. 3
figure 3

Comparing the mean TGRD rates in Taraba State with other states in Nigeria.

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The box-and-whisker plots in Fig. 4 compared the variations of TGRD rates in Taraba State based on the geological formations of the state. From the plots, it was observed the TGRD rates in UBG (undifferentiated basement complex and granitoid) and BSS (Bima sandstone) formations were normally distributed compared to the TGRD rates in ALL (alluivium), BTR (basalt, trachyte and rhyolites) and BLS (alkaline, biotite and pegmatite) formations, which were all right skewed. The TGRD rates were more dispersed in BLS and least dispersed in ALL. The outliers in ALL and BSS were all real, representing the lowest and the highest measured TGRD rates in the respective formations, which lie outside the range of the rest of the TGRD rates.

Fig. 4
figure 4

Box-and-whisker plots, comparing the TGRD rates in geological formations of Taraba.

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Isodose map of Taraba State

Maps showing the regions of equal TGRD rates in Taraba State were drawn using Esri ArcGIS software version 10.7.1. The software is incorporated with 3D Spatial Analyst tools of Kernel Density Function (KDF), Inverse Distance Weighted (IDW) and surface contour39. The IDW interpolation was used to generate the isodose map using the Spatial Analyst tool in the ArcGIS environment, as shown by ref40. The KDF was used to show the distribution of the TGRD rates from the sampling points.

Prior to plotting, it was ensured the coordinates are in degree decimals and the World Geodetic System (WGS) 1984 was used to define the coordinate system of the ArcGIS software as detailed in the work of33. The generated isodose maps from these analyses were presented in (Figs. 5 and 6). The isodose map in (Fig. 5), being the KDF map, indicated that TGRD rates within \(\:220\:\text{a}\text{n}\text{d}\:8500\:\text{n}\text{G}\text{y}{h}^{-1}\) were recorded around the Yorro and Zing local government areas of the state and were designated with red colour on the map. This observation correlates with the results in (Table 1). Similarly, the TGRD contour map (Fig. 6) of the state represents the TGRD rates projected onto the xy-plane through lines of equal TGRD rates. The contour lines clustered around the Yorro and Zing local government areas, representing the region of highest elevated TGRD rates in the state.

Fig. 5
figure 5

Isodose map of Taraba State.

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Fig. 6
figure 6

TGRD contour map of Taraba State.

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Outdoor annual effective dose (OAED)

The results of a year of outdoor effective dose of TGRD rates in Taraba were summarised in (Table 3). This table represents the stochastic health risk to a whole body, which is the probability of cancer risk and genetic effects of low levels of ionising radiation. From the table, the OAED in the entire state ranged from \(\:0.01\:\text{t}\text{o}\:10.38\:\text{m}\text{S}\text{v}{y}^{-1}\) with an average value of \(\:0.13\:\text{m}\text{S}\text{v}{y}^{-1}\). This average value is 65.25% higher than the world average value \(\:0.07\:\text{m}\text{S}\text{v}{y}^{-1}\), which means the risk of cancer induction and genetic effects as a result of TGRD rates in Taraba State is 65.25%.

Table 3 Outdoor annual effective dose rates according to Taraba geology.
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The average OAEDs in the state’s formations were compared with each other in (Fig. 7). The chart depicted that the average outdoor annual effective dose in UBG formation is highest among the formations, with 21, 25, 27 and 32%, respectively higher than BSS, BLS, BTR and ALL formations respectively.

Fig. 7
figure 7

Comparing the average of outdoor annual effective dose in Taraba geology.

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Excess lifetime cancer risk (ELCR)

The results of excess lifetime cancer risk in Taraba due to exposure to TGRD rates in the state were presented in (Table 4). The table revealed that excess cancer risk in one’s life in the state ranged from 0.007 to 36.34 with a mean of 0.46 \(\:\pm\:\) 0.01. This is relatively 1,594.66 times higher than the world average of 0.29\(\:\times\:{10}^{-3}\) for an outdoor exposure. The mean ELCR in all geological formations of the state were higher than the United Nations world average value for outdoor exposure. Likewise, comparing the mean excess cancer risk of all the geological formations of Taraba State in (Fig. 8), it was observed that the mean ELCR in UBG formation was 21, 26, 26 and 32% higher than the mean ELCR in BSS, BLS, BTR and ALL formations respectively. This means that the risk of excess lifetime cancer on residents residing within the UBG formation is higher than in other formations and was least in ALL formation.

Table 4 Geological expression of excess lifetime cancer risk (ELCR) due to TGRD rates in Taraba State.
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Fig. 8
figure 8

Comparing the average ELCR among the state formations.

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The relationships existing between TGRD rates and excess lifetime cancer risk in Taraba State were analysed using Pearson correlation analysis, which is a statistical tool that measures the direction of the linear relationship between two data sets. A Pearson correlation coefficient of [\(\:\text{r}\left(877\right)\:=\:1.000,\:\text{p}\:=\:.000\)] was observed between the TGRD and ELCR in the state. This indicated there were perfect, significant positive relationships between TGRD rates and ELCR in the study area. This means the activities of TGRD rates in the ambient air of Taraba State could be a contributing factor to high annual effective dose rates and excess lifetime cancer risks in the state.

Conclusion

This study has investigated the TGRD rates and its associated hazards in Taraba State. It has shown that the mean TGRD rate in the state is \(\:108\:\pm\:13\:nGy{h}^{-1}\), which is greater than the world average of \(\:59\:nGy{h}^{-1}\). It also revealed that UBG formation had the highest mean TGRD rates among the rest of the formations. Radiological health assessments due to the presence of TGRD rates in the studied area revealed that mean outdoor effective dose per annum and excess lifetime cancer risk were \(\:0.13\:\pm\:0.02\:mSv{y}^{-1}\) and \(\:0.46\:\pm\:0.06\) respectively, which are greater than their respective world average values of \(\:0.07\:mS{y}^{-1}\) and \(\:0.29\:\times\:\:{10}^{-3}\) respectively. The isodose map of the state revealed that the regions mainly affected by the elevated TGRD rates above the world average TGRD rates in Taraba State were the Yorro, Zing, Lau, Jalingo, Wukari, Donga and Saraduna local government areas of the state. This implies that inhabitants within the above-mentioned locations are at higher risk of excess cancer and other radiation-related illnesses in their lifetime in the state. A detailed Pearson correlation statistical analysis performed revealed a perfect positive significant relationship between TGRD rates in Taraba State and ELCR with a correlation coefficient of [\(\:\text{r}\left(877\right)\:=\:1.000,\:\text{p}\:=\:.000\)]. The p-value of less than 0.001 indicated that the relationship is statistically significant. This led to the reject of the null hypothesis and accepts the alternative “there is a significant relation between TGRD rates and the excess cancer risk in Taraba State”.