Osun State is an inland state in Southwestern Nigeria with coordinates 6˚55'N 4˚06'E and 8˚07'N 5˚05'E and a total area of 9251 km² (3572 sq mi). Its capital is Osogbo. It has boundary with Kwara State in the North, in the east partly by Ekiti State and partly by Ondo State, in the south by Ogun State and in the west by Oyo State [10]. Osun is underlain by metamorphic rocks of the basement complex, which is visible on the surface over many parts. Rocks of the basement complex observed here are schists, associated with quartzite ridges of the type obtained in Ilesa area. The metamorphic rocks are chiefly not different, though two particular rock groups could still be pinpointed. The first group consists of the migmatite complex, including banded magmatic and augen gneisses and pegmatites with outcrops in Ilesa and Ife areas. Metasediments consisting of schists and quartzites, calsilicates, meta-conglomerates, amphibolites, and metamorphic iron beds make up the second group. They are found in Iwo and Ikire areas. Other parts of the state are underlain by undifferentiated metamorphic rocks [11] [12]. Figure 1 is a map of Osun State Nigeria. The coloured regions are the five local governments where quarry sites under study are located.

A total of eighty granite samples, stone-dust (24 samples), a half-inch (32 samples) and twenty-four samples of a three-quarter-inch (24 samples) were collected from eight functional quarry sites in Osun State. The quarries are distributed across five Local government areas of the State. Each granite sample was packed into a polyethylene bag and clearly labeled to avoid cross-contamination. The label contained sample location, code and collection date. The polythene bags were tightly sealed with cellophane. Table 1 shows the quarry names, sample code, sample number and GPS location. The samples were crushed with a Laboratory Jaw Crusher. The samples were transferred to the laboratory and prepared into 1 litre Marinelli beakers. The beakers were thick enough to check the imbuing of radon. The beakers were closed by screw caps and the plastic tape was wrapped over the caps and then stored for a month to allow time for ²²²Rn to reach a state of secular equilibrium with its short-lived daughters before gamma spectroscopy. This step was necessary to ensure that radon gas was confined within the volume and that the progenies also remained in the sample.

The activity concentrations of the samples were determined by using a computerized gamma-ray spectrometry system with high purity germanium (HPGe). The relative efficiency of the detector system was 40%, and the resolution of 1.8 keV at 1.33 MeV of ⁶⁰Co. The gamma spectrometer is coupled to conventional electronics connected to a multichannel analyzer card (MCA) installed in a desktop computer. A software program called MAESTRO-32 was used to accumulate and analyze the data. The detector is located inside a cylindrical lead shield of 5 cm thickness with an internal diameter of 24 cm and a height of 60 cm. The lead shield is lined with various layers of copper, cadmium, and Plexiglas, each 3 mm thick. A counting time of 36,000 seconds (10 h) was used to acquire spectral data for each sample. The activity concentrations of the uranium-series were determined using γ-ray emissions of ²¹⁴Pb at 351.9 keV (35.8%) and ²¹⁴Bi at 609.3 keV (44.8%) for ²³⁸U, and for the ²³²Th-series, the emissions of ²²⁸Ac at 911 keV (26.6%), ²¹²Pb at 238.6 keV (43.3%) and ²⁰⁸Tl at 583 keV (30.1%) were used. The ⁴⁰K activity concentration was determined directly from its emission line at 1460.8 keV (10.7%) (See Figure 2).

The specific activity concentrations (A_sp) of ²³⁸U, ²³²Th, ⁴⁰K in Bq·kg⁻¹ for the rock samples were determined using the following expression [14].

A s p = N s a m γ E ⋅ ε ⋅ T C ⋅ M (1)

where N_sam is net counts of the radionuclide in the sample, γ E is gamma-ray emission probability (gamma yield), ε is total counting efficiency of the detector system, T_c is sample counting time and M is mass of sample (kg).

Quarry names, sample code, sample number and GPS location are presented in Table 1. Table 2 summarized mean activity concentrations of ²³⁸U, ²³²Th, and ⁴⁰K in granite samples from each quarry together with their corresponding uncertainties. Mean activity concentrations of ²³⁸U, ²³²Th, ⁴⁰K in the samples for different sizes in all locations are shown in Table 3. The activity concentration of ²³⁸U ranged from 3.47 Bq·kg⁻¹ to 58.98 Bq·kg⁻¹ in three-quarter-inch granite, it ranged from 1.53 Bq·kg⁻¹ to 27.51 Bq·kg⁻¹ in a half-inch granite, while the range is 1.60 Bq·kg⁻¹ to 9.67 Bq·kg⁻¹ in stone-dust. Based on location, The mean activity concentrations of ²³⁸U in three-quarter inch, a half-inch and stone-dust are 12.64 Bq·kg⁻¹, 8.76 Bq·kg⁻¹, and 5.10 Bq·kg⁻¹ respectively as shown in Table 3. This implies that the larger the size the higher the mean activity. Aggregate size distribution likely has effects on radiation levels since the stone-dust has the least mean activity concentration. This is an indication that fine-grain aggregates allow easy escape of radiations than the coarse aggregates thereby giving a shorter period of hazard compared with other sizes [7]. The activity concentrations of ²³⁸U in all locations are below the world average value of 33 Bq·kg⁻¹ [4].

The activity concentration of ²³²Th ranged from 5.23 Bq·kg⁻¹ to 77.85 Bq·kg⁻¹ with mean value of 16.93 Bq·kg⁻¹ in three-quarter inch granite, it ranged from 5.15 Bq·kg⁻¹ to 43.86 Bq·kg⁻¹ with mean value of 13.69 Bq·kg⁻¹ in half-inch granite, while the range of activity concentration in stone-dust is 1.62 Bq·kg⁻¹ to 16.55 Bq·kg⁻¹ with mean value of 8.97 Bq·kg⁻¹ (see Table 3). The average activity concentrations of ²³⁸Th in all locations and sizes are below the worldwide average activity concentrations of 45 Bq·kg⁻¹ [15]. For ²³²Th, the mean activity concentration decreases as the granite size decreases as shown in Table 3. Three-quarter has the highest mean activity concentration of ²³²Th while stone-dust has the lowest.

Of the three granite sizes, three-quarter-inch has the highest average activity of ⁴⁰K followed by stone-dust which is slightly greater than that of half-inch.

The range and mean activity concentration of ⁴⁰K for three-quarter inch, half inch granite and stone-dust respectively are (90.21 - 672.54) 266.19 Bq·kg⁻¹, (56.53 - 426.68) 151.85 Bq·kg⁻¹ and (57.88 - 379.77) 164.27 Bq·kg⁻¹ respectively (see Table 3).

All the mean activity concentrations values of ⁴⁰K are below the worldwide average value of 412 Bq·kg⁻¹ [15].

The mean activity concentrations of ²³⁸U, ²³²Th, and ⁴⁰K in this work were compared with the reported values obtained by other researchers such as [4] [7] [26] [27] [28], etc.

The result is comparison in Figure 3. It is important to note that the values shown are not representative values for the countries mentioned, but only for the regions in which the samples were collected.

To establish the health risk due to the exposure to natural radionuclides in granite on quarry workers, granite end-users and the general public in the study area, the radiation hazard indices were calculated (Table 4).

The mean absorbed dose rate in air in the study area ranged from 4.56 nGy·h⁻¹ to 38.10 nGy·h⁻¹. This is attributed to the size of the samples and geology of the study areas. The absorbed dose rate in air in stone-dust is least in Wolid, and Slava Yetidepe (WQ and YQ) whereas highest in Ife/Modakeke and Espro (IQ and EQ). This is because the activity concentration of natural radionuclides in granite is proportional to the size of the granite. The larger the granite sizes the higher the activity concentrations of natural radionuclides in the granite of study areas, as shown in Table 3.

⁴⁰K is the highest contributor to the absorbed dose rate in air, followed by ²³²Th and then ²³⁸U. The chart showing the contributions of ²³⁴U, ²³²Th and ⁴⁰K to the absorbed dose rate in air in the study area is shown in Figure 4. ⁴⁰K accounts for 90% of the absorbed dose rate in air, while ²³²Th and ²³⁸U account for 6% and 4% respectively. The absorbed dose rate in air in all locations is below the world average value of 60 nGy·h⁻¹ [4].

The absorbed dose rate in the air at about 1 m above the ground surface does not directly provide the radiological risk to which an individual is exposed [16].

The annual effective dose equivalent (AEDE) as presented in Table 5 for the selected quarry sites in this study varied from 5.60 to 46.73 µSv·y⁻¹. AEDE for 3/4 inch, 1/2 inch and stone-dust ranged from 10.80 to 46.73, 6.64 to 30.69 and 5.60 to 20.18 µSv·y⁻¹ respectively with mean values of 17.10, 7.45 and 7.55 µSv·y⁻¹ respectively. When compared with a worldwide effective dose of 0.07 mSv·y⁻¹ [15], the results in this work is lower. The highest AEDE was found in 3/4 inch granite from Espro and the least in stone dust from Wolid quarry site. According to [24], the acceptable annual effective dose rate recommended for members of the public without constraint is 1.0 mSv·y⁻¹.

The radium equivalent activity (Ra_eq) ranged from 28.79 Bq·kg⁻¹ to 162.43 Bq·kg⁻¹, 21.64 Bq·kg⁻¹ to 108 Bq·kg⁻¹ and 17.13 Bq·kg⁻¹ to 46.32 Bq·kg⁻¹, with mean values of 43.85 Bq·kg⁻¹, 27.44 Bq·kg⁻¹ and 22.76 Bq·kg⁻¹ for 3/4 inch granite, 1/2 inch granite and stone-dust respectively. This is below the permissible maximum value of 370 Bq·kg⁻¹ [4] which corresponds to an effective dose of 1 mSv for the general public. The radium equivalent activity (Ra_eq) is highest in three-quarter inch granite from Espro (EQ). The radium equivalent activity (Ra_eq) is lowest in stone-dust from Wolid (WQ).

The mean annual gonadal equivalent dose (AGED) ranged from 98.32 mSv·y⁻¹ to 517.90 mSv·y⁻¹, 72.15 mSv·y⁻¹ to 351.12 mSv·y⁻¹ and 58.40 mSv·y⁻¹ to 155.02 mSv·y⁻¹, with mean values of 150.98 mSv·y⁻¹, 88.85 mSv·y⁻¹ and 76.54 mSv·y⁻¹ respectively for 3/4 inch granite, 1/2 inch granite and stone-dust respectively. The AGED is highest in three-quarter inch granite from Espro (EQ), and is at lowest in stone-dust from Wolid (WQ). An increase in AGED has been known to affect the bone marrow, destroying the red blood cells that are then replaced by white blood cells. This situation results in a blood cancer called leukemia which is fatal. The mean values are lower than the maximum permissible value of 300 mSv·y⁻¹ [15]. Hence, the quarry workers, granite end-users and the general populace in the study area are not at risk of developing blood cancer due to the exposure to the natural radionuclides present in the study area.

The external hazard index (H_ex) ranged from 0.08 to 0.44, 0.06 to 0.29 and 0.05 to 0.13, with mean values of 0.12, 0.07 and 0.06 for 3/4 inch granite, 1/2 inch granite and stone-dust respectively. While the internal hazard index (H_in) ranged from 0.10 to 0.56, 0.07 to 0.35 and 0.05 to 0.15, with mean values of 0.14, 0.09 and 0.07 for 3/4 inch granite, 1/2 inch granite and stone-dust respectively. These values do not exceed the acceptable limit value of unity (ICRP, 2010), thus, suggesting that radiation hazard due to the exposure to natural radionuclides in the study area is negligible.

The external hazard index (H_ex) and the internal hazard index (H_in) are highest in three-quarter inch granite from Espro (EQ). The external hazard index (H_ex) is lowest in both Ayofe and Wolid (AQ and WQ) while the internal hazard index (H_in) is lowest in stone-dust from wolid (WQ).

The representative gamma index (I_γ) ranged from 0.22 to 1.17, 0.20 to 0.79 and 0.13 to 0.35, with mean values of 0.33, 0.20 and 0.17 for 3/4 inch granite, 1/2 inch granite and stone-dust respectively. An increase in the representative gamma index greater than the universal standard of unity may result in radiation risk leading to the deformation of epithelial and blood cells, thereby causing cancer [29]. Since the values of the representative gamma index (I_γ) is lower than 1 in all the quarries except Espro (I_γ = 1.17 for 3/4 inch granite), thus, the populace of the study area does not suffer significant health risk due to the exposure to radiation from natural radionuclides in the granite of the study area.

The excess lifetime cancer risk (ECLR) has to do with the probability of developing cancer over a lifetime at a given exposure level. The excess lifetime cancer risk (ECLR) ranged from 0.04 to 0.16 with a mean value of 0.06 for three-quarter-inch granite, ranged from 0.02 to 0.11 with a mean value of 0.03 for half-inch granite and from 0.02 to 0.07 with a mean value of 0.03 for stone-dust. These values are significantly lower than unity, so the risk that the general public of the study area would develop cancer due to the exposure to radiation emitted from natural radionuclides is very low or insignificant.

The excess lifetime cancer risk is highest in 3/4 inch granite and lowest in stone-dust (<1/2 inch granite).

The excess lifetime cancer risk (ELCR) is highest at the Espro (EQ), closely followed by the Clario quarry (CQ). The excess lifetime cancer risk is lowest at Wolid (WQ).

The values of the radiation hazard indices in all locations are below the maximum permissible limit set by ICRP. Hence, radiation emitted from natural radionuclides (²³⁸U, ²³²Th, and ⁴⁰K) in the granite of the study areas does not pose a serious health risk to the quarry workers, granite end-users and the general public of the study area.

The Pearson correlation results of ²³⁸U, ²³²Th and ⁴⁰K in a three-quarter, half-inch granite, and stone-dust as shown in Table 4 are 0.984 0.998 and 0.707 respectively indicating a strong positive linear relationship between the mean activity concentrations of ²³⁸U, ²³²Th and ⁴⁰K respectively and granite size. The r² values are 0.968, 0.996 and 0.500, implying that 96.8%, 99.6% 50% of the mean activity concentration of ²³⁸U, ²³²Th and ⁴⁰K can be explained by granite size.