Assessing the carcinogenic and non-carcinogenic health risks associated with potentially toxic elements (PTEs) in vegetables from Gombe Markets, Northern Nigeria

Abstract

Vegetables are essential for human nutrition due to their low fat content and high levels of vitamins, minerals, and dietary fiber. However, studies have shown that vegetables are vulnerable to PTE contamination as a result of anthropogenic activities. This study determined the concentrations and health impacts associated with potentially toxic elements (PTEs) in specific vegetables (green beans, spinach, green pepper, carrots, and onions). A total of 90 vegetable samples were randomly selected and purchased from local markets and analyzed for potentially toxic elements (PTEs) (Cd, Fe, Cr, Pb, Cu, Zn, Co, and Ni) using an atomic absorption spectrophotometer (AAS). The mean concentrations (mg·kg^-1) of PTEs ranged from 0.006 - 0.021 for Cd, 2.27 - 12.32 for Fe, 0.05 - 0.150 for Cr, 0.087 - 0.254 for Pb, 0.035 - 0.062 for Cu, 2.65 - 15.61 for Zn, 0.010 - 0.050 for Co, and 0.012 - 0.058 for Ni. The abundance of PTEs was found to be in the following declining order: Zn >Fe >Cu >Cr >Ni >Co >Pb >Cd. The hazard index (HI) for both children and adults was <1, suggesting that there is no likely non-carcinogenic effect from consuming these vegetables. Similarly, the carcinogenic risk was below the acceptable value range of 1.0 × 10^-6 - 1.0 × 10^-4. Based on the results of this study, it is unlikely that the vegetables analyzed pose a health risk to consumers. However, monitoring and continuous stringent regulations of PTEs on foodstuff for public health protection.

Introduction

Vegetables are edible plants with nutrient reserves in leaves, roots, steams, and fruits that provide essential minerals and vitamins for a healthy life [1]. Vegetables are a trendy food among consumers due to their importance in the human diet and their provision of essential nutrients, including fiber, vitamins, minerals, proteins, and carbohydrates, as well as their antioxidative effects. These nutrients are necessary for human health and can help lower the risk of chronic diseases [2-4]. However, several studies have found that vegetables are susceptible to contamination with potentially toxic elements (PTEs) [3,5,6]. PTEs are significant contaminants that can enter the environment through the application of pesticides and chemical fertilizers, mining, and irrigation water are the main sources of PTEs in agricultural produce [7-9]. Studies conducted in Nigeria and other parts of the world have reported significant concentrations of PTEs in different vegetables [2,3,10-13].

The consumption of these PTE contaminated vegetables is considered an important route of exposure and can lead to various illnesses in consumers [14-17]. PTEs pose significant health risks to humans as they can enter the body through the food chain [9]. Food contamination with PTEs can cause the accumulation of PTEs in the kidneys and livers of individuals, interfering with numerous biochemical processes that can result in diseases of the heart, brain, kidneys, and bones [18-20]. For example, exposure to Cd can lead to cancer and other harmful health effects, such as harm to the testicles, brain, liver, lung, and blood system [21-25]. Copper causes anemia, metabolic problems, liver and kidney damage, and stomach pain [26,27]. Lead (Pb) affects the nervous system, particularly resulting in deficiencies in the intelligence quotient (IQ) in children [9,28,29]. Therefore, it is important to investigate PTE contamination in vegetables and evaluate the harm to human health.

However, several studies have found that vegetables are susceptible to contamination with PTEs in Nigeria and other parts of the world have reported significant concentrations of PTEs in different vegetables [2,3,30-35]. However, limited or no studies have been reported on the carcinogenic and non-carcinogenic risk assessments of the PTE on the studied vegetables from the study area. Hence, it is important to determine PTE contamination and evaluate the health risk of PTEs in vegetables to assess the safety of the vegetables consumed. The aim of this study was to determine the concentrations of PTEs in vegetables sold in Gombe markets, Nigeria and to assess the health risks associated with the consumption of these vegetable samples.

Materials and Methods

Study Area

The study was conducted in the old and main markets within the Gombe metropolis, the capital of Gombe State in northern Nigeria (Fig. 1). The markets in the metropolis draw regional items from the farming and business sectors in the surrounding districts. The metropolis spans an area of roughly 52 km² and is located between latitudes 10°17′05.88''N and 11°10′36.78''E. The study area was characterized by two distinct wet and dry seasons: May-October and November-April, respectively. The research region has a tropical climate. The meteorological parameters include an annual temperature range of 18.5-39°C, an average precipitation of 903 mm, and a relative humidity that varies from 15% to 20% in December to 70% to 80% in August [36].

Figure 1.

A map of the study area showing sample points.

Sample collection, preparation, and analysis

Five vegetables: green beans (Pisum sativum), spinach (Amaranthus hybridus), green pepper (Capsicum annuum), carrots (Daucus carota subsp. sativus), and onion (Allium cepa) were purchased at random from three major vendors at the main and old markets (grown locally). A total of 90 samples, consisting of six samples for each of the five vegetables, were collected three times over the course of a month. The samples were obtained from both the main market and the old market and were analyzed for PTE concentrations. The vegetable samples were placed in polybags and transported to the laboratory. The samples were dried in an oven at 80 °C for 2 to 3 days, and their weight was periodically checked until it reached a constant value. The dried samples were first pulverized using a mortar and pestle until they became a fine powder. The sample powder was then sifted through a 2 mm filter and finally sealed in polythene bags.

Approximately 1.0 g of each dried vegetable sample was measured and placed into the digestion flasks. Then, 5 mL of H₂SO₄ and 15 mL of HNO₃ were added. In addition, a blank sample was produced in the same way. The flasks were placed on an electric hot plate and heated at 80 to 90 °C for 2 h. The samples were boiled until a clear solution was achieved [3]. The digest was allowed to cool and filtered using Whatman' No. 42. The resulting solutions were then topped up to 50 mL with deionized water and analyzed for the PTE concentration using an atomic absorption spectrophotometer (AAS model AA320N, Wincom Coy Ltd., China).

Quality assurance

All reagents used were of analytical grade, and quality control procedures were implemented to ensure the validity of the study results. For the recovery investigation, the concentrations of PTEs in both the spiked and unspiked samples were measured in triplicate. A blank solution was analyzed in triplicate to determine the quantification and detection limits of AAS. The conditions of the operations are summarized in Table 1. The detection limit (LOD) for each PTE was evaluated as follows: LOD = 3.3 standard deviation (SD)/b [37]. The mean recovery of the PTEs ranged from 93% to 114%, and the limits of detection for Cd, Fe, Ni, Cr, Pb, Cu, Zn, and Co were 0.001, 0.008, 0.005, 0.008, 0.010, 0.007, 0.008, 0.004, and mg/L, respectively.

Table 1.

Operation conditions of AAS for the analysis of the potentially toxic elements.

Elements	λ (nm)	Slit (nm)	Flame type	Lamp current (mA)
Cd	228.8	0.7	Air-acetylene	8
Fe	248.3	0.2	Air-acetylene	10
Cr	357.9	0.7	Air-acetylene	10
Pb	283.3	0.7	Air-acetylene	10
Cu	324.7	0.7	Air-acetylene	10
Zn	213.9	0.7	Air-acetylene	10
Co	193.7	0.2	Air-acetylene	10
Ni	232.0	0.2	Air-acetylene	10

Data analysis

Descriptive statistics (mean and standard deviation) and the obtained mean values were compared with the permissible limit, and inferential statistics (Duncan, Person correlation, and principal components analysis) were used to measure the relationship and possible sources of the investigated PTEs using SPSS version 25.

Health risk assessment

Health risk assessment involves assessing data related to human health [27,38] and provides useful information about exposure to contaminants. The risk exposure of PTEs is estimated via oral, inhalation, and dermal contact, but the exposure assessment carried out in this study is via oral exposure for children and adults were used in this study.

To evaluate the potential health hazards associated with the prolonged consumption of PTE contaminated vegetables, the estimated daily intake (EDI) of PTEs was calculated using the USEPA model and data from previous studies [2,4,12,32], with the equation 1 below.

$$\text{EDI=}\frac{\text{C×IR×EF×ED}}{\text{BW×AT}}$$ (1)

The hazard quotient (HQ) of the PTEs in the vegetables was used to estimate the non-carcinogenic health risk.

$$\text{HQ=}\frac{\text{EDI}}{\text{RfD}}$$ (2)

For any PTE under investigation, the hazard index (HI) is the sum of the HQ values. If the value of HI <1, there are no probable non-carcinogenic consequences [27].

$$\text{HI=}{\displaystyle {\sum }_{\text{n}}^{\text{i}}{\text{HQ}}_{\text{i}}}$$ (3)

The carcinogenic risk (CR) is estimated by evaluating the likelihood of developing cancer over a lifetime. The estimation is calculated using the equation provided below.

$$\text{CR = EDI× CSF}$$ (4)

A CR greater than 1 × 10⁻⁴ is considered unacceptable, risks less than 1 × 10⁻⁶ are considered to pose no major health hazards, and risks falling within the safe range are those between 1 × 10⁻⁴ and 1 × 10⁻⁶. where EDI = estimated daily intake dose of PTEs, HQ = hazard quotient, HI = hazard index, RfD = reference dose, and CSF = cancer slope factor. The meaning and units of all variables used in the equations are presented in Table 2 and S1 (in the supplementary information).

Table 2.

The parameters and input assumptions for exposure assessment of elements via ingestion routes.

Parameters	Meaning	Units	Value
			Non Cancer		Cancer
			Adults	Children
C	Measured elements concentration	mg/kg
IRing	Daily consumption rate	mg/day	0.75	0.25
BW	Body weight	kg	70	15
EF	Exposure frequency	days/year	365	365
ED	Exposure duration	years	24	6	70
ATnc	Average time for non-carcinogens	days	ED × 365 days	ED × 365 days
ATc	Average time for carcinogens	days	ED × 365 days	ED × 365 days	70 × 365 days

Results and Discussion

Potentially toxic elements (PTEs) concentrations in vegetables

Tables 3 and 4 present the mean concentrations of PTEs in vegetable samples from the Gombe markets in northern Nigeria. The mean concentrations of PTEs in the vegetable samples ranged from 0.006 - 0.021 mg·kg^-1, 2.27 - 12.32 mg·kg^-1, 0.05 - 0.150 mg·kg^-1, 0.087 - 0.254 mg·kg^-1, 0.035 - 0.062 mg·kg^-1, 2.65 - 15.61 mg·kg^-1, 0.010 - 0.050 mg·kg^-1, and 0.012 - 0.058 mg·kg^-1 for Cd, Fe, Cr, Pb, Cu, Zn, Co, and Ni, respectively, in the vegetable samples. The mean PTE concentrations showed the following trend: Zn > Fe > Cu > Cr > Ni > Co > Pb > Cd.

Table 3.

Descriptive statistics of potentially toxic elements (mg·kg^-1) in vegetable samples from the main market.

Vegetables	Statistic	Cd	Fe	Cr	Pb	Cu	Zn	Co	Ni
Green beans	Min.	0.005	10.530	0.020	0.083	0.051	10.050	0.011	0.011
	Max	0.009	11.380	0.080	0.090	0.060	11.000	0.013	0.013
	Mean	0.007	10.955	0.050	0.087	0.056	10.525	0.012	0.012
	SD	0.003	0.601	0.042	0.005	0.006	0.672	0.001	0.001
Spinach	Min.	0.009	9.050	0.051	0.090	0.030	9.980	0.009	0.015
	Max	0.010	10.370	0.080	0.135	0.040	11.340	0.012	0.023
	Mean	0.010	9.710	0.066	0.113	0.035	10.660	0.011	0.019
	SD	0.001	0.933	0.021	0.032	0.007	0.962	0.002	0.006
Green pepper	Min.	0.008	7.020	0.110	0.210	0.046	6.830	0.008	0.020
	Max	0.010	8.010	0.130	0.230	0.051	7.870	0.018	0.031
	Mean	0.009	7.515	0.120	0.220	0.049	7.350	0.013	0.026
	SD	0.001	0.700	0.014	0.014	0.004	0.735	0.007	0.008
Carrot	Min.	0.003	8.530	0.110	0.115	0.038	4.080	0.005	0.013
	Max	0.008	10.310	0.170	0.150	0.053	5.310	0.018	0.060
	Mean	0.006	9.420	0.140	0.133	0.046	4.695	0.012	0.037
	SD	0.004	1.259	0.042	0.025	0.011	0.870	0.009	0.033
Onion	Min.	0.006	3.230	0.090	0.089	0.043	3.890	0.006	0.025
	Max	0.009	5.380	0.110	0.123	0.054	4.530	0.019	0.033
	Mean	0.008	4.305	0.100	0.106	0.049	4.210	0.013	0.029
	SD	0.002	1.520	0.014	0.024	0.008	0.453	0.009	0.006
WHO/FAO (2007)		0.2	450	5	0.3	40	50	50	10

WHO/FAO values represent the maximum permissible limit of PTEs in the vegetables.

Table 4.

Descriptive statistics of potentially toxic elements (mg·kg^-1) in vegetable samples from the old Market.

Vegetables	Statistic	Cd	Fe	Cr	Pb	Cu	Zn	Co	Ni
Green beans	Min.	0.009	11.830	0.025	0.090	0.050	2.560	0.015	0.019
	Max	0.011	12.810	0.092	0.110	0.061	2.740	0.018	0.020
	Mean	0.010	12.320	0.059	0.100	0.056	2.650	0.017	0.020
	SD	0.001	0.693	0.047	0.014	0.008	0.127	0.002	0.001
Spinach	Min.	0.008	8.980	0.058	0.150	0.030	15.370	0.009	0.023
	Max	0.012	10.530	0.084	0.170	0.050	15.850	0.011	0.027
	Mean	0.010	9.755	0.071	0.160	0.040	15.610	0.010	0.025
	SD	0.003	1.096	0.018	0.014	0.014	0.339	0.001	0.003
Green pepper	Min.	0.010	2.090	0.101	0.190	0.053	11.580	0.048	0.019
	Max	0.011	2.450	0.130	0.230	0.061	12.830	0.050	0.023
	Mean	0.011	2.270	0.116	0.210	0.057	12.205	0.049	0.021
	SD	0.001	0.255	0.021	0.028	0.006	0.884	0.001	0.003
Carrot	Min.	0.013	8.990	0.130	0.228	0.059	10.550	0.011	0.056
	Max	0.020	11.000	0.170	0.280	0.065	11.080	0.017	0.060
	Mean	0.017	9.995	0.150	0.254	0.062	10.815	0.014	0.058
	SD	0.005	1.421	0.028	0.037	0.004	0.375	0.004	0.003
Onion	Min.	0.019	9.530	0.110	0.150	0.051	3.530	0.043	0.048
	Max	0.023	10.350	0.140	0.190	0.058	5.350	0.057	0.051
	Mean	0.021	9.940	0.125	0.170	0.055	4.440	0.050	0.050
	SD	0.003	0.580	0.021	0.028	0.005	1.287	0.010	0.002
WHO/FAO (2007)		0.2	450	5	0.3	40	50	50	10

WHO/FAO values represent the maximum permissible limit of PTEs in the vegetables.

The mean concentration of Cd in this study was 17.90% - 19.40% lower than the permissible limit of 0.2 mg·kg^-1 [39]. However, the mean Cd concentration determined in onion (0.021 ± 0.003 mg·kg^-1) and carrot (0.017 ± 0.005 mg·kg^-1) from the old market was higher (p < 0.05) than those from the main market. This may be attributed to the cultivation of vegetables in contaminated soil or the excessive application of chemical fertilizers [40,41]. Moreover, the Cd levels in green beans, spinach, and green pepper were not significantly different (p < 0.05). In contrast, the mean Cd concentrations of 0.30 mg·kg^-1 (spinach) and 0.05 mg·kg^-1 (onion), 0.20 mg·kg^-1 (pepper), and 0.05 mg·kg^-1 (leafy vegetables), reported by Zarei et al. [41], Abubakar et al. [33], and Moa et al. [4], respectively, were higher than those obtained in this study. It has been reported that even a low level of exposure to Cd can cause hearing and vision loss and be detrimental to the kidneys, liver, skeletal system, and cardiovascular system [23,26,43,44].

The highest Fe concentration was obtained in amaranth (12.320 ± 0.693 mg·kg^-1) from the old market (Table 4). The concentration of Fe in various vegetables was significantly higher (p < 0.05) than those from the main market, except for spinach. This may be attributed to the use of irrigation water containing suspended Fr-rich sediments, which can enhance Fe accumulation in plants. Similarly, Taiwo et al. [1] measured high concentrations of Fe in spinach samples from southwestern Nigeria. It has been reported that Fe is one of the most abundant elements in Nigerian soil [45], which can result in high uptake and accumulation in the vegetables. It has been reported that spinach naturally accumulates high levels of Fe due to its physiological characteristics [46]. However, the Fe concentrations obtained in the study were 885% - 358,668% lower than those reported previously [12,31,33]. On the other hand, the values obtained were 548% - 785% higher than those reported by Ametepey et al. [2] and Lere et al. [43]. Iron is an essential element that plays a significant role in an organism’s metabolism [47]. However, elevated intake of Fe can result in gastrointestinal issues such as cramping and bleeding and have a negative impact on the metabolic processes and cardiovascular systems [47,48].

Chromium is essential for regulating the metabolism of nucleic acids, carbohydrates, and lipoproteins, as well as the enhancing the insulin action for glucose metabolism [49,50]. However, Cr is a biogenic microelement in the form of Cr (III). In contrast, Cr (VI) is highly toxic and carcinogenic to humans [51,52]. Moreover, excessive intake of Cr can lead to cardiovascular disorders, reduced blood glucose levels, and digestive problems [53]. In addition, long-term consumption of Cr has been associated with an increased risk of gastrointestinal cancer [54,56]. Fortunately, the concentration of Cr in the samples examined was found to be within the permissible limit of 5 mg·kg^-1 [39]. The shighest concentration of Cr was observed in amaranth (0.150 ± 0.028 mg·kg^-1) from the old market (Table 4). Furthermore, the levels of Cr in green beans, onions, carrots, spinach, and green pepper were found to be significantly different (p < 0.05). The old market environments are frequently exposed to vehicular emissions and dust, which can settle on the vegetables surface and contribute to elevated Cr concentrations. In a study conducted by Ametepey et al. in Tamale Metropolis, Ghana, the concentration of Cr in vegetables ranged from 0.03-0.64 mg·kg^-1 [2]. Moreover, the mean concentrations of Cr reported by Ashraf et al. [31], Bambara et al. [57], Najmi et al. [17], and Osae et al. [12] were 0.41 - 0.67 mg·kg^-1, 4.00 mg·kg^-1, 0.22 - 4.46 mg·kg^-1, and 1.49 - 17.42 mg·kg^-1, respectively, which exceeded the values obtained in this study.

The mean concentrations of Pb, Cu, Zn, and Co were below the permissible limits of 0.3 mg·kg^-1, 40 mg·kg^-1, 50 mg·kg^-1, and 50 mg·kg^-1, respectively [39]. The vegetable may have grown in minimally contaminated soil, reducing element accumulation and phytotoxicity risk, which also minimizes exposure through consumption of the vegetable samples. However, there were significant differences (p < 0.05) in the levels of Pb, Zn, and Co in amaranth, onion, carrot, spinach, and green beans. Copper content, on the other hand, was generally higher in vegetables from the old market, with no significant difference (p < 0.05) observed in the content in green bean, onion, carrot, spinach, and green pepper samples between the two markets. In comparison to previous studies, Ametepey et al. [2] and Fabiano et al. [57] were consistent with the results of our study. However, the values reported by Ashraf et al. [31], Lere et al. [45], Gupta et al. [32], Najimi et al. [17], and Bambara et al. [53] were 19.60% - 1,126% higher than the findings in this study. Lead has detrimental effects on the nervous system, particularly causing IQ deficiencies in children [19,27,28]. Furthermore, the concentrations of Cu, Zn, and Co in this study were notably lower than the values reported by Ashraf et al. [31], Najmi et al. [17], and Adhikari et al. [34], with differences ranging from 158.8% - 4,645.8%, 475% - 22,303%, and 6% - 632%, respectively.

Nickel (Ni) plays a vital role as an activator of the enzyme urease, making it a vital micronutrient for plants [57]. However, high concentrations of Ni can have negative effects on seed germination and overall plant growth, including the growth of roots, shoots, biomass, and final production [59]. In this study, the highest average Ni concentration was observed in carrots (0.058 ± 0.003 mg·kg^-1) from the old market (Table 3). In addition, the levels of Cr in green beans, carrots, spinach, green pepper, and onions varied significantly (p < 0.05). A previous study has shown that vegetables have lower levels of Ni contamination [12,31,32,56]. In fact, the concentration of Ni in the samples studied was below the permissible limit of 10 mg·kg^-1 [39]. Similarly, Ashraf et al. [31] reported low Ni concentrations ranging from 1.36 - 3.12 mg·kg^-1. Furthermore, other studies by Gupta et al. [32], Osae et al. [12], and Bambara et al. [56] reported mean Ni concentrations of 0.54 - 0.89 mg·kg^-1, 8.82 mg·kg^-1, and 0.38 - 6.5 mg·kg^-1, respectively. However, Adhikari et al. [34] found significantly higher mean Ni concentrations ranging from 8.00 - 198.71 mg·kg^-1, exceeding the permissible limit of 10 mg·kg^-1 [39]. The observed Ni levels may be associated with the application of phosphate fertilizers, which are recognized s major sources of Ni to agricultural soil, along with other potential sources such as wastewater irrigation, and industrial emissions that can further enhance Ni accumulation in vegetables.

Source identification of PTEs in vegetables

Pearson’s correlation analysis was used to determine the interrelationships between the PTEs in the vegetable samples, and the results are presented in Table 5. Most element pairs demonstrated strong positive correlations (r > 0.6), including Pb-Cr, Cu-Cd, Co-Cr, and Ni-Cr (p < 0.001). Additionally, the Co-Pb, Ni-Pb, and Ni-Co pairs showed a significant correlation at p < 0.05. The strong positive correlation coefficient among the PTEs indicates a similar source and nearly identical PTE accumulation in the samples.

Table 5.

Correlation analysis for the potentially toxic elements in the vegetable samples.

Elements	Cd	Fe	Cr	Pb	Cu	Zn	Co	Ni
Cd	1
Fe	0.133	1
Cr	0.513	-0.284	1
Pb	0.342	-0.483	.653a	1
Cu	.705a	0.099	0.183	-0.044	1
Zn	-0.328	-0.461	0.074	0.478	-0.365	1
Co	0.342	-0.483	.653a	1.000b	-0.044	0.478	1
Ni	0.342	-0.483	.653a	1.000b	-0.044	0.478	1.000b	1

^aCorrelation is significant at the 0.05 level (2-tailed).;

^bCorrelation is significant at the 0.01 level (2-tailed).

The results of the principal components analysis (PCA) are presented in Table 6 and Fig. 2. The results identified two principal components (PC) with a total cumulative variance of 92.603% of the source analysis for the various PTEs in the samples. PC1 shows a significant correlation between Cr, Pb, Zn, Co, and Ni, with a total variance of 51.781%. PC2 was correlated with Cd and Cu, with a total variance of 27.033%. The sources of PC1 and PC2 are considered to be a combination of fertilizer application and other anthropogenic activities, such as industrial pollution and vehicle emissions.

Table 6.

Principal component analysis of PTEs in the vegetable samples.

Elements	Component
	1	2
Cd	0.321	0.896
Fe	-0.593	0.336
Cr	0.715	0.397
Pb	0.980	0.030
Cu	-0.034	0.831
Zn	0.540	-0.628
Co	0.980	0.030
Ni	0.980	0.030
Eigenvalues	4.142	2.163
% of Variance	51.781	27.033
Cumulative %	51.781	78.814

Figure 2.

Rotated component loading plot of component analysis of PTEs of vegetable samples.

Health risk assessment

The USEPA has implemented health risk parameters, including carcinogenic and non-carcinogenic health risks, to assess the potential health risks associated with prolonged exposure to elements [31]. Tables S2 and S3, and S4 and S5 present the values of EDI and CDI for non-carcinogenic and carcinogenic risks, respectively. The EDI values of Cd, Fe, Cr, Pb, Cu, Zn, Co, and Ni were below their respective oral RfD values of 0.001, 0.07, 0.003, 0.004, 0.035, 0.04, 0.02, and 0.02 mg·kg^-1day-1, respectively [32,56]. The EDI values of the elements obtained are comparable with those of similar studies previously reported by Ametepey et al. [2] and Gupta et al. [32], but lower than those reported by Ashraf et al. [31]; Manwani et al. [10], Bambara et al. [56], and Fabiano et al. [57].

Additionally, the HQs and HIs of the PTEs in vegetables are presented in Tables 7 and 8. The HQ values for both children and adults are ranked in the following order: Pb > Zn > Cr > Fe > Cd > Ni > Co > Cu for the main market, and Pb > Cr > Zn > Fe > Cd > Ni > Co > Cu for the old market, in both children and adults. The hazard quotient (HQ) was used to evaluate the samples' non-carcinogenic risk in both adults and children. In all the investigated samples, the HQ values for each PTE monitored in the samples were <1. The values of HI in the studied samples ranged from 4.06×10^-05 - 2.07×10^-03.

Table 7.

Target hazard quotient (THQ) and hazard index (HI) of PTEs in vegetables from the main market.

Vegetables	Age group	HQ								HI
		Cd	Fe	Cr	Pb	Cu	Zn	Co	Ni
Green beans	Children	4.67 × 10-05	1.04 × 10-04	1.11 × 10-04	1.66 × 10-04	9.33 × 10-07	2.34 × 10-04	4.00 × 10-06	1.42 × 10-06	1.19 × 10-03
	Adults	2.08 × 10-05	4.47 × 10-05	4.77 × 10-05	7.11 × 10-05	4.00 × 10-07	1.00 × 10-04	1.72 × 10-06	6.35 × 10-06	5.02 × 10-04
Spinach	Children	6.67 × 10-05	9.24 × 10-05	1.47 × 10-04	2.15 × 10-04	5.83 × 10-07	2.37 × 10-04	3.67 × 10-06	2.72 × 10-06	1.27 × 10-03
	Adults	2.86 × 10-05	3.96 × 10-05	6.23 × 10-05	9.23 × 10-04	2.50 × 10-07	1.02 × 10-04	1.57 × 10-06	6.35 × 10-06	2.26 × 10-03
Green pepper	Children	6.00 × 10-05	7.16 × 10-05	2.67 × 10-05	4.20 × 10-04	8.15 × 10-07	1.63 × 10-04	4.33 × 10-06	3.72 × 10-06	1.37 × 10-03
	Adults	2.57 × 10-05	3.07 × 10-05	1.14 × 10-05	1.80 × 10-04	3.50 × 10-07	7.05 × 10-05	1.86 × 10-06	8.65 × 10-06	6.02 × 10-04
Carrot	Children	4.00 × 10-05	8.97 × 10-05	3.11 × 10-04	2.53 × 10-04	7.18 × 10-07	1.04 × 10-04	4.00 × 10-06	5.30 × 10-06	1.49 × 10-03
	Adults	1.71 × 10-05	3.84 × 10-05	1.33 × 10-05	1.09 × 10-04	3.28 × 10-07	4.47 × 10-05	1.72 × 10-06	1.24 × 10-06	3.96 × 10-04
Onion	Children	5.33 × 10-05	3.67 × 10-05	2.22 × 10-04	2.02 × 10-04	8.18 × 10-07	9.37 × 10-05	4.33 × 10-06	4.15 × 10-06	1.14 × 10-03
	Adults	2.29 × 10-05	1.76 × 10-05	9.53 × 10-05	8.66 × 10-04	3.50 × 10-07	4.00 × 10-05	1.86 × 10-06	9.65 × 10-06	2.07 × 10-03

Table 8.

Target hazard quotient (THQ) and hazard index (HI) of PTEs in vegetables from the old market.

Vegetables	Age group	HQ								HI
		Cd	Fe	Cr	Pb	Cu	Zn	Co	Ni
Green beans	Children	6.67 × 10-05	1.17 × 10-04	1.31 × 10-04	1.91 × 10-04	9.33 × 10-07	5.90 × 10-05	5.65 × 10-06	2.86 × 10-06	9.65 × 10-04
	Adults	2.86 × 10-05	5.03 × 10-05	5.63 × 10-05	8.17 × 10-05	4.00 × 10-07	2.52 × 10-05	2.44 × 10-06	6.65 × 10-06	4.24 × 10-04
Spinach	Children	6.67 × 10-05	8.56 × 10-05	1.58 × 10-04	3.06 × 10-04	6.68 × 10-07	3.47 × 10-05	3.34 × 10-06	3.57 × 10-06	1.16 × 10-03
	Adults	2.86 × 10-05	3.99 × 10-05	6.77 × 10-05	1.31 × 10-04	2.85 × 10-07	1.49 × 10-05	1.43 × 10-06	8.35 × 10-06	5.16 × 10-04
Green pepper	Children	7.33 × 10-05	2.16 × 10-05	2.58 × 10-04	4.04 × 10-04	9.50 × 10-07	2.71 × 10-04	1.14 × 10-05	3.00 × 10-06	4.13 × 10-03
	Adults	3.14 × 10-05	9.27 × 10-06	1.10 × 10-04	1.71 × 10-04	4.08 × 10-07	1.49 × 10-05	7.00 × 10-06	7.00 × 10-06	6.61 × 10-04
Carrot	Children	1.13 × 10-04	9.51 × 10-05	3.33 × 10-04	2.07 × 10-04	1.03 × 10-07	2.40 × 10-04	4.67 × 10-06	8.30 × 10-06	1.79 × 10-03
	Adults	4.86 × 10-05	4.09 × 10-05	1.43 × 10-04	3.33 × 10-05	4.43 × 10-07	1.49 × 10-05	2.00 × 10-06	1.84 × 10-05	5.14 × 10-04
Onion	Children	1.40 × 10-04	9.47 × 10-05	2.78 × 10-04	1.39 × 10-04	9.23 × 10-07	9.87 × 10-05	1.80 × 10-06	7.15 × 10-06	1.29 × 10-03
	Adults	6.00 × 10-05	4.06 × 10-05	1.19 × 10-04	2.78 × 10-04	3.93 × 10-07	4.23 × 10-05	7.15 × 10-04	1.67 × 10-05	4.06 × 10-05

These HI values were all below 1, suggesting no probable non-carcinogenic adverse health risks. Conversely, children were more susceptible to PTE contamination than adults.

The estimates for the CR of the PTEs studied in children and adults are presented in Table 9. For children, the CR values ranged from 9.91 × 10^-09 - 5.62 × 10^-6, while for adults they ranged from 9.16 × 10^-10 - 3.28 × 10^-6 in the main market, and 4.77 × 10^-8 - 6.03 × 10^-6 in children, while for adults the range was from 5.92 × 10^-9 - 3.51 × 10^-6 in the old market. The examined PTEs in this study for children and adults were found to be lower than the threshold value of 1 × 10^-6 – 1 × 10^-4, suggesting that the risk of cancer development is unlikely to occur. However, monitoring of PTEs on foodstuffs to reduce the impact of bioaccumulation and protect public health. The contribution to cancer by the investigated PTEs in the vegetable samples is shown in Figs. 3 and 4. The contribution of individual PTEs to HI (Fig. 3) and CR (Fig. 4) were calculated by expressing HI and CR value of each element as a percentage for each vegetable. Chromium makes the highest fraction of the CR in carrot, onion and green pepper from both old and main markets for children. Cr constituted of 25.76% - 94.92%, Pb contributed about 9.05% - 25.66%. The Cd and Ni contributed a lower fraction of around 0.53% - 65.00% and 0.04% - 1.14%, respectively. Notable differences in contribution between main and old markets samples were observed. For instance, Cr was generally higher in vegetables from the old market compared with main market. These differences may be attributed to environmental and anthropogenic factors such as the use of wastewater for irrigation, phosphate fertilizer application, or atmospheric deposition from vehicular emissions, which are more pronounced around the old market. Post-harvest handling and storage conditions, which differ between the two markets, may also play a role in influencing the observed variations . The CR values obtained in our study are consistent with the trends reported in a previous study reported by Abubakar et al. [33] and Said et al. [60], but lower than the values reported by Taiwo et al. [1], Ashraf et al. [31], Manwani et al. [10], Bambara et al. [56], and Adhikari et al. [34].

Table 9.

Cancer risk (CR) of PTEs in vegetables from main and old markets.

Vegetables	Age group	Main market (CR)				Old market (CR)
		Cd	Pb	Cr	Ni	Cd	Pb	Cr	Ni
Green beans	Children	4.18×10-08	7.24×10-07	2.01×10-06	9.91×10-09	5.98×10-06	8.33×10-07	2.37×10-06	1.65×10-08
	Adults	2.44×10-08	4.22×10-07	1.17×10-06	5.76×10-09	3.48×10-08	4.85×10-07	1.38×10-06	1.18×10-08
Spinach	Children	5.98×10-08	9.44×10-07	2.65×10-06	1.56×10-08	5.98×10-06	1.33×10-06	2.85×10-06	2.06×10-08
	Adults	3.48×10-08	5.47×10-07	1.55×10-06	9.16×10-10	3.48×10-08	7.77×10-07	1.71×10-06	1.20×10-08
Green pepper	Children	5.38×10-08	1.84×10-07	4.84×10-06	2.14×10-08	6.69×10-06	1.75×10-06	4.67×10-06	1.73×10-08
	Adults	3.14×10-08	1.07×10-07	2.81×10-06	1.25×10-08	3.84×10-08	1.02×10-06	2.72×10-06	1.01×10-08
Carrot	Children	3.59×10-08	1.11×10-07	5.62×10-06	3.04×10-08	1.02×10-07	2.12×10-06	6.03×10-06	4.77×10-08
	Adults	2.09×10-08	6.46×10-07	3.28×10-06	1.77×10-08	5.92×10-09	1.23×10-06	3.51×10-06	2.78×10-08
Onion	Children	4.18×10-08	7.24×10-07	2.01×10-06	9.91×10-09	5.98×10-06	8.33×10-07	2.37×10-06	1.65×10-08
	Adults	2.44×10-08	4.22×10-07	1.17×10-06	5.76×10-09	3.48×10-08	4.85×10-07	1.38×10-06	1.18×10-08

Figure 3.

Contribution of cancer risk by PTEs in vegetables from main market.

Figure 4.

Contribution of cancer risk by PTEs in vegetables from old market.

Conclusions

This study determined the concentration and assessed the health risk associated with PTEs (Cd, Fe, Cr, Pb, Cu, Zn, Co, and Ni) in vegetables sold in markets in Gombe, Northern Nigeria. The results revealed that the mean concentrations of Pb, Cu, Zn, and Co were below the permissible limits set by the FAO/WHO. The concentration treads followed the order: Zn > Fe > Cu > Cr > Ni > Co > Pb > Cd, with Zn and Fe being the most abundant elements among the investigated PTEs. The health risk assessment showed that both non-carcinogenic (HI < 1) and carcinogenic health risks for children and adults were lower than the threshold value of 1 × 10^-6 - 1 × 10^-4, suggesting no probable non-carcinogenic and carcinogenic health risks. However, children showed higher susceptibility to the potential risks than adults. Despite the negligible current risk, the potential for bioaccumulation and long-term exposure underscores the need to continually monitor the vegetables being sold in the market to ensure that only wholesome and safe vegetables are available for human consumption.

These findings make a significant contribution to health and food safety research by bridging a key knowledge gap in Northern Nigeria, particularly in Gombe State, where there is limited assessment of non-carcinogenic and carcinogenic health risks associated with PTEs in the vegetables, advancing knowledge on food safety, risk communication and offering data for regulation decision-making and public health policy. However, the study was limited to PTEs in vegetable samples from markets in the Gombe metropolis, and no speciation study was conducted. We recommend that future work include multi-seasonal motoring across urban and rural markets and to evaluate the chemical speciation.

Supplementary Material

This material is available online at www.eaht.org.

Table S1.

The parameters and input assumptions for exposure assessment of metals via ingestion routes.

Table S2.

Estimated daily intakes (EDI) of the PTEs in the in vegetable samples from main market.

Table S3.

Estimated daily intakes (EDI) of the PTEs in the in vegetable samples from old market.

Table S4.

Chronic daily intake (CDI) and cancer slope factor (CSF) of PTEs in the in vegetable samples from main market.

Table S5.

Chronic daily intake (CDI) and cancer slope factor (CSF) of PTEs in the in vegetable samples from old market.