Heavy metal contamination of food crops from industrial untreated wastewater is a serious problem and a reason for concern in Nigeria [1]. In the global scene however, approximately 10% of the world’s crops are cultivated through wastewater irrigation [2], and approximately 200 million hectares of farmlands are irrigated annually with any kind of water that is readily available or easily accessible irrespective of whether it is treated, not treated or hygienically safe. Thus, often times untreated and contaminated wastewater is used for irrigation, especially during the dry season [3]. Even though some industrial wastewater has nutrient constituents that could improve soil fertility and subsequently crop yield [4], the high metal contents and its associated toxicity and hazards to humans could be life-threatening. The long-term implication of prolonged and continuous use of wastewater for irrigation of crops and the build-up of metals in soil and subsequent transfer along the ecological food chain has caused severe consequences to human health [5].

Continuous health risk assessment is therefore required to guarantee safety and to assist in the management and evaluation of soil, crops, and resource quality generally [1]. For the dietary exposure of heavy metal contamination to humans, it is important to consider a thorough health risk assessment which typically involves four different steps: hazard identification, exposure assessment, dose-response assessment, and risk characterization [6]. Two main criteria viz, carcinogenic and non-carcinogenic are typically taken into account in health risk assessment [6]. The former refers to the likelihood that a person will be exposed to carcinogenic pollutants that could cause cancer, while the latter describes the possibility that a person will be exposed to non-carcinogenic pollutants that could cause chronic illnesses or injuries to tissues. Evaluation models and techniques created by the US-EPA and IRIS are utilized to determine the dangers to human health that are both carcinogenic and non-carcinogenic.

The consumption of contaminated food crops with heavy metals is the main pathway through which heavy metals enter human tissue [7]. Most of the food crops produced in this region of our study area are irrigated with wastewater which contains heavy metals and are transported and distributed to other parts of the country without any form of screening [8]. There are some studies that found high concentrations of heavy metals above the regulatory limits in some food crops irrigated with wastewater in northern Nigeria [9]. Excessive uptake of these toxic metals in the human system could lead to severe detrimental impacts and even death, especially to the vulnerable children and infants [4].

Despite the alarming levels of toxic metals detected in some vegetables irrigated with wastewater and pesticides, there is paucity of information on the health risk to humans in both adults and children. Information from this study will be a remarkable contribution to heavy metal toxicity of crops and could provide baseline data on the carcinogenic and non-carcinogenic health risk associated with heavy metal levels in vegetables that are consumed daily. Most studies in this region focused on the assessment of concentrations of heavy metals in food crops, while the human health risks are usually not considered. This study will, therefore, aim at determining the potential human risk of developing cancer and non-carcinogenic risk from the consumption of some common vegetables irrigated with wastewater.

The concentrations of the metals in the vegetables are presented in Table 2. The concentration of Cd differed significantly at p < 0.05 in all the vegetables. The Cd concentrations ranged between 26.87 mg/kg (spinach) and 33.50 mg/kg (pepper) and exceeded WHO permissible limit in all the vegetables.

Pb concentrations in the vegetables ranged between 4.17 mg/kg (spinach) and 10.90 mg/kg (tomato) and all the samples exceeded WHO permissible limits.

The concentrations of Cr ranged between 27.00 mg/kg (bell pepper) and 38.67 mg/kg (lettuce) and exceeded WHO permissible limit in all the vegetables.

Cu concentrations ranged between 10.60 mg/kg (onion) and 24.38 mg/kg (lettuce) and were below WHO permissible limit in all the vegetables.

Zn concentrations ranged between 1.77 mg/kg (pepper) and 3.42 mg/kg (spinach) and were below WHO permissible limit.

This study shows a high accumulation of heavy metals in the edible parts of the vegetables which was linked to the industrial wastewater used for irrigation of crops. The mean concentrations of Cd, Pb, and Cr in the vegetables exceeded the permissible limits set by WHO [17]. This could be attributed to the use of industrial wastewater that is contaminated with heavy metals in Bassa province. The wastewater from the coal and tin mining industries is usually discharged to streams and locals use it for irrigation of crops. The bioaccumulation of these metals by the vegetables could pose potential human health risks. Carcinogenic metals such as Cd, Pb, and Cr recorded at high levels in the vegetables are a source of concern. These metals even at low levels have adverse effects on humans [20]. The threats to health risks could even go beyond Bassa province since vegetables from this region are transported and sold in other regions of Nigeria. The consumption of metal-contaminated food crops is the main pathway into the human system and has been shown to cause several human health illnesses such as kidney and liver damage, blood pressure, and cardiovascular disease among others [21].

The highest concentration of Cd content was recorded in pepper. Pepper is one of the major spices that is used daily in all parts of Nigeria and the high Cd recorded should be a serious concern. There were, however, other studies that reported Cd levels in vegetables with varying concentrations. For example, in Zaria, Nigeria, Tanimu et al. [21] found 72 mg/kg in cabbage leaves irrigated with wastewater. This value was much higher than what was obtained in this study. In contrast, lower values of 1.76 mg/kg were reported in Pakistan [22]. In Bangladesh however, a much lower Cd (0.041 mg/kg) in some vegetables was reported [23]. Lower values of Cd in vegetables could be attributed to either the source of the water for irrigation which might have very low Cd concentration or perhaps a strict regulation and control of toxic metals by the government agency which might include treatment of any metal-contaminated water before discharge. Cr and Pb are among the carcinogenic metals, and long-term exposure could lead to the development of intestinal and nervous system cancers [1]. This study recorded high Cr and Pb levels in the vegetables which exceeded the WHO permissible limits [17]. There were other studies though that recorded lower values of Cr, Pb, and other metals in Nigeria [20, 21]. These lower values of metals in the vegetables were linked with lower metal contamination from the source of the water used for irrigation of crops. However, there were similar records of higher values of metal-contaminated vegetables in other parts of the developing countries. For example, 15.99 mg/kg concentrations of Cr were observed in spinach irrigated with wastewater in Pakistan [24]. The concentration of Zn in all the studied vegetables was generally very low with no health risk due to its values below the tolerable limit [16]. Heavy metal uptake in vegetables also depends on vegetable physiology, metal concentration in the soil, soil attributes and chemistry, and plant uptake mechanism [25]. There were reports of much higher Zn in other varieties of vegetables in Zaria [22, 23]. Cu concentrations in the vegetables do not pose any cancer risk due to their relatively lower values, unlike a previous study that reported higher values with potentially carcinogenic risk in Nigeria [21].

The EDI values of Cd, and Pb, for both the children and adult population had exceeded the oral RFD (the daily exposure of an individual to toxins or pollutants that can pose no appreciable hazard over a lifetime) and are likely to cause human health risks. The EDI for Cu exceeded the reference value in spinach, cucumber, carrot, tomato, lettuce, and bell pepper and thus put children at high hazard risk. There were other studies though that reported lower EDI values from the consumption of vegetables irrigated with industrial wastewater [23]. These lower values could be due to government’s policy of regulation and treatment of effluents before discharge.

The HQ of Cd for both children and adults was found to be greater than the threshold value of 1. This is an indication of likelihood of potential human health risks. However, HQ values of Cd in children were several-fold higher compared to HQ values obtained through adult consumption. Thus, children will experience a greater risk of Cd contamination than adults. Similar studies reported HQ > 1 for Cd, Pb, Cr, and Cu in vegetables irrigated with wastewater [22, 24]. In Bangladesh, for example, HQ values greater than 1 were reported for children viz 2.720, 2.629, and 0.269 for Pb, Cd, and Cr, and 1.807, 1.203, and 1.746 for adults respectively [23].

The HI values for both adults and children populations were all > 1 and thus revealed severe cumulative health risks of Cd, Pb, Cr, Cu, and Zn. A greater than 1 HQ value of vegetables irrigated with wastewater in Pakistan was reported [22] and in Bangladesh for Pb and Cr [23].

This study revealed higher values of HI in children compared to that of adults. Thus, the children population in this region is most susceptible to the cumulative effect of heavy metals through the uptake of the studied vegetables. This risk could extend beyond this region since vegetables are transported from the local markets to other parts of Nigeria.

Based on the TCR values of this study, adults and children are susceptible to carcinogenic risks posed by Cd and Cr exposure from consumption of the vegetables. Similar TCR values of 9.3E–05 and 6.1E–05 for Cd and Pb were reported by [24, 25]. The TCR values for lead could be regarded as within the safe limits.

In conclusion, this study has found that both adults and children could be exposed to non cancer and cancer risks from the consumption of the studied vegetables that were irrigated with contaminated industrial wastewater in the Bassa region of Plateau State located in northern Nigeria. Regulation should be put in place to enforce compliance of treating industrial wastewater before discharge. This would drastically control and reduce metal contamination of water bodies in channels and streams and the eventual transfer of metals to food chain through irrigation. The need for effective and sustainable monitoring cannot be over emphasized.