Systematic literature review of radionuclides, heavy metals, and organochlorine pesticides in Nigerian food crops: Assessment of carcinogenic and non-carcinogenic health risk

Literature search strategy

A comprehensive search was conducted to identify published scientific articles reporting quantitative data (in mean or range) on heavy metals, organochlorine pesticides (OCPs), and radionuclide contamination in plant-based food items from Nigeria. Articles were retrieved from the following scientific databases: Google Scholar, PubMed, Scopus, Directory of Open Access Journals (DOAJ), and Web of Science, with publications considered through July 2024. The following keywords, phrases, and clauses: “heavy metals in plants from Nigeria” AND “OCPs accumulation in plants from Nigeria” AND“ heavy metal accumulation in food crops from Nigeria” AND“ pesticide accumulation in food crops from Nigeria” AND“ Radionuclides in plants from Nigeria”, “Ionizing radiation in food crops from Nigeria” AND “chemical contaminants of food from Nigeria” AND“ specific names of the individual metals, radionuclide elements and OCPs (Pb, Cr, Cd, Cu, aldrin, lindane, heptachlor, etc.), “contaminated food crops in Nigeria” AND “crop plants collected from contaminated Nigerian environment” were employed for article search. The retrieved articles were then screened according to the study's inclusion and exclusion criteria to remove duplicates and unrelated studies, ensuring a focused and relevant selection for review.

Inclusion and exclusion criteria

The scientific reports evaluated in this study include both field and laboratory-based publications. An initial search yielded over 500 articles, necessitating the application of specific inclusion and exclusion criteria to identify studies most relevant to the study’s aims and objectives. Details of these criteria are provided in Figure 2.

The inclusive criteria include:

1. Articles reporting xenobiotic (chemical and physical) contaminants in plant-based food items commonly consumed in Nigeria,

2. Articles published in English and peer-reviewed journals,

3. Articles providing quantitative, accessible contaminant concentrations in mean or range format,

4. Studies conducted on food crops consumed in Nigeria,

5. Full-length articles available for review.

Exclusion criteria:

1. Articles reporting contaminants in foods of animal origin,

2. Articles on contaminants in food crops but from studies conducted outside Nigeria,

3. Articles containing duplicate content,

4. Articles written in languages other than English,

5. Articles available only as abstracts (e.g., conference abstract books),

6. Articles that are reviews of original research.

This study focuses exclusively on xenobiotic pollutants in crop plants commonly consumed in Nigerian such as cereals, legumes, fruits, and vegetables. Animal-based foods were excluded due to distinctions in contamination pathways and parameters used in the risk assessment of xenobiotics contaminants between plant- and animal based food items. Combining both plant and animal sources may increase the volume of the manuscript beyond the size many journals can accommodate. By narrowing the scope of the present review to plant-based food crops, we ensure a more homogenous analysis of pollution sources and their associated health risks.

Data extraction

Two independent investigators retrieved data on the mean concentrations of individual heavy metals and OCPs reported in Nigerian food crops. These data were used to calculate the mean concentrations and standard deviations for each metal and OCP. Additionally, the investigators gathered information on radionuclide contaminants from the selected articles. An initial search of scientific databases yielded 568 articles, 58 of which were excluded before screening due to duplication (Figure 2). The remaining papers were subjected to preliminary screening, and 418 did not meet the inclusion criteria. Of the 92 full-text articles further assessed, 66 met the selection criteria, comprising 36 on heavy metal concentrations, 20 on OCP residues, and 10 on radionuclides. The process of article identification, screening, and inclusion is summarized in Figure 2.

Statistical analysis

The concentrations of individual heavy metals and OCPs reported in the reviewed studies were analyzed using GraphPad Prism version 8.0™, with results presented as Mean ± Standard Deviation (SD). These data were used to model the carcinogenic and non-carcinogenic health risks associated with the consumption of metal- and OCP-contaminated crops, following established frameworks recommended by USEPA [19] for human health risk assessment. Radionuclide concentrations across various crop types (tubers, grains, legumes, etc.) were also analyzed using descriptive statistical methods, including mean, standard deviation, and range, for effective summarization of the data.

Human health risk assessment

Route of exposure to food crops contaminated by metals and OCPs in humans is mainly oral. Therefore, the non-carcinogenic and carcinogenic health risk assessment is determined based on the estimation of daily intake of the contaminants (metals and OCPs) via ingestion of food crops in accordance with United States Environmental Protection Agency model using the threshold values [19].

Possible human exposure to radionuclides through the ingestion of food crops

Surveys of natural radioactivity in food crops focus on assessing radionuclide concentrations in food substances consumed by human populations. Such surveys are essential for understanding food safety and evaluating potential health risks associated with radionuclide accumulation in the human body over time. Environmental radionuclides can enter the human body through multiple sources and pathways. Beyond direct inhalation from contaminated air—a known health hazard—another critical exposure route involves the accumulation and biomagnification of radionuclides along the food chain, ultimately affecting tertiary consumers, including humans. Radionuclides present in soils are absorbed by plant roots, translocated, and bioaccumulated in the various components of the foliage of crop plants [30-32].

Table 1 presents findings from the reviewed articles on radionuclide as a contaminant in food crops. Many studies reported elevated concentrations of naturally occurring radionuclide materials (NORM) and artificial radionuclides in food crops, contravening the United Nations’ goal for sustainable food security, that is, access to sufficient, nutritionally adequate, and safe food [33]. Oladele et al. [32] observed radionuclide accumulation in several crops, including yam, cassava, rice, maize, groundnut, cowpea, okra, pumpkin leaf, banana, and pawpaw, cultivated on farmlands in southwestern Nigeria. Their findings suggested potential health risks due to the biomagnification of ionizing radiation in humans consuming these crops. Similarly, Avwiri et al. [31] examined staple food crops—white yam (Dioscorea rotundata), maize (Zea mays), cassava (Manihot esculenta), beans (Phaseolus vulgaris), rice (Oryza sativa), sweet potato (Ipomea batatas), groundnut (Arachis hypogaea), banana (Musa sapientum), plantain (Musa paradisiaca), and cocoyam (Xanthosoma sp.)—for radioactive radionuclides. They deduced from their findings that the activity concentration of ²²⁶Ra (radium), ²³²Th (thorium), and ⁴⁰K (potassium)varied across these staple foods, depending on the contamination level of their growing locations.

Table 1.

Studies on activity concentrations of radionuclides in crop plants and their radiological health impacts.

Radionuclide bioaccumulation in crops also depended on crop type, with aerial versus root crops showing differing bioaccumulation rates. Descriptive statistical analysis of the data in Table 1, categorized by crop type, indicated that mean radionuclide activities (mean ± SD) were as follows: for tuber crops,⁴⁰K= 283.80 ± 139.70 (range: 37.84–684.50), ²³⁵U= 10.03 ± 1.48 (range: 8.55–11.50), ²³²Th= 20.35 ± 17.30 (range: 1.14–89.80) and ²²⁶Ra= 29.74 ± 27.88 (range: 1.72–85.50); for grains and cereals, ⁴⁰K= 105.20 ± 74.24 (range: 30.92–179.40), ²³²Th= 5.11 ± 2.99 (range: 2.12–8.10) and ²²⁶Ra= 3.59 ± 1.11 (range: 2.49–4.70); and for vegetables, ⁴⁰K= 575.80 ± 338.60 (range: 72.96–327.20), ²³²Th= 53.11 ± 14.72 (range: 0.85–111.30) and ²²⁶Ra= 22.46 ± 5.01 (range: 2.08–43.07). Comparable values for ⁴⁰K in food crops and meals have been documented globally; for instance, Jayasinghe et al. [40] found activity concentrations of ⁴⁰K ranging from 80.56 ± 17.53 to 143.41 ± 24.60 Bq/kg in meals from Sri Lanka. Badran et al. [41] reported values between 55.0 ± 11.0 and 328.0 ± 147.0 Bq/kg in vegetables consumed in Egypt, while Shanthi et al. [42] observed 29.64 ± 9.10 to 181.10 ± 14.30 Bq/kgin food crops from southwestern India. Venturini and Sordi [43] reported 35–380 Bq/kg in foodstuff from Brazil, and Ban-Nai et al. [44] reported ⁴⁰K concentrations ranging from 38.40 ± 0.40 to 299.50 ± 3.50 Bq/kg in edible mushrooms in Japan. Some studies, including Ibikunle et al. [37], found effective doses exceeding recommended standards for human radiological safety. As indicated in Table 1, radionuclide contamination in food crops in Nigeria was primarily linked to anthropogenic activities, including oil spills, extensive fertilizer use, wastewater irrigation, and tin mining in Plateau State.

Human exposure to heavy metals

Heavy metals are significant environmental hazards due to their persistence, low degradability, and ability to bioaccumulate in plants [27, 45, 46]. Metals such as Zn, Fe, Cu, Mn, and Ni are essential in trace amounts, supporting vital physiological functions in both plants and animals [13, 47]. However, elevated exposure to these metals through ingestion of contaminated foods accelerates their accumulation within the body, disrupting key physiological processes. This accumulation may result in cardiovascular, respiratory, neurological, renal, and skeletal damage in animals and similar disruptions in plants [48-51]. Additionally, these metals are linked to cancer, genotoxicity, mutagenesis, and teratogenesis [52-56]. Non-essential metals like Pb, Cd, As, and Cr pose significant health risks even at low concentrations [17, 47, 53, 54].

Different plant species vary in their capacity for heavy metal absorption and elimination; certain species are known to accumulate specific metals from contaminated soils or polluted air, which poses a substantial health risk when these plants are consumed [45, 57, 58]. Plants absorb trace amounts of heavy metals in solution along with essential nutrients and transport them via fruits, leaves, roots, and seeds, ultimately reaching tertiary consumers in the food chain [50, 59, 60]. Orisakwe et al. [61] found that plant-based foods contribute approximately 50% of the total ingestion of Pb, Cd, and Hg. Rice, a staple food in Nigeria, has been identified as a considerable source of Cd and Pb, both associated with renal damage in humans [58]. The NAFDAC recently issued a public warning against consuming fruits artificially ripened with calcium carbide, as it may contain harmful metals like Pb and As, posing serious health risks [62].

Table 2 displays the mean concentrations (mg/kg) of various metals extracted from the reviewed studies. The presence of metals was observed in leaves, fruits, stems, and tubers of grain and root crops, which are common food sources in Nigeria. The mean concentrations of the metals increased in the following order: Cd (0.81) < Co (0.99) < Ni (3.00) < Cr (5.83) < Pb (6.69) < Cu (10.72) < Zn (17.97) < Mn (26.82) < Fe (44.58). The essential metals—Cu, Zn, Mn, Ni, and Fe—required for physiological processes in plants and animals, showed the highest mean concentrations. The mean concentrations for certain essential nutrients such as Cu, Zn, and Ni were within WHO/FAO permissible limits for food crops [63] (Table 2), suggesting general safety for the average Nigerian consumer. However, it is important to note that these essential metals are prone to bioaccumulation in bodily tissues and organs, as they do not readily degrade. Therefore, health risk assessment models were employed to evaluate the long-term impact of metal exposure through food crop consumption, assessing potential carcinogenic and non-carcinogenic health risks.

Table 2.

Metal concentrations (mg/kg) in Nigerian food crops obtained from a systematic review of published articles in comparison with standard permissible limits.

Pb and Cd, both highly toxic with no known biological role in humans, animals, or plants, exhibited high concentrations in commonly consumed Nigerian food crops. The concentrations of these metals, as well as Cr and Co, exceeded WHO/FAO set permissible limits [63] (Table 2). These metals are also on the priority list of hazardous substances recommended for routine toxicological monitoring [64]. Given the historical health impacts, including mortality, linked to these metals [65, 66], their carcinogenic and non-carcinogenic risks were evaluated using health risk assessment models to assess the potential effects of prolonged dietary exposure.

Human exposure to OCPs

The use of pesticides to enhance agricultural productivity is a significant source of environmental contamination and has jeopardized human health through the consumption of pesticide-contaminated food crops [13]. Pesticide residues accumulate in plants through absorption from contaminated soil, direct application on crops in the field, or treatment during storage [67, 68]. Additionally, pesticides can contaminate groundwater and surface water through runoff after rainfall, thereby amplifying the risk of environmental pollution [11].

Pesticides consist of chemical and metallic residues that are persistent in the environment and pose serious health risks to both humans and wildlife [69]. Although they have been largely prohibited from agricultural use in many industrialized countries, pesticides continue to be widely used in developing nations to boost crop yields. As a result, pesticide residues are increasingly detected in food crops in these regions, posing health risks to humans and wildlife alike [24, 46, 67]. For example, high levels of toxic pesticide residues, including lindane (an OCP), organophosphates, and carbamates, were recently identified by NAFDAC in beans produced in Taraba and Gombe States of northeastern Nigeria [68]. Additionally, 112 individuals, including two children, were hospitalized after consuming pesticide-treated cowpea in Cross River State, Nigeria [70]. Human exposure to pesticide residues through food consumption is anticipated to be approximately five orders of magnitude greater than exposure via other routes, such as inhalation or ingestion of contaminated water [71].

Table 3 presents the mean concentrations (mg/kg) of OCP residues found in food crops, as reported in the reviewed studies. The OCP residues, in ascending order of concentration, are as follows: Lindane (0.23) < Heptachlor epoxide (0.30) < Methoxychlor (0.37) < p,p’-DDE, Heptachlor (0.47) < Endrin (0.62) < Dieldrin (0.82) < p,p’-DDD (1.31) < Endosulfan I (1.42) < Aldrin (1.69) < Endosulfan II (3.85) < p,p’-DDT (4.56). These concentrations were all above the maximum residue limits (MRLs) set by FAO/WHO and the European Commission (EC) for related food crops (Table 3).

Table 3.

Data of the concentrations of organochlorine pesticide residues (mg/kg) in Nigerian food crops obtained from a systematic review of published articles.

Persistent OCPs such as aldrin, dieldrin, heptachlor, and endosulfan have been associated with acute health effects, including headaches, dizziness, irritability, vomiting, agitation, disorientation, nausea, and convulsions [72-75], as well as chronic health effects, such as reproductive issues, neurotoxicity, tremors, and cancer [70, 76]. Furthermore, the International Agency for Research on Cancer (IARC) and the United States Environmental Protection Agency (USEPA) classify dieldrin, aldrin, and heptachlor as Group 2B, probably carcinogenic to humans [76, 77]. Numerous studies have reported OCP contamination in Nigerian food crops, including vegetables, tubers, and legumes in southwestern Nigeria [24, 71, 74, 77]; cocoa in southwestern Nigeria [78]; kolanuts in southwestern Nigeria [25]; and fruits and vegetables in northern Nigeria [11, 69].

Estimated health risk assessment from consumption of metal- and pesticide-contaminated food crops

The EDI, which forms the basis for assessing both carcinogenic and non-carcinogenic health risks of heavy metals, is shown in Table 4, while Table 5 presents EDI values for OCPs. For heavy metals, EDI values ranged from 4.66 to 256.30 μg/kg/day for adults and 5.75 to 316.28 μg/kg/day for children, with Cd and Fe representing the lowest and highest values, respectively, for both age groups. For OCPs, EDI values ranged from 1.32 to 26.22 μg/kg/day for adults and 1.63 to 32.35 μg/kg/day for children. Lindane exhibited the lowest EDI values, while p,p'-DDT recorded the highest for both adults and children.

Table 4.

Estimated daily intake, hazard quotient, hazard index and carcinogenic risk of heavy metals in food crops from Nigeria.

Table 5.

Non-carcinogenic health risk estimation of OCPs.

Estimated non-carcinogenic health risk assessment from consumption of metal- and pesticide-contaminated food crops

The HQ analysis for heavy metals in the selected reviewed articles ranged from 0.02 to 9.63 for adults and 0.03 to 11.87 for children (Table 4) while that of OCPs ranged from 0.43 to 97.17 for adults and 0.53 to 119.90 for children (Table 5). The HI for the adult (18.71) and children (23.07), which described the sum of all HQs for the analyzed heavy metals, exceeded the acceptable limit of 1 for both the adult and children’s categories (Table 4). For the heavy metals, Cr and Pb respectively recorded the lowest and highest HQ for both age groups (Table 4), while methoxychlor and aldrin were the OCPs that had the lowest and highest HQ for both age groups respectively (Table 5). In the adult group, the HQs for Pb, Cd, Cu and Mn exceeded the safe limit of 1 while the HQs for Cr, Ni, Zn, Co and Fe were within the safe limit. Albeit for the children group, the HQs for Pb, Cd, Cu, Ni and Mn exceeded the safe limit of 1, while HQs for Cr, Zn, Co and Fe were within the safe limit (Table 4). With the exception of methoxychlor, the HQs for all the analyzed OCPs for both age groups were higher than the recommended limit of 1 (Table 5).

Estimated carcinogenic health risk assessment after consuming metal- and pesticide-contaminated food crops

Tables 4 and 6 present the carcinogenic health risk estimated for metals and OCPs, respectively. The carcinogenic risk (CR) of the metals ranged from 3.27E-04 to 1.68E-02 for adults and 4.03E-04 to 2.07E-02 for children. The CR for Pb, Cd, Cr and Ni exceeded the acceptable limit of between 1E-06 to 1E-04 for both age groups (Table 4). The hazard ratio (HR), which estimated the carcinogenic risk posed by the OCPs, ranged from 5.29 to 952.69 for adults and 8.04 to 1446.32 for children, indicating that all HRocp > 1. Aldrin and p,p'-DDE recorded the highest and lowest HR for both age groups, respectively (Table 6).

Table 6.

Carcinogenic health risk estimation of OCPs.

Radionuclide accumulation in food crops and associated health risk among Nigerians

International organizations, such as the World Health Organization (WHO) and the European Food Safety Authority (EFSA), have incorporated risk assessment of consumable goods into their operational goals to enhance food safety systems, thereby mitigating health risks associated with contaminated food consumption [103, 104]. In Nigeria, radionuclide assessment in food and the environment appears to have gained momentum following early research by Ogunranti [105], who observed significant hematological abnormalities, including macrocytic anemia and leukopenia, in 26 workers exposed to radioactive waste in Koko village, Nigeria. The findings from the report by Ogunranti [105] suggest a notable risk for leukemia (a type of blood cancer) in exposed individuals. Furthermore, reports that approximately two million Nigerians face serious health risks due to exposure to radioactive materials near abandoned mining sites in Plateau State [106] have underscored the need for systematic radionuclide assessment in food crops.

The consumption of food crops with accumulated radionuclides results in the bioconcentration of ionizing radiation within body tissues. Avwiri et al. [31] estimated that approximately 15 out of 1,000,000 adults could develop various forms of cancer (based on lifetime fatality cancer risk standards), while 39 out of 1,000,000 may suffer hereditary disorders over their lifetime, suggesting a heightened cancer risk for those consuming radionuclide-laden food crops over extended periods. Oyekunle et al. [107] also reported health risks associated with prolonged exposure to radionuclides, including muscular weakness, paralysis, kidney and liver disease, cardiovascular disorders, chromosomal aberrations, leukemia, benign tumors, and cancers of the bone and pancreas. These findings align with Akhter et al. [108], who documented that 238Uaccumulates in the kidneys and lungs, ⁴⁰K in muscle tissues, and ²³²Th in skeletal, lungs and liver tissues in Pakistani adults. At certain threshold concentrations, these radionuclides can have harmful effects, including immunosuppression and an increase in systemic and genetic diseases and mortality [108]. The radiotoxicity of radioactive elements and nuclides is mainly driven by emitting ionizing radiations, including α-particles, β-particles, X-rays, and γ-rays. These radiations readily generate oxygen and nitrogen containing free radicals in cells and tissues. These free radicals elicit oxidative stress which indirectly cause damage to cellular DNA and other biomolecules including proteins and carbohydrates. These radiations are also capable of directly interacting with DNA strands to induce strand breaks, base modifications and or DNA-protein crosslinks, due to their chemical configurations. DNA damage can enhance genetic instability and pathophysiological damage including cancer [32, 107].

Nduka et al. [86] observed that the average annual effective dose (AED) and excess lifetime cancer risk (ELCR) from plant ingestion in Ishiagu and Ezillo, Nigeria, were (0.0811 mSv/y, 0.283 ×10^-3) and (0.0731 mSv/y, 0.255 ×10^-3), respectively. Some plants recorded ELCR values > 0.29 ×10^-3, suggesting potential cancer or radionuclide-related health risks with prolonged consumption of such plants. Due to the long half-lives of stable radionuclides, their uptake through continuous food consumption can reach levels that may lead to acute radiation syndrome or radiation poisoning. Therefore, monitoring radionuclide levels in food crops is essential for assessing potential public health impacts. Such monitoring also provides critical data for establishing regulatory guidelines to improve food quality and safety, benefiting national and international bodies, including NAFDAC, WHO, and EFSA.

Heavy metal and OCP accumulation in food crops

Heavy metals are among the most extensively studied environmental chemicals globally due to their pervasive presence and significant public health risks [7, 109]. Numerous sources contribute to metal contamination in the environment, which, in turn, leads to metal accumulation in food crops [5, 52]. Among the diverse classes of pesticides, OCPs are the most used and are applied both in the field and during storage to enhance food crop production and availability [68]. The persistence and slow degradation of OCPs [11] make them particularly persistent environmental contaminants. The reviewed studies report high but variable concentrations (Mean ± SD) of heavy metals and OCPs in food crops, which can be attributed to factors such as environmental contamination levels, plant absorption capacities, regional soil physicochemical properties, plant type, planting season, and soil organic matter content across different regions of Nigeria [13, 77, 101].

Among the metals, Fe has the highest mean concentration (Table 2), likely due to its high natural abundance in the earth’s crust and the prevalence of Fe-containing waste materials in the environment [80, 81]. Although Cd shows the lowest mean concentration, it is known to exert toxic effects even at extremely low levels [13, 17]. Plants readily absorb essential micronutrients such as Fe, Cu, Mn, Zn, and Cr in trace amounts, which support vital physiological processes including DNA synthesis, protein synthesis, immune function, and tissue repair [7, 13]. However, when these metals accumulate beyond permissible limits in the body, they can disrupt normal physiological processes, leading to metal-induced toxicosis [7].

The consumption of food crops with elevated metal concentrations is a major route for human metal toxicity (Figure 3). For instance, Cd can accumulate in human bone, lung, liver, kidney, and nerve tissues, resulting in tissue damage and dysfunction [27]. Exposure to Pb has been linked to neurological disorders, hypertension, cognitive decline, renal dysfunction, arthritis, hallucinations, and vertigo [27, 46] (Figure 3). Exposure to Cr is associated with chromosomal abnormalities, ulcers, nasal septal perforations, lung cancer, and alterations in DNA transcription and replication [46].

Figure 3.

Pathways of toxicity and translocation of contaminants from food crops exposed to heavy metals, pesticides, and ionizing radiation. Ingested contaminants enter the gastrointestinal tract (GIT) and are absorbed through the epithelial lining. They are then transported via the hepatic portal vein to the liver, where they may be metabolized before entering systemic circulation through the blood and lymphatic systems. Contaminants may also return to the GIT through bile excretion. Once in the bloodstream, contaminants and their metabolites can be distributed to various tissues and organs throughout the body).

The detection of these OCP residues in food crops highlights the ongoing use of OCPs on Nigerian farms, despite their ban by NAFDAC [96]. This also suggests the persistence of these compounds in the environment from past applications [69]. Notably, p,p’-DDT—banned in Nigeria since 2008 [11, 69]—was the most highly accumulated OCP in Nigerian food crops, indicating either its long-term environmental persistence or its continued illegal use by some farmers. Endosulfan, still widely available in agricultural stores as a substitute for the banned lindane, also exhibited high concentrations in Nigerian food crops, likely reflecting its continued usage [99]. Many pesticides detected in Nigerian food crops are known to be neurotoxic, teratogenic, carcinogenic, and immunosuppressive [25, 67]. Specific health effects have been associated with some OCPs, including increased d-glutaric acid metabolism (aldrin) and skin sensitization, enzyme induction, allergic reactions, and contact dermatitis (lindane) [101].

Agricultural practices contribute significantly to the rising levels of hazardous metals and OCPs in the environment and food crops [23, 94]. There is evidence suggesting a strong link between these two types of contaminants, as Bawa et al. [46] indicate that pesticide application may facilitate the accumulation of heavy metals in soil, which are subsequently absorbed by plants. Some pesticides contain metals as biocidal components [17]. Several studies included in this review report that the concentrations of heavy metals [4, 13, 17, 26, 46, 49, 58, 80, 93] and OCP residues [11, 25, 67, 69, 71, 75, 77] in Nigerian food crops exceed FAO/WHO regulatory limits, highlighting contamination as a significant public health concern.

Risk assessment of human health

While multiple exposure pathways exist for heavy metals and OCPs, food crop consumption is recognized as a primary route of exposure [58, 61]. Consequently, assessing the potential health risks associated with the consumption of heavy metal- and pesticide-laden food is essential, particularly given the possible adverse effects of long-term exposure. The EDI is a critical tool for evaluating health risks posed by contaminants in food crops [23], and it was employed in this study to calculate concentrations of heavy metals and OCPs in food crops consumed by a 60 kg adult and a 32.7 kg child. Higher EDI values recorded for children raise concerns for future health due to their prolonged potential exposure through dietary consumption of such food crops. The Hazard Quotient (HQ) for certain metals (Pb, Cd, Cu, Mn) in both adults and children was > 1, indicating an elevated risk of metal-induced toxicity for consumers of these crops. The elevated HQ values for Pb and Cd are particularly concerning given that these metals have no known biological function in humans [23]. Although the HQ values for other metals, such as Cr, Zn, Co, and Fe, fell within safe limits, the Hazard Index (HI) assessing the combined non-carcinogenic health effect of all analyzed metals exceeded one, suggesting a heightened risk of systemic toxicity among Nigerian consumers of these food crops.

Most reviewed studies compared OCPs residue in food crops to MRLs set by regulatory authorities; however, MRLs are not toxicological thresholds [25]. In this study, OCP exposure levels (EDI) were evaluated against the Acceptable Daily Intakes (ADI) established by the WHO Joint Meeting on Pesticide Residues and the Australian Pesticides and Veterinary Medicines Authority to assess health risks from food consumption [110]. It was found that the HQ values for both adults and children was > 1 for 92% of the evaluated OCPs, indicating substantial health risks posed by these pesticide residues. Similar findings were reported in South Africa and India [111,112].

The range of 10-6 – 10-4 represents the estimated permissible lifetime risk range for carcinogens [27]. A Cancer Risk (CR) level of one per million (1x10-6) implies that among 1 million exposed individuals, at least one case of cancer is expected [7]. The estimated CR values, calculated for carcinogens (Pb, Cd, Cr, Ni), exceeded the tolerable threshold of 1x10^-4 for both adults and children, consistent with studies conducted in Pakistan and Iran [113,114]. A Hazard Ratio (HR) > 1 for OCP residues suggests a long-term risk of harm, particularly cancer, due to pesticide residues in food crops [24]. The Stockholm Convention, which identified aldrin, dieldrin, heptachlor, and heptachlor epoxide as hazardous chemicals in 2001 (adding endosulfan in 2011), suggests that these substances are highly detrimental to both human health and the environment and should be excluded from agricultural use [25, 77]. These findings indicate that lifetime exposure to heavy metals and OCPs through contaminated food crop consumption may present carcinogenic risks to both children and adults at levels exceeding one in a million. Buah-Kwofie et al. [111] obtained a similar outcome in food crops consumed by members of a rural community in South Africa, as did Chourasiya et al. [112] in a peri-urban location in India.

The EDI, HQ, HI, HR and CR—standard health risk models used in this study—demonstrate a substantial exposure to heavy metals and OCPs, posing a health threat to Nigerians (both adults and children) consuming these contaminated food crops. This necessitates urgent action from Nigerian food safety regulatory agencies to promote safer agricultural practices. Children are particularly vulnerable to pesticide and heavy metal toxicity due to their higher food intake relative to body weight and the immaturity of their detoxification systems [7, 99]. These factors likely contribute to the higher hazard quotient and ratio observed in children in this study, emphasizing the need for rigorous monitoring of contaminants in food crops consumed by this demographic [77].

Reports on xenobiotic-contaminated food consumption and poisoning incidents

Nriagu [109], in the late twentieth century, described the prevalence of heavy metals in the environment as a "silent epidemic" of environmental metal poisoning. Decades later, contamination of food crops by heavy metals, agrochemicals, and radioactive substances continues to result in poisoning incidents and fatalities among healthy Nigerians [10, 115] and other populations worldwide. Table 7 summarizes documented cases of metal- and pesticide-induced poisoning from 1958 to 2024 due to the consumption of contaminated food crops in Nigeria. According to this data, there have been at least eight incidents resulting in multiple fatalities.

Table 7.

Case reports of poisoning incidences induced by contaminated food crops in Nigeria (1958 - 2024).

The recurrence of morbidity and mortality from the ingestion of metal-contaminated [115, 120] and pesticide-contaminated [116-119] food crops over the time span of 1958 and 2024 highlights the need for stricter regulatory enforcement by Nigerian food safety agencies to prevent intentional and unintentional food adulteration. Additionally, Nigerian environmental regulatory authorities should implement stringent measures to prevent the indiscriminate discharge of toxic metals and agrochemicals into the environment, which subsequently enter the food chain.

This study also has international relevance, as reports of poisoning through metal- or agrochemical-contaminated food crops are increasing globally. For example, from 1910 to 2007, over 100 individuals died, and many others suffered various deformities from Itai-itai disease (cadmium poisoning) after consuming Cd-contaminated rice grown in the Jinzu River basin, Toyama, Japan [65]. Also, pesticide poisoning, a persistent public health issue, remains highly relevant to date. Boedeker et al. [121] found that pesticide poisoning continues to pose a significant health risk, with the World Health Organization’s 1990 estimate of 20,000 annual deaths from approximately one million unintentional poisonings still relevant thirty years later, largely due to the increasing global use of pesticides. Their review, covering 141 countries, reported approximately 740,000 annual cases of unintentional pesticide poisoning, with the highest incidence rates in southern Asia, followed by southeastern Asia and eastern Africa [121]. In contrast, foodborne poisoning from radiation and radioactive substances appears to be under-researched and underreported. This gap underscores the need for robust regulatory oversight in monitoring food production processes globally to prevent contaminant-induced food poisoning.

This systematic review has some limitations that should be acknowledged and considered for future research. A significant limitation is the variability and inconsistency in methodologies, sampling techniques, and analytical procedures employed across the included studies. These differences may affect the comparability and reliability of the findings, potentially leading to discrepancies in the reported concentrations of contaminants. Additionally, the review primarily relied on available published studies, which may not comprehensively capture the extent of contamination across all regions of Nigeria, particularly in remote or understudied areas. Incomplete datasets from the reviewed studies also posed a challenge, as specific contaminants of interest (e.g., heavy metals, OCPs, and radionuclides) were not consistently reported across all crop types and locations. Moreover, while staple crops were prioritized, the exclusion of less-consumed but culturally or nutritionally important crops limits the understanding of localized exposure risks. Furthermore, the health risk assessment models applied in this review (e.g., EDI, HQ, CR) are based on estimated consumption rates and body weights, which may not fully account for the diverse dietary habits and physiological characteristics of the Nigerian population, particularly among vulnerable groups such as children. For example, USEPA’s estimated intake rates may not accurately reflect the higher consumption of Nigeria’s staple crops like cassava. Also, the distinct processing methods of local staple foods such as cassava flakes (local name–garri) alter the bioavailability of pollutants but are not accounted for in USEPA models. Addressing these limitations in future research by applying standardized methodologies, expanding the diversity of crop type and incorporating locally derived exposure factors would provide a more comprehensive understanding of the contamination levels in Nigerian food crops and the subsequent health risks posed by xenobiotic pollutants.