Assessment of heavy metal contamination in fish, fruits, and vegetables in Southwest Nigeria: A systematic review

1. Introduction

Heavy metals cause build-ups and biological magnification in plethora food chains because they are sturdy and loiter in the ecosystem for a long period. In the Southwestern part of Nigeria, where industrialization, rapid urbanization, and agricultural activities greatly contribute to environmental pollution (Odafivwotu and Abel, 2015; Akinnifesi et al., 2021; Akinsola et al., 2022) and stringent environmental regulations are seldom implemented, this presents a long-term debilitating health risk. The population is more dependent on locally produced food. Application of fertilizers, use of pesticides possessing heavy metal compounds, and use of unadulterated irrigation water can all contribute to heavy metal contamination in the soil, thereby affecting humans via consumption of agricultural produce. Fish in aquatic systems become contaminated by heavy metals in contaminated water bodies, frequently due to mining operations or industrial effluents. Although lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), aluminum (Al), and mercury (Hg) are not necessary for normal human functioning (Boyd and Rajakaruna, 2013), food contamination by heavy metals poses a serious health risk to consumers, especially because these metals bio accumulate in the food chain (Hussain et al., 2019; Nkwunonwo et al., 2020; Bawa-Allah 2023).

In Nigeria, fish, fruits, and vegetables contain dangerously high concentrations of heavy metals, often surpassing internationally acceptable limits set by regulatory bodies such as the Food and Agriculture Organization (FAO) and the World Health Organization (WHO). These findings have also been reported in other studies. Adegbola et al. (2021) and Abidemi-Iromini et al. (2022) conducted studies on fish from rivers in Lagos and Ogun states and revealed high concentrations of heavy metals, which are extremely harmful even in small amounts. Similarly, consuming grains (such as rice) and plants (including lettuce, amaranth, water spinach, and cowpea) cultivated in soil contaminated with mercury over an extended period of time is harmful to human health. Studies conducted in Southwest Nigeria have shown that commonly consumed food items contain elevated concentrations of heavy metals, frequently surpassing the permissible limits set by the Food and Agriculture Organization (FAO) and World Health Organization (WHO) (Adekunle et al., 2009; Akinola et al., 2020). For example, fish samples from rivers near industrial zones in Lagos were found to contain large amounts of Pb and Cd by Olayinka and Adedeji (2018), whereas vegetables irrigated with contaminated water sources had high levels of arsenic (Ogundele et al., 2015).

Exposure to heavy metals is associated with numerous health hazards. Lead is a common heavy metal that contaminates the environment and enters the body through absorption, bioavailability, bioconcentration, and biomagnification. It causes problems in the reproductive, neurological, skeletal, hematological, renal, and cardiovascular systems (Collin et al., 2022). Mercury exposure causes neurological damage, including ataxia, muscle weakness, numb limbs, disturbance in speech, chewing, and swallowing, as well as brisk and increased tendon reflexes in patients exposed to massive amounts of Me-Hg (Balali-Mood et al., 2021). Cd exposure can result in a variety of adverse effects, including renal and hepatic dysfunction, pulmonary edema, testicular damage, osteomalacia, and damage to the adrenal and hematopoietic systems (Genchi et al., 2020). Long-term arsenic exposure can cause cancer, developmental problems, and skin damage. Additionally, long-term exposure to arsenic may suppress the immune system by causing keratinocyte apoptosis through the interaction of Fas and Fas-ligand, lowering the percentage of peripheral blood CD4+ T cells, reducing the number of Langerhans cells, and altering their migration (Tang et al., 2023). Understanding the distribution and concentrations of heavy metals in regularly consumed foods is essential for safeguarding public health in Southwest Nigeria, given their possible health consequences.

Furthermore, it is impossible to ignore how heavy metal poisoning affects society economically. Agriculture and fishing in Southwest Nigeria provide a substantial portion of the population’s income. These food supplies are contaminated, which puts the public’s health in danger, as well as the local economy, food security, and consumer confidence in the safety of locally made commodities at risk. This might result in more imported food products, which would drive up food prices and worsen poverty in areas that already have serious economic difficulties.

A rigorous study of the degree of heavy metal contamination is important in Southwest Nigeria because of the growing industrial and agricultural operations there, as well as possible health repercussions. The purpose of this systematic study was to evaluate the level of heavy metal contamination in Southwest Nigerian fish, fruits, and vegetables. This study reviews previous research in an effort to provide readers with a thorough understanding of contamination levels, pinpoint possible heavy metal sources, and assess the health hazards of consuming them. In addition, this study seeks to identify areas of unmet need in the field, indicate future research directions, and offer mitigation measures for heavy metal pollution in the area’s food supply. Designing policies and actions to lower public health risks, guarantee food safety, and advance environmental sustainability in Southwest Nigeria also require an understanding of the extent and effects of heavy metal contamination in food supplies.

2. Methods

3. Results

Among the 64 eligible articles selected for this systematic review, 30 were carried out in Lagos, eight in Ondo, and seven in Ogun State of Nigeria. Three, six, and seven studies were conducted in the Oyo, Ekiti, and Osun states, respectively. The majority (40) of the studies included in this review were carried out on fish, while 20 and 4 studies were carried out on vegetables and fruits, respectively.

4. Discussion

The results of this review revealed widespread contamination of fruits, vegetables, and fish by heavy metals across southwestern Nigeria, with no significant public health concerns. Heavy metals, such as lead (Pb), cadmium (Cd), chromium (Cr), nickel (Ni), and zinc (Zn), were found to exceed the WHO safety limits for fish, fruits, and vegetables. These findings raise global concerns regarding the persistence of heavy metals in the environment due to pollution, industrial activities, and agricultural practices. In addition, widespread heavy metal contamination across different types of fruits in this systematic review study was highly reflected in this study, posing debilitating health risks, for example, from consuming fruits (Ojezele et al., 2021). Nonetheless, some articles have revealed fruits with heavy metal contamination within the WHO safe limits. Citrullus lanatus (Watermelon) displays safe levels of heavy metals, including cadmium (0.004 mg/kg) and lead (Atikpo et al., 2021).

Additionally, fish species, distinctly Tilapia zilli, Chrysichthys nigrodigitatus, and Clarias gariepinus, frequently exceeded the WHO limits for heavy metals, particularly Pb, Cr, and Ni. Results from this study in terms fish contamination in an article by Yahaya et al. (2020) revealed lead, and chromium contamination at levels exceeding WHO linits in Tilapia zilli with 4.36 mg/kg and 5.40 mg/kg respectively. This was similar to the results from a study by Putshaka et al. (2015), who reported contamination levels in lead and chromium at 1.132 mg/kg and 1.845, respectively, exceeding. The Pb level in the study by Yahaya et al. (2020) was much higher than that reported by Putshaka et al. (2015). The results of this study by Yahaya et al. (2020) were dissimilar to results from a study by Inalegwu et al. (2019), who reported lower levels of Pb (0.347 mg/kg) and Cr (0.848 mg/kg) contamination in Tilapia zilli when compared to the study by Yahaya et al. (2020). This suggests that the aquatic environments in which these fish are harvested are heavily contaminated, most likely from industrial runoff, mining activities, and illegitimate waste disposal. This raises public health concerns, as fish are a primary source of dietary protein globally, and long-term consumption of polluted or contaminated fish may lead to bioaccumulation of toxic metals in humans, resulting in kidney damage, congenital defects, and an increased risk of cancer.

Yahaya et al. (2020) reported Pb, Cu, and Ni contamination in Chrysichthys nigrodigitatus exceeding the WHO limits of 3.63 mg/kg, 1.28 mg/kg and 4.16 mg/kg respectively. The results from Yahaya et al. (2020) were similar to the results from Unaeze et al. (2023). Although, the level of Pb contamination was lower (2.00 mg/kg) in the study by Unaeze et al. (2023) when compared to the study by Yahaya et al. (2020) but exceeding WHO limit, also the concentration for Zinc in Chrysichthys nigrodigitatus was extremely high (240.00 mg/kg) in the study by Unaeze et al. (2023) unlike results from Yahaya et al. (2020) having Zinc level in the same fish as 1.28 mg/kg. Also Cu levels in Unaeze et al. (2023) (2.32 mg/kg) was higher than the level reported by Yahaya et al. 2020 (1.28 mg/kg) with both exceeding WHO limits.

Clarias gariepinus is a commercially liked fish species, which was reflected several times in the reviewed articles selected for this study, probably because of its significance in the local markets in Nigeria. The high levels of Cd and Pb found in this species underscore the need for stringent monitoring of fish harvested from polluted water bodies. In this review article, seven articles revealed heavy metal contamination in Clarias gariepinus (Olurin, 2024; Ogungbemi et al., 2022; Utete and Fregene, 2020; Yahaya et al., 2022; Ayanda et al., 2020; Adubiaro and Animashaun, 2021, Adewunmi et al., 2017), while, four articles (Olurin, 2024; Utete and Fregene, 2020; Yahaya et al., 2022; Ayanda et al., 2020) reported heavy metal contamination exceeding WHO limits, and three articles (Ogungbemi et al., 2022; Adubiaro and Animashaun, 2021; Adewunmi et al., 2017) reported heavy metal contamination within the WHO permissible limits. In this study, heavy metal contamination in Clarias gariepinus exceeding WHO limits is similar to the study by Odey et al. (2020), with Lead (Pb) being at the forefront, frequently appearing in these articles. Articles from this review reporting Pb contamination revealed Pb levels at levels of 9.14 mg/kg, 0.20 mg/kg, 3.23 mg/kg, 9.43 mg/kg, 11.10 mg/kg, 92.08 mg/kg, and 1.10 mg/kg in studies by Yahaya et al. (2022), Ogungbemi et al. (2022), Olurin (2024), Adewunmi et al. (2017), Utete and Fregene (2020), Ayanda et al. (2020) and Adubiaro and Animashaun (2021) respectively. Pb level (92.08 mg/kg) reported in Clarias gariepinus reported by Ayanda et al. (2020) was higher than that reported by Odey et al.(2020), although they all exceeded WHO permissible limits. However the Pb level reported by Olurin (2024) was lower than what was reported by Odey et al. (2020) More so, articles by Adubiaro and Animashaun (2021), Ogungbemi et al. (2022), Adewunmi et al. (2017) reported heavy metal contamination within WHO permissible limits in all selected metals that was assayed for, results in these studies were similar to results reported by Abubakar and Adeshına (2019). The presence of Pb in the main culprits of this fish implies that ingestion of Clarias gariepinus may be poisonous to individuals in the southwest region of Nigeria, as long-term ingestion of Pb via this fish leads to the accumulation of Pb, which may result in coma, convulsions, and even death (Ray & Vashishth, 2024).

Furthermore, some articles reviewed in this study found metals such as zinc and iron exceeding WHO safety limits (Ajibare et al., 2021; Oghenechuko et al., 2022; Taiwo et al., 2019; Wangboje & Miller 2018; Taiwo et al., 2017; Taiwo et al., 2019; Olowu et al., 2015) Articles that reported heavy Zn conatamination in fish were similar to results reported by Mahidi and Albashr (2023); Yalwa et al. (2024) and Balooch et al. (2021) in terms of exceeding WHO limits Furthermore, Ajibare et al. (2021) reported Znc (112.19 mg/kg) contamination in Oreochromis which was higher than Znc level reported Mahidi and Albashr (2023); Yalwa et al. (2024) and Balooch et al. (2021) in fish at a level of 37.17 mg/kg, 50.84 mg/kg and 13.36 mh/kg respectively. Bioaccumulation of Zn may cause debilitating effects in the human body, resulting in reproductive disorders and Alzheimer’s disease (Kiouri et al., 2023) Although, Fe is an essential element for humans as it participates in a wide variety of metabolic processes, including oxygen transport, deoxyribonucleic acid (DNA) synthesis, and electron transport (Abbaspour et al., 2014). For Fe contamination of fish, reports from studies by Taiwo et al. (2017); Olowu et al. (2015); Taiwo et al. (2019) and Oghenechuko et al. (2022) revealed higher level of Fe concentration in fish at level of 8.05 mg/kg/, 8.02 mg/kg, 8.68 mg/kg and 5.51 mg/kg respectively when compared to Fe levels that was reported in studies by Bobar et al. (2022) and Salam et al. (2019) having with 0.23 mg/kg and 4.30 mg/kg respectively; suggesting a great concern in the southwestern region in Nigeria, as bioaccumulation of Fe. The ingestion of fish by humans may lead to the formation of free radicals and excessive tissue damage in the body (Abbaspour et al., 2014). Shellfish species like Callinectes danae and Cardisoma armatum also revealed levels of copper contamination, indicating that both fish and crustaceans from these regions may pose a health risk if consumed in large quantities (Oladunjoye et al., 2021). Because of the continual nature of heavy metals in aquatic habitats, the continuous consumption of contaminated fish could have long-term health implications for the southwestern people of Nigeria.

Vegetables and fruits are very much essential components of human diets (Rodriguez-Casado, 2016) Results from this review showed substantial contamination with heavy metals, particularly lead and cadmium. Carica papaya (pawpaw) and Mangifera indica (mango), the two frequently reported fruits captured in this systematic review, were found to contain elevated levels of lead, significantly exceeding the WHO limits. Imperatively, Pb levels in mangoes reach 1.620 mg/kg, posing a serious health risk, as chronic exposure to Pb can lead to neurological disorders, developmental delays in children, and cardiovascular diseases in adults (Atikpo et al., 2021). In the same fashion, levels in pawpaw (0.077 mg/kg) approached the safety threshold, raising concerns over kidney damage and bone demineralization with long-term consumption leading to bioaccumulation. Watermelon (Citrullus lanatus) also exhibited notable heavy metal contamination, with Pb levels reaching 1.760 mg/kg, which is significantly higher than the permissible limits. This finding is concerning given the popularity of watermelon in Southwestern Nigeria, suggesting that regular consumption could lead to bioaccumulation of heavy metals in humans, who are invariably consumers. While some fruits such as apples (Malus domestica) and mixed fruit samples showed contamination with metals such as chromium and nickel, other fruits such as oranges (Citrus sinensis) revealed metal levels exceeding WHO safe limits (Omoyajowo et al., 2017).

Similarly, vegetables, a very important food in the diets of individuals in southwestern Nigeria, revealed alarming levels of contamination. Talinum triangulare, green peas (Pisum sativum), and Celosia argentea were reported to contain high concentrations of Cd, Cr, Zn, and Pb. The presence of a plethora of heavy metals in these vegetables, most importantly lead and cadmium, is also of great concern, as these metals can cause cumulative dangerous effects over time, leading to severe health issues, including neurotoxicity, reproductive issues, and carcinogenic effects (Rodriguez-Casado, 2016).

The high levels of aluminum in green peas and mushrooms in this systematic review (Ojezele et al., 2021) further suggest the potential for widespread heavy metal contamination, as aluminum has been implicated in anemia and neurological disorders (Cirovic et al., 2023). In addition to aluminum, lead was also reported in a study by Ojezele et al. (2021), which contaminates green peas and mushrooms at high levels. Precisely, for green peas Al (14.0 mg/kg), Pb (1.68 mg/kg), and for mushroom Al (11.0 mg/kg) Pb (1.38 mg/kg). The results from the study by Ojezele et al. (2021) were dissimilar to those of Pb levels reported by Oloruntoba et al. (2017), which were below detectable limits. For mushroom, the Pb level(1.15 mg/kg) reported by Orywal et al. (2021) was slightly lower than the Pb (1.38 mg/kg) level in mushroom reported by Ojezele et al. (2021), with mushrooms being of great concern in the southwest region of Nigeria. The accumulation of aluminum in vegetables may lead to anemia and neurological disorders, making it a metal of great concern. Overall, Amaranthus hybridus (Pb and Cd), Telfairia occidentalis (Cu and Pb), and Celosia argentea (Cr, Zn, and Cu) had heavy metal concentrations exceeding the WHO limits, having a robust tendency to cause debilitating effects on human health. Furthermore, some vegetables, such as red peppers, chilli peppers, and tomatoes, were found to have heavy metal levels according to the WHO safety guidelines, suggesting possible regional variations in contamination. The remarkable levels of contamination among various food groups indicate that stringent measures by the government, private organizations, and environmental management strategies must be strengthened to reduce these risks. This review highlights the persistent contamination of essential food items in Southwest Nigeria, which may have profound long-term health consequences. Chronic exposure to elevated levels of lead, cadmium, and other heavy metals is linked to a range of adverse health effects, including neurotoxicity (Mitra et al., 2022) cardiovascular diseases, and increased cancer risks (Ebrahimi, et al., 2020) Lead exposure, particularly in children, can cause irreversible cognitive impairments, whereas cadmium accumulation in the body is associated with kidney dysfunction and osteoporosis (Ma et al., 2022).

5. Conclusion

This systematic review highlights the prevalence and persistence of heavy metal contamination in fish, fruits, and vegetables in Southwest Nigeria between 2014 and 2024. Most of the reviewed studies revealed significant contamination in fish, fruits, and vegetables. The reviewed studies revealed that the concentrations of chromium and nickel frequently exceeded the permissible limits of the World Health Organization (WHO), which could pose significant health risks to consumers. A number of fish species (Tilapia zilli, Chrysichthys nigrodigitatus, and Clarias gariepinus) in the reviewed studies, showed high levels of contamination, thus making them critical indicators of environmental pollution in the region. Although vegetables and fruits have been less frequently studied in the region, the reviewed studies showed that they still exhibited significant levels of heavy metal contamination in some instances. Therefore, this review highlights the significant risks posed by the presence of heavy metals in food products and underscores the importance of stringent environmental monitoring and the adoption of appropriate regulatory mechanisms for health and environmental risk mitigation. This could help in the formulation of appropriate policy implementation strategies geared towards mitigating heavy metal contamination in the region’s food supply.

Although the results constantly showed heavy metal contamination among food samples (fish, fruits and vegetables), a plethora of variations in detection methods and research methodology may contribute to deviations in reported contamination levels. Peer-reviewed studies generally stuck to higher methodological standards, fortifying the reliability of their findings. In addition, the included articles were observed to have uneven geographic representation and there was inconsistent use of advanced detection techniques, which may have affected the generalization of the findings. In addition, the study relied on English-language publications and grey literatures were excluded, which may have omitted relevant data, thence giving room to potential selection bias.

Ethics and consent

Ethical approval and consent were not required.