HEAVY METAL CONTAMINATION AND ASSOCIATED HUMAN HEALTH RISKS IN DRINKING WATER, SOIL, AND EDIBLE VEGETABLES IN NIGERIA: A SYSTEMATIC REVIEW

Abstract

This systematic review examined heavy metal contamination in drinking water, soil, and edible vegetables in Nigeria, along with the associated human health risks. Heavy metal pollution from mining, oil exploration, and wastewater irrigation poses serious threats to food safety and public health; however, comprehensive synthesis of contamination levels and risk assessments across the country remains limited. This systematic review, guided by PRISMA 2020, assessed contamination patterns and health risks across Nigeria. Comprehensive searches were conducted in PubMed, Scopus, Web of Science, ScienceDirect, African Journals Online (AJOL), and Google Scholar for studies published between 1 January 2011 and 30 April 2026. From 1,248 records, 18 unique studies (derived from 21 references after duplicate removal) were included. These comprised cross-sectional surveys and contamination analyses reporting quantitative heavy metal concentrations in water, soil, and/or edible vegetables together with standard health risk indices (EDI, THQ, HI, TLCR, BAF, or TF).Findings revealed widespread exceedance of FAO/WHO permissible limits, particularly for cadmium and lead in leafy vegetables such as spinach, lettuce, cabbage, and amaranth. Hazard indices (HI) exceeded 1 in the majority of studies, while lifetime cancer risks (TLCR) for arsenic frequently surpassed the acceptable threshold of 10⁻⁴, especially in northern mining zones (Kano, Jos, Kaduna) and the Niger Delta. Nigerian studies showed higher exceedance rates (Pb: 83%, Cd: 78%) and elevated risk levels compared with some other West African contexts. Leafy vegetables consistently demonstrated the highest bioaccumulation. Interventions and monitoring efforts remain limited, and systemic challenges such as unregulated irrigation and weak enforcement of environmental standards continue to exacerbate exposure risks, particularly for children and frequent consumers of locally grown produce. Urgent, context-specific interventions are required, including stricter regulation of irrigation water and soil quality, targeted remediation in high-risk zones, safer agricultural practices, and enhanced public awareness. These measures are essential to reduce dietary heavy metal exposure, protect public health, and support sustainable food safety in Nigeria.