Heavy Metal Contamination of African Leafy Vegetables Grown in Urban and Peri-Urban Agricultural Systems- A Review

Abstract

Urban and peri-urban agriculture is the major source of the fresh produce sold in African cities, including indigenous African leafy vegetables such as Solanum nigrum, Cleome gynandra, and Vigna unguiculata. Many urban and peri-urban agricultural activities are practiced close to wastewater sources, industrial activities, major roads, and waste dumps, exposing the vegetables to cadmium, lead, chromium, nickel, copper, zinc, arsenic, and mercury contamination. Reported concentrations of heavy metals vary widely with irrigation practice, soil characteristics, contamination source, and vegetable species. Cadmium and lead are the metals most frequently reported to exceed food safety limits. Additionally, chromium and zinc have also been noted to exceed safe limits in wastewater-irrigated and roadside-cultivated vegetables. This review examines the soil and plant factors governing metal uptake and evaluates human health risks using Target Hazard Quotient, Hazard Index, and Lifetime Cancer Risk approaches. This study highlights the significant risks posed by the presence of heavy metals in vegetables and emphasizes the importance of stringent environmental monitoring and the implementation of appropriate regulatory mechanisms for human health and environmental risk mitigation. Comparability across studies remains limited by inconsistent health-risk-assessment parameters and uneven geographic and species coverage, with several widely consumed African leafy vegetable species and West, Central, and North African countries remaining underrepresented in the reviewed literature. The findings could help formulate appropriate policy implementation strategies aimed at mitigating heavy metal contamination in Africa’s food supply. However, further research is needed to cover underutilized vegetable species. Moreover, studies from more countries should be included in future research.