Heavy metal contamination in urban roadside vegetables: origins, exposure pathways, and health implications

Abstract Urban agriculture along major highways often exposes food crops to heavy metal contamination, primarily from vehicular emissions. This review examines the origins, exposure pathways, and health risks associated with heavy metals in roadside vegetables. Sources include both tailpipe emissions, such as lead (Pb) and cadmium (Cd), and non-tailpipe emissions from tyre and brake wear, which release zinc (Zn) and copper (Cu). Contamination occurs through atmospheric deposition on plant surfaces and root uptake from polluted soils. A systematic synthesis of global case studies reveals alarming levels of heavy metals, often exceeding WHO/FAO thresholds. For instance, vegetables cultivated along the Accra-Tema motorway in Ghana contained cadmium levels up to 279 mg/kg, posing significant health risks. Similarly, roadside vegetables in India, Nigeria, and China demonstrated bioaccumulation of toxic elements, with health implications including carcinogenic and non-carcinogenic effects. The review highlights the direct and indirect exposure pathways for humans, such as dietary intake of contaminated vegetables, inhalation of particulates, and dermal contact. Health consequences include gastrointestinal distress, neurotoxicity, reproductive issues, and elevated cancer risks. To mitigate these risks, the review advocates for the prohibition of vegetable farming near high-traffic highways, along with the implementation of stringent monitoring programs to assess and control heavy metal levels. Future research should address ecological impacts and integrate diverse contamination sources to develop effective risk management strategies. This work is a critical resource for policymakers, researchers, and urban planners aiming to enhance food safety and public health in urban settings.