Clinical Pharmacist and Master’s student in Clinical Pharmacy with research interests in pharmacovigilance, behavioral interventions in mental health, and AI applications in clinical decision support. Experience includes digital health research with Bloomsbury Health (London) and pharmacovigilance practice in patient support programs. Published work covers drug awareness among healthcare providers, postpartum depression management, and patient safety reporting. 
Clinical Pharmacist and Master’s student in Clinical Pharmacy with research interests in pharmacovigilance, behavioral interventions in mental health, and AI applications in clinical decision support. Experience includes digital health research with Bloomsbury Health (London) and pharmacovigilance practice in patient support programs. Published work covers drug awareness among healthcare providers, postpartum depression management, and patient safety reporting. What was reviewed?
This review synthesizes global evidence on heavy metal contamination in roadside vegetables, focusing on sources, exposure pathways, concentrations in edible crops, and health risk metrics relevant to certification and regulation. Vehicular emissions are parsed into tailpipe and non-tailpipe contributions, with metals such as Pb, Cd, Zn, Cu, Ni, Cr, As, Hg traced from road dust and aerosols into soils, plant foliar surfaces, and edible tissues. The review collates case studies from Africa, Asia, Europe, and South America, highlighting exceedances of WHO/FAO thresholds, hazard indices above unity, and carcinogenic risk estimates tied to cadmium and chromium. A mechanistic diagram and multi-country data tables ground the synthesis in observed concentrations, distance-decay patterns from road edges, and plant morphologies that enhance particulate capture.
Who was reviewed?
The reviewed corpus spans observational field studies of leafy and fruiting vegetables grown adjacent to highways and urban arterials, along with comparative market surveys of roadside versus retail produce. Geographies include Ghana’s Accra–Tema motorway, Johannesburg, urban districts in India, Nigeria, Libya, Brazil, Germany, Malaysia, Turkey, Pakistan, Bangladesh, and China. Populations implicitly affected are urban consumers, including children and pregnant women, exposed via diet, inhalation of resuspended particulates, and dermal contact; risk characterizations report elevated hazard indices and lifetime cancer risks where Cd and Cr exceed permissible limits. Methodologies comprise soil–plant transects by distance from roads, foliar versus root uptake analyses, and bioaccessibility-based health risk assessments. The review also details environmental covariates traffic volume, road surface, wind, rainfall, and soil pH, that modulate deposition and uptake.
Most important findings
| Critical point | Details |
|---|---|
| Extreme exceedances near highways | Ghana motorway vegetables: Cd 167.73–279.03 mg/kg; Pb 24.97–393.12 mg/kg; HI >10; elevated total carcinogenic risk for Cd and Cr. |
| Roadside market risk | Johannesburg roadside leafy vegetables showed Cd 51.47, Cr 74.31, Hg 2.45, Ni 198.71 mg/kg, with highest HI ≈11.77. |
| Distance buffers | Accumulation above background up to ~33 m; conservative exclusion ≥100 m on both sides of roads recommended. |
| Dominant sources | Tailpipe (Pb, Cd) and non-tailpipe brake/tire wear (Zn, Cu, Cd); concrete motorways and high speeds increase wear. |
| Key pathways | Foliar deposition and root uptake; dietary intake dominates human exposure; inhalation and dermal routes contribute. |
| Modulating factors | Johannesburg roadside leafy vegetables showed Cd 51.47, Cr 74.31, Hg 2.45, Ni 198.71 mg/kg, with the highest HI ≈11.77. |
| Visual evidence | As, Cd, Cr, Pb, Hg were identified as known/probable carcinogens relevant to certification thresholds. |
| Regulatory carcinogens | As, Cd, Cr, Pb, Hg identified as known/probable carcinogens relevant to certification thresholds. |
Key implications
For regulatory impacts, the evidence mandates exclusion zones for urban agriculture at least 100 m from high-traffic roads, with heavy metal contamination in roadside vegetables used as a sentinel metric. Certification requirements should set crop- and metal-specific action limits, require site history screening, soil pH management, and foliar deposition controls. Industry applications include audited sourcing away from corridors and periodic lab testing. Research gaps include co-pollutant mixtures and bioaccessibility. Practical recommendations emphasize routine surveillance and public advisories.
Citation
Boahen E. Heavy metal contamination in urban roadside vegetables: origins, exposure pathways, and health implications. Discover Environment. 2024;2:145. doi:10.1007/s44274-024-00182-7
Heavy metals are high-density elements that accumulate in the body and environment, disrupting biological processes. Lead, cadmium, arsenic, mercury, nickel, tin, aluminum, and chromium are of greatest concern due to persistence, bioaccumulation, and health risks, making them central to the HMTC program’s safety standards.
Lead is a neurotoxic heavy metal with no safe exposure level. It contaminates food, consumer goods and drinking water, causing cognitive deficits, birth defects and cardiovascular disease. HMTC’s rigorous lead testing applies ALARA principles to protect infants and consumers and to prepare brands for tightening regulations.
Cadmium is a persistent heavy metal that accumulates in kidneys and bones. Dietary sources include cereals, cocoa, shellfish and vegetables, while smokers and industrial workers receive higher exposures. Studies link cadmium to kidney dysfunction, bone fractures and cancer.
Nickel is a widely used transition metal found in alloys, batteries, and consumer products that also contaminates food and water. High exposure is linked to allergic contact dermatitis, organ toxicity, and developmental effects, with children often exceeding EFSA’s tolerable daily intake of 3 μg/kg bw. Emerging evidence shows nickel crosses the placenta, elevating risks of preterm birth and congenital heart defects, underscoring HMTC’s stricter limits to safeguard vulnerable populations.
Mercury (Hg) is a neurotoxic heavy metal found in various consumer products and environmental sources, making it a major public health concern. Its regulation is critical to protect vulnerable populations from long-term health effects, such as neurological impairment and cardiovascular disease. The HMTC program ensures that products meet the highest standards for mercury safety.
