Adhikari & Struwig 2024 — Heavy metal concentrations in leafy vegetables from roadside markets vs. large stores in Johannesburg
This study compared concentrations of ten elements (Al, As, Cd, Co, Cr, Cu, Hg, Ni, Pb, Zn) in five leafy vegetables (coriander, lettuce, mint, spring onion, swiss chard) from roadside open-air markets (unwashed and washed) versus supermarkets and vegetable markets in Johannesburg, South Africa, and assessed associated human health risks. Unwashed open-air-market leaves showed the highest concentrations of all elements, with Cd, Hg, Ni, and Pb being significantly elevated relative to washed samples, reflecting anthropogenic deposition rather than soil uptake alone. Hazard index exceeded 1 in all sample groups (OMUW: 11.77; OMW: 1.83; SMW: 1.29; VMW: 1.01), while cancer risk for Cd and As exceeded 1×10⁻⁶ in both washed and unwashed open-air-market leaves. Thorough washing reduced non-carcinogenic risk by 84% and cancer risk by 74–87%.
Key numbers
- n = 20 composite samples per group (OMUW, OMW, SMW, VMW) across five vegetable types and four market types
- OMUW (unwashed roadside market): highest concentrations across all elements; Cd, Hg, Ni, Pb significantly higher than all washed groups (p < 0.05)
- Hazard quotient above safe threshold (HQ > 1) for Cd, Cr, As, Ni only in OMUW group
- Hazard index: OMUW 11.77 > OMW 1.83 > SMW 1.29 > VMW 1.01
- Cancer risk for Cd and As: > 1×10⁻⁶ in both washed and unwashed open-air-market leaves
- Washing reduced non-carcinogenic risk 84%; cancer risk 74–87%
- Elemental order by mean concentration in OMUW: Al > Zn > Ni > Cr > Cd > Cu > Co > As > Hg > Pb
- ICP-MS analysis; multivariate analysis indicated anthropogenic origin for most elements in open-air-market samples
Methods (brief)
ICP-MS for all ten elements; composite samples from five vegetable species across four market categories (roadside open-air markets unwashed and washed, supermarkets washed, vegetable markets washed); bi- and multivariate source analysis. Standard reference materials used for QC. Location: Johannesburg, South Africa (Witwatersrand industrial/gold mining region). Published in Environmental Monitoring and Assessment.
Implications
Certification: Leafy vegetables sourced from open-air urban markets in mining-adjacent areas carry elevated heavy metal deposition loads not fully removed by ordinary washing. Supply-chain screening for leafy vegetables should include market/sourcing type as a risk factor. Courses: Demonstrates the importance of deposition as a contamination pathway distinct from soil uptake; washing efficacy data is directly pedagogically useful. App: Leafy-vegetable risk profiles for urban-sourced versus large-store produce differ materially; sourcing context modifies contamination profile. Microbiome: Not applicable.