Oduro et al. 2023 — Heavy metals in cereal-based breakfast meals, Kumasi, Ghana
This study measured total concentrations of As, Cd, Cr, Mn, Ni, and Pb in 54 commercially sold cereal-based breakfast products (wheat, rice, maize, oats, millet and sorghum) from markets in the Kumasi Metropolis of Ghana, and assessed carcinogenic and non-carcinogenic health risks across six age groups using estimated dietary intake (EDI) and Monte Carlo simulation. All six metals exceeded FAO/WHO permissible limits for cereal-based foods in every cereal class tested. In-vitro bioaccessibility extracts were below detection limits for all metals, suggesting that the total-metal concentrations represent a worst-case exposure estimate; relative bioavailability of 1 was assumed for risk calculations. Cadmium and nickel hazard quotients exceeded 1 for all age groups, with toddlers bearing the highest burden, while cancer risk values for As and Cd exceeded the de minimis threshold (10⁻⁶) across all age groups.
Key numbers
Total metal concentrations (mg/kg dry weight, mean ± SD across five cereal classes):
| Metal | Wheat | Rice | Millet & Sorghum | Maize | Oats | FAO/WHO max |
|---|---|---|---|---|---|---|
| As | 0.63 ± 0.04 | 0.64 ± 0.02 | 0.69 ± 0.08 | 0.65 ± 0.09 | 0.59 ± 0.03 | 0.2 mg/kg |
| Cd | 1.29 ± 0.07 | 1.32 ± 0.05 | 1.41 ± 0.13 | 1.30 ± 0.16 | 1.27 ± 0.08 | 0.1 mg/kg |
| Cr | 5.29 ± 0.27 | 5.06 ± 0.23 | 9.85 ± 10.70 | 4.66 ± 2.08 | 5.41 ± 0.26 | 1 mg/kg (China standard) |
| Mn | 8.30 ± 0.51 | 8.57 ± 0.69 | 9.73 ± 1.51 | 8.43 ± 1.68 | 8.26 ± 0.82 | — |
| Ni | 5.15 ± 0.42 | 5.22 ± 0.24 | 5.81 ± 1.02 | 5.22 ± 0.87 | 5.01 ± 0.29 | — |
| Pb | 0.85 ± 0.05 | 0.85 ± 0.02 | 0.92 ± 0.10 | 0.87 ± 0.11 | 0.83 ± 0.03 | 0.2 mg/kg |
Note: All concentrations are reported in mg/kg (= ppm); to convert to ppb (µg/kg), multiply by 1,000. As values reported are total As (tAs), not speciated; no iAs/organic As split was performed.
Bioaccessibility: In-vitro extracts below detection limits: As < 1.7 µg/L, Cd < 2.2 µg/L, Ni < 2 µg/L, Pb < 1.4 µg/L.
Estimated dietary intake (EDI), toddlers (highest exposure group):
- As: 6.55 × 10⁻³ mg/kg bw/day (millet and sorghum)
- Cd: 13.59 × 10⁻³ mg/kg bw/day (wheat, highest); ranges 12.14–13.43 × 10⁻³ across cereals
- Cr: 50.38–51.61 × 10⁻³ mg/kg bw/day
- Mn: 78.57–84.8 × 10⁻³ mg/kg bw/day
- Ni: 47.83–55.40 × 10⁻³ mg/kg bw/day
- Pb: 7.94–8.77 × 10⁻³ mg/kg bw/day
Hazard quotients (all cereal classes, selected age groups):
- Cd HQ: 4.10–5.81 (infants), 4.18–5.70 (toddlers); all age groups HQ > 1
- Ni HQ: 0.43–0.93 (infants); all below 1 in some calculations, but declared a concern
- Cr HQ: 0.01–0.03 (all age groups); low non-carcinogenic risk
- Mn HQ: 0.17–0.30 (infants), below 1 for all age groups
- Pb HQ: below 1 for all age groups
Cancer risk:
- As CR (toddlers): 8.90–9.82 × 10⁻³ (above de minimis 10⁻⁶)
- Cd CR (toddlers): 185.40–185.43 × 10⁻³ (above de minimis)
- Pb CR (all ages): 6.31–7.46 × 10⁻⁵ (above de minimis 10⁻⁶ but below USEPA threshold for unacceptable risk)
Methods (brief)
Analytical methods: XRF (Niton XL3t GOLDD+, calibrated with NIST 2711) for screening; ICP-MS after acid microwave digestion (USEPA Method 3052; EPA Method 200.8) for quantification. In-vitro bioaccessibility used USEPA SOP glycine extraction at pH 1.5, 37°C, 1 hour, filtered through 0.45 µm, quantified by ICP-MS. Recovery of NIST 1575a reference material: As 92 ± 11%, Cd 85 ± 5%, Cr 91 ± 16%, Mn 90 ± 9%, Ni 98 ± 6%, Pb 99 ± 6%. Method detection limits: As 0.5 mg/kg to Mn 3.4 mg/kg. No arsenic speciation performed; all As values are total arsenic. Health risk calculations used Monte Carlo simulation (Palisade @Risk) and USEPA oral reference doses.
Implications
Certification: Cd concentrations in all five cereal classes (1.27–1.41 mg/kg = 1,270–1,410 ppb) are 13–14× the Codex/EU 0.1 mg/kg limit for cereals, and 6.4–7× the 0.2 mg/kg limit that applies under some frameworks. Any HMT&C certification threshold for cereals from West African sourcing regions must account for these dramatically elevated levels. Pb values (0.83–0.92 mg/kg = 830–920 ppb) similarly exceed the 0.2 mg/kg standard by 4–5×. These are processed market products (not raw grain), suggesting that milling or packaging may contribute additional contamination beyond field levels, though PCA indicates soil as the primary source for most metals. Millet and sorghum consistently show the highest concentrations for all metals; this sub-commodity warrants its own risk tier.
Courses: Illustrates the geographic and agricultural-context dependence of heavy metal contamination in cereals. The Kumasi data (all metals above permissible limits, Cd 13× over) contrasts sharply with European surveillance data, demonstrating that region of origin is a fundamental sourcing variable. The study also demonstrates the gap between total-metal concentrations and actual bioavailable exposure — the bioaccessibility findings (all extracts below LOD) complicate the risk picture and warrant class discussion.
App: tAs concentrations of 590–690 ppb (0.59–0.69 mg/kg) for cereals from Ghana are substantially above what US-market data typically show; the app’s contamination profiles for wheat, rice, maize, and oats should flag geographic origin as a high-variance driver. The absence of speciation is a limitation: these tAs values cannot be directly compared to iAs-specific regulatory thresholds. For Cd, the 1,270–1,410 ppb range is the highest multi-cereal survey value in the current corpus.