Human Exposure to Toxic Metals (Cd, Pb, Hg) and Nitrates (NO3−) from Seaweed Consumption

Martín-León and colleagues (2021) measured cadmium, lead, total mercury, and nitrate in 72 edible algae samples marketed in Tenerife (Canary Islands, Spain), partitioning species by declared geographic origin into an Asian-sourced group (Japan, Korea) and a European-sourced group (Galicia, Spain). Metals were quantified by graphite-furnace and cold-vapor atomic absorption spectrophotometry after microwave acid digestion; nitrate by ion chromatography. Cadmium concentrations were notably higher in Asian-sourced algae, with hijiki (1.196 mg/kg dry weight) and nori (1.005 mg/kg dw) the highest species means in the dataset. Lead was highest in Asian-sourced wakame (0.119 mg/kg dw). Total mercury was highest in European-sourced arame (0.055 mg/kg dw). Nitrate concentrations exceeded 8000 mg/kg in some nori samples and reached a maximum of 6400 mg/kg in kombu; mean nitrate in kombu (3084 mg/kg) and nori (3183 mg/kg) was within the 6000–7000 mg/kg range that EU Regulation 1258/2011 sets as the maximum for rocket/arugula, the leafy vegetable category nearest to seaweed for nitrate regulatory comparison (seaweed itself is not regulated for nitrate). The authors compute estimated daily intake at the manufacturer’s recommended 4 g/day dry-seaweed consumption and a mean Spanish adult body weight of 68.48 kg; under those assumptions, no analyte exceeds its EFSA tolerable weekly intake or benchmark dose for adult consumers, with the highest contributions coming from Asian hijiki Cd (19.6% of TWI) and Asian nori Cd (16.4% of TWI).

Key numbers

Concentrations reported as mean ± SD in mg/kg dry weight. Estimated daily intake (EDI) computed at 4 g/day dry-seaweed consumption and 68.48 kg body weight. LOQs of the analytical method: Cd 0.020, Pb 0.040, Hg 0.10 mg/kg.

Cadmium (Table 4):

Lead (Table 5); % BMDL columns reference the EFSA Pb BMDL01 of 0.63 µg/kg bw/day (nephrotoxicity, “Np”) and 1.5 µg/kg bw/day (cardiovascular, “Card”):

Total mercury (Table 6); the paper measured total Hg by cold-vapor AAS and did not perform speciation. The two %TWI columns in Table 6 are not separate Hg measurements — they apply the same measured total-Hg concentration against the EFSA TWI for methylmercury (1.3 µg/kg bw/week, “Org”) and against the EFSA TWI for inorganic mercury (4 µg/kg bw/week, “Inorg”):

Nitrate (Table 7); LOQ 500 mg/kg; %ADI references EFSA/WHO ADI of 3.7 mg/kg bw/day:

Aggregate findings reported in the discussion (§3.1):

• Asia versus Europe contrast was statistically significant (Mann–Whitney, p < 0.05) for all three metals; concentrations were higher in Asian-sourced algae across Cd, Pb, and Hg, with the species-level exception that mean Hg was highest in European arame.
• Margin of Exposure (MoE) values reported by the authors, at 4 g/day dry-seaweed consumption: Cd MoE 2.1 for hijiki and 2.5 for nori, against a NOAEL of 0.01 mg/kg bw/day; Pb MoE 3.15 for wakame consumption against a BMDL01 of 0.0015 mg/kg bw/day; Hg MoE 182 for arame consumption against a methylmercury NOAEL of 0.04 mg/kg bw/day.
• Some kombu samples (max 6400 mg/kg) and some nori samples (max >8000 mg/kg) exceed the EU rocket/arugula nitrate ceiling (6000–7000 mg/kg, Regulation 1258/2011), the leafy-vegetable category nearest to seaweed for regulatory comparison; seaweed itself is unregulated for nitrate at EU level.

Methods (brief)

Seventy-two algae samples (one gram per sample) were microwave-digested in Teflon vessels (Anton Paar Multiwave GO Plus) using 4 mL 65% HNO₃ + 2 mL H₂O₂, with a five-step ramp ending at 180 °C (temperature limit 200 °C). Three replicates per sample. Digests were brought to 10 mL with Milli-Q water.

Cadmium and lead were quantified by graphite-furnace atomic absorption spectrophotometry (PerkinElmer AS-800 with HGA-800 graphite chamber) at 228.8 nm (Cd) and 283.3 nm (Pb). Total mercury was quantified by cold-vapor atomic absorption spectrophotometry (PerkinElmer AS-800 with FIMS-400 flow injection) at 253.7 nm. The method follows Commission Regulation 333/2007/EC (later modified by 836/2011). Instrumental LOQs: Cd 0.020 mg/kg, Pb 0.040 mg/kg, Hg 0.10 mg/kg. Quality control used certified reference materials BCR-279 (Sea Lettuce, British Certified Reference) for Cd and Pb, and NIST SRM 1577 BL and BCR-278 R MT for Hg. Recoveries were greater than 95%.

Nitrates were determined at the Public Health Laboratory of Las Palmas de Gran Canaria following EN 12014-4:2005, accredited to ISO/IEC 17025 by the Spanish National Accreditation Entity (ENAC). 2.5 ± 0.2 g of homogenized sample was extracted in ultrapure water at 70 °C with saturated sodium tetraborate, boiled 15 min, centrifuged, and cleaned on C18 cartridges preconditioned with methanol. Quantification was by ion chromatography on a Waters 2695 system with a PDA detector (210 nm), anion IC-Pak column (50 mm × 4.6 mm ID, 10 µm), phosphate-buffer/ultrapure-water mobile phases in isocratic mode at 1.0 mL/min, 25 °C. Nitrate LOQ 500 mg/kg; relative measurement uncertainty 14%; recoveries 96.7–105.4% across 500–8000 mg/kg.

Statistical analysis used GraphPad 8.4.3; normality was assessed (Anderson–Darling, D’Agostino–Pearson, Shapiro–Wilk, Kolmogorov–Smirnov), and where data were non-normal, the Mann–Whitney two-tailed test was applied with α = 0.05.

Speciation: total Hg only (no methylmercury / inorganic mercury speciation performed). The “Org” and “Inorg” columns of Table 6 are not separate measurements — they are the same measured total-Hg value compared against two different EFSA TWIs. Arsenic was not measured. Chromium speciation not applicable.

Dietary-intake assumptions: dry-seaweed consumption of 4 g/day (the manufacturer’s recommended portion stated on the packaging); Spanish adult mean body weight 68.48 kg (AECOSAN 2006).

Implications

The source contributes a 72-sample, ten-species, two-origin occurrence dataset for Cd, Pb, total Hg, and nitrate in edible algae marketed in a European retail channel. The Asia-versus-Europe contrast — Asian-sourced hijiki, nori, kombu, and wakame consistently showing higher Cd, Pb, and Hg than the same or related species sourced from Galicia — is internally significant at p < 0.05 and is consistent across all three metals (with the species-level exception of European arame for Hg). The species panel is comparatively wide for European-marketed seaweed studies, covering brown algae (Laminaria, Undaria, Sargassum, Eisenia, Himanthalia), red algae (Porphyra, Gracilaria, Mastocarpus), and green algae (Ulva).

For seaweed-kelp-foods occurrence routing, the species-level means reported here populate dry-weight Cd, Pb, and total-Hg distributions for hijiki, nori, kombu, wakame, arame, sea spaghetti, sea lettuce, starry moss, and mixed salad. The nitrate data populate the same occurrence channel for kombu, nori, sea lettuce, and mixed salad. The paper itself does not perform Hg speciation, so its Hg values flow into total-Hg distributions only; the methylmercury column in Table 6 is a reference-value comparison, not a measurement.

The 4 g/day exposure assumption is the manufacturer’s recommended portion size for dry seaweed and is not necessarily reflective of consumer practice across all species — the authors note global-diet Cd intake may be considerably higher than the seaweed-only EDI under different consumption assumptions.

Verification notes

• Fresh ingest 2026-06-04 from KADC manual-fetch folder 05_Kelp_Seaweed_Canned. No prior wiki source page existed for this DOI, raw_handle, or cite-key (three identity checks clean).
• License: CC BY 4.0 per the MDPI Applied Sciences publisher boilerplate on page 1 of the PDF. DOI 10.3390/app11156934 verified against the publisher footer.
• Speciation discipline (CLAUDE.md Part 14): the paper measures total Hg by CV-AAS and reports it as a single concentration value per species. Table 6’s two %TWI columns (“Hg (Org.)” and “Hg (Inorg.)”) apply the same total-Hg measurement against two different EFSA TWIs; they are not separate methylmercury / inorganic-Hg measurements. Frontmatter metals: therefore records tHg, not MeHg. The body table preserves the source’s column labels and adds an explicit note that both columns derive from the same total-Hg measurement.
• Arsenic was not measured in this study; frontmatter omits both tAs and iAs.
• Brand firewall (Part 12): no commercial brand names attached to contamination values appear in the paper’s results tables. The instrument vendors (PerkinElmer, Anton Paar, Waters), reference-material providers (BCR, NIST), and statistical software (GraphPad) named in the methods section are scientific-reproducibility vendor names, permitted under Exception 2 (Part 12 strict reading, 2026-05-17).
• Wiki/HMTc firewall (Part 2): the paper’s MoE and %TWI/%BMDL/%ADI calculations are reported as the authors compute them (against EFSA reference values), not converted into HMTc threshold suggestions. The Implications section describes occurrence-channel contributions to seaweed-kelp-foods, not HMTc thresholds.
• Matrices: edible-seaweed covers the full species panel; macroalgae covers the marine algae taxon; kelp is included because Laminaria ochroleuca (kombu) is a kelp species; dry-weight records the reporting basis the paper uses across Tables 4–6. Nitrate data in Table 7 are reported on the same dry-weight basis as the metals tables (the paper does not switch basis).
• Jurisdictions: ES (sale and analysis in Tenerife, Canary Islands), EU (Galicia/Spain origin for the European subset), JP and KR (declared origin for the Asian-sourced subset). Geographic origin “Unknown” rows in Table 1 are not separately routed.
• Reference-comparison reasoning for the nitrate ceiling: the paper itself cites EU Regulation 1258/2011 modifying 1881/2006 (rocket/arugula 6000–7000 mg/kg) as the only EU nitrate ceiling potentially comparable to seaweed. The wiki page reports the comparison as the authors describe it; it does not assert that the arugula ceiling applies to seaweed.
• EDI values in Tables 4–6 are reproduced exactly as the paper reports them. Unit note: EDI is given in µg/day for metals and mg/day for nitrate, consistent with the source tables.
• BMDL values: the paper cites the EFSA Pb BMDL01 of 0.63 µg/kg bw/day (nephrotoxicity) and 1.5 µg/kg bw/day (cardiovascular). The body table’s ”% BMDL Np” and ”% BMDL Card” columns label these explicitly.
• Audit subagent (2026-06-04, verdict PROMOTE) ran all five audit-prompt checks and found all numerical fidelity (Tables 4–7 and §3.1 MoE narratives), speciation/methods, brand firewall, and wiki/HMTc firewall checks ✅ clean. One ⚠️ on Check 2 flagged dry-weight as a reporting basis rather than a matrix; verified against corpus convention — dry-weight is in active use as a matrices value across the existing seaweed source pages (e.g., mosusu2023-edible-seaweeds-japan-metals.md, aziz2021-gracilaria-seaweed-bangladesh.md). The basis-vs-matrix critique is structurally valid but reflects established Heavy Metal Index convention rather than a defect of this page. No correction applied; logged as false positive against current corpus convention.

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