Heavy metals in Seaweed

Seaweed overview

Inorganic arsenic decides most seaweed questions, and it depends far more on the type of seaweed than on where it was bought. Hijiki is the clear outlier: nine retail hijiki products in the United Kingdom averaged 77 mg/kg inorganic arsenic as sold, and WHO GEMS/Food hijiki submissions had a median of 47 mg/kg (10 results, 2010–2024).^{1Rose M, Lewis J, Langford N, Baxter M, Origgi S, Barber M, MacBain H, Thomas K · 2007Arsenic in seaweed—Forms, concentration and dietary exposureFood and Chemical ToxicologyRead source →2World Health Organization · 2026GEMS/Food Contaminants database heavy-metal exportsGEMS/Food Contamination Monitoring and Assessment ProgrammeRead source →} Nori, wakame and most kombu sold in Australia and Europe measured about 0.1–0.3 mg/kg, two to three orders of magnitude lower on the same dried-as-sold basis.^{3Food Standards Australia New Zealand · 2013Survey of Inorganic Arsenic in Seaweed and Seaweed-Containing Products Available in AustraliaFood Standards Australia New Zealand (FSANZ), Survey reportRead source →5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →}

Total arsenic is high in almost every seaweed but is mostly organic arsenosugars, so a total arsenic result cannot stand in for inorganic arsenic.^{5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →8García-Salgado S, Quijano MA, Bonilla MM · 2012Arsenic speciation in edible alga samples by microwave-assisted extraction and high performance liquid chromatography coupled to atomic fluorescence spectrometryAnalytica Chimica ActaRead source →9Taylor VF, Jackson BP · 2016Concentrations and speciation of arsenic in New England seaweed species harvested for food and agricultureChemosphereRead source →} Dried kombu in GEMS/Food had a median of 47 mg/kg total arsenic while the same dried-kombu category had a median of 0.23 mg/kg inorganic arsenic.^{2World Health Organization · 2026GEMS/Food Contaminants database heavy-metal exportsGEMS/Food Contamination Monitoring and Assessment ProgrammeRead source →} Two kelps break the pattern: Norwegian oarweed (Laminaria digitata) had a median of 21 mg/kg inorganic arsenic dry weight, against 0.037 mg/kg in tangle (Laminaria hyperborea) from the same monitoring programme.^{4Duinker A, Kleppe M, Fjare E, Biancarosa I, Heldal HE, Dahl L, Lunestad BT · 2020Knowledge update on macroalgae food and feed safetyRapport fra HavforskningenRead source →}

Cadmium is the second metal to watch and it ranks the types differently. In European monitoring, dried laver (nori) averaged 1.68 mg/kg cadmium and dried wakame 1.28 mg/kg, against 0.46 mg/kg in dried kombu, so the lowest inorganic arsenic types are not the lowest cadmium types.^{5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →} Lead and mercury are usually low: most dried-seaweed mercury results in GEMS/Food were below a 50 µg/kg detection limit, and dried nori lead had a median of about 0.14 mg/kg.^{2World Health Organization · 2026GEMS/Food Contaminants database heavy-metal exportsGEMS/Food Contamination Monitoring and Assessment ProgrammeRead source →}

For buyers, the supported priorities are to avoid hijiki or require an inorganic arsenic result for it, to specify the kelp species, and to ask for a cadmium result on nori, wakame and laver products.^{1Rose M, Lewis J, Langford N, Baxter M, Origgi S, Barber M, MacBain H, Thomas K · 2007Arsenic in seaweed—Forms, concentration and dietary exposureFood and Chemical ToxicologyRead source →4Duinker A, Kleppe M, Fjare E, Biancarosa I, Heldal HE, Dahl L, Lunestad BT · 2020Knowledge update on macroalgae food and feed safetyRapport fra HavforskningenRead source →5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →} Soaking and boiling remove a large share of arsenic from hijiki and other brown seaweeds into the water, which must then be discarded, but they leave a measurable residue and do little for cadmium.^{7Ichikawa S, Kamoshida M, Hanaoka K, Hamano M, Maitani T, Kaise T · 2006Decrease of arsenic in edible brown algae Hijikia fusiforme by the cooking processApplied Organometallic ChemistryRead source →10Laparra JM, Velez D, Montoro R, Barbera R, Farre R · 2003Estimation of Arsenic Bioaccessibility in Edible Seaweed by an in Vitro Digestion MethodJournal of Agricultural and Food ChemistryRead source →11Ownsworth E, Selby D, Ottley CJ, Unsworth E, Raab A, Feldmann J, Sproson AD, Kuroda J, Faidutti C, Bucker P · 2019Tracing the natural and anthropogenic influence on the trace elemental chemistry of estuarine macroalgae and the implications for human consumptionScience of the Total EnvironmentRead source →12Sund R, Holdt S L, Bang-Berthelsen C H, Stevant P, Sloth J J, Skipnes D · 2026Pulsed electric field processing as an alternative to warm water treatment for the reduction of potentially toxic elements in Saccharina latissimaJournal of Applied PhycologyRead source →}

Seaweed identity and scope

This page covers edible marine macroalgae: brown seaweeds such as kombu and other kelps (Laminaria, Saccharina), wakame (Undaria), hijiki (Sargassum fusiforme), arame and the wracks (Fucus, Ascophyllum); red seaweeds such as nori and laver (Porphyra, Pyropia), dulse and Irish moss; and green seaweeds such as sea lettuce (Ulva).^{4Duinker A, Kleppe M, Fjare E, Biancarosa I, Heldal HE, Dahl L, Lunestad BT · 2020Knowledge update on macroalgae food and feed safetyRapport fra HavforskningenRead source →5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →} Results stay separate by seaweed type because types differ systematically, and kelp species are named where the source names them.^{4Duinker A, Kleppe M, Fjare E, Biancarosa I, Heldal HE, Dahl L, Lunestad BT · 2020Knowledge update on macroalgae food and feed safetyRapport fra HavforskningenRead source →13Ronan JM, Stengel DB, Raab A, Feldmann J, O’Hea L, Bralatei E, McGovern E · 2017High proportions of inorganic arsenic in Laminaria digitata but not in Ascophyllum nodosum samples from IrelandChemosphereRead source →}

Spirulina, chlorella and other microalgae or cyanobacteria are not seaweed and are excluded, as are foods that only contain seaweed, such as sushi rolls, seaweed snacks mixed with other ingredients, dashi broth and seasoning salts, unless the seaweed itself was measured.^{2World Health Organization · 2026GEMS/Food Contaminants database heavy-metal exportsGEMS/Food Contamination Monitoring and Assessment ProgrammeRead source →5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →14Cheyns K, Demaegdt H, Waegeneers N, Ruttens A · 2021Intake of food supplements based on algae or cyanobacteria may pose a health risk due to elevated concentrations of arsenic speciesFood Additives & Contaminants: Part ARead source →} Kelp supplements made from dried seaweed are shown as their own form and are never pooled with seaweed foods.^{14Cheyns K, Demaegdt H, Waegeneers N, Ruttens A · 2021Intake of food supplements based on algae or cyanobacteria may pose a health risk due to elevated concentrations of arsenic speciesFood Additives & Contaminants: Part ARead source →15Nischwitz V, Pergantis SA · 2006Improved Arsenic Speciation Analysis for Extracts of Commercially Available Edible Marine Algae Using HPLC-ES-MS/MSJournal of Agricultural and Food Chemistry 54(18):6507-6519Read source →}

The basis matters as much as the type. Fresh seaweed is roughly 85% water, so a fresh-weight concentration is several times lower than a dry-weight one for the same plant, and Australia and New Zealand express their seaweed limit at 85% hydration.^{3Food Standards Australia New Zealand · 2013Survey of Inorganic Arsenic in Seaweed and Seaweed-Containing Products Available in AustraliaFood Standards Australia New Zealand (FSANZ), Survey reportRead source →16Food Standards Australia New Zealand · 2024Australia New Zealand Food Standards Code - Schedule 19 - Maximum levels of contaminants and natural toxicantsFederal Register of Legislation, Authorised Version F2024C00908Read source →} This page keeps dry-weight laboratory results, dried products as sold, fresh or wet results and prepared (soaked or cooked) results apart, and never converts between them without the measured moisture of that sample.^{1Rose M, Lewis J, Langford N, Baxter M, Origgi S, Barber M, MacBain H, Thomas K · 2007Arsenic in seaweed—Forms, concentration and dietary exposureFood and Chemical ToxicologyRead source →5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →}

Heavy metal contamination profile of seaweed

The table holds every seaweed result extracted from 78 sources: WHO GEMS/Food monitoring summarised by seaweed type, product form, basis and WHO region, agency reports and published surveys.^{2World Health Organization · 2026GEMS/Food Contaminants database heavy-metal exportsGEMS/Food Contamination Monitoring and Assessment ProgrammeRead source →3Food Standards Australia New Zealand · 2013Survey of Inorganic Arsenic in Seaweed and Seaweed-Containing Products Available in AustraliaFood Standards Australia New Zealand (FSANZ), Survey reportRead source →4Duinker A, Kleppe M, Fjare E, Biancarosa I, Heldal HE, Dahl L, Lunestad BT · 2020Knowledge update on macroalgae food and feed safetyRapport fra HavforskningenRead source →5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →} Each row is one statistic a source reports for one seaweed group and metal, so a mean, a median and individual samples appear as separate rows.^{5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →6Shaughnessy B K, Jackson B P, Byrnes J E K · 2023Evidence of elevated heavy metals concentrations in wild and farmed sugar kelp (Saccharina latissima) in New EnglandScientific ReportsRead source →17Hahn ME, Schultz IR, Ylitalo GM, Boyd D, Sloan CA, Niewolny LA, Bolton JL, Haigh T, Incardona JP · 2022Chemical contaminant levels in edible seaweeds of the Salish Sea and implications for their consumptionPLOS ONERead source →} Total arsenic, inorganic arsenic and dimethylarsinic acid are separate metals in the table, and other arsenic species are kept in the evidence records rather than shown as rows.^{5European Food Safety Authority (EFSA), Dujardin B, Ferreira de Sousa R, Gomez Ruiz JA · 2023Dietary exposure to heavy metals and iodine intake via consumption of seaweeds and halophytes in the European populationEFSA JournalRead source →8García-Salgado S, Quijano MA, Bonilla MM · 2012Arsenic speciation in edible alga samples by microwave-assisted extraction and high performance liquid chromatography coupled to atomic fluorescence spectrometryAnalytica Chimica ActaRead source →}

Ratings place a mean, median or single result against a pooled HMI seaweed reference for the same metal and basis, built from every compatible sampling family: GEMS/Food subgroup medians, individual sample values and published subgroup medians.^{2World Health Organization · 2026GEMS/Food Contaminants database heavy-metal exportsGEMS/Food Contamination Monitoring and Assessment ProgrammeRead source →4Duinker A, Kleppe M, Fjare E, Biancarosa I, Heldal HE, Dahl L, Lunestad BT · 2020Knowledge update on macroalgae food and feed safetyRapport fra HavforskningenRead source →6Shaughnessy B K, Jackson B P, Byrnes J E K · 2023Evidence of elevated heavy metals concentrations in wild and farmed sugar kelp (Saccharina latissima) in New EnglandScientific ReportsRead source →17Hahn ME, Schultz IR, Ylitalo GM, Boyd D, Sloan CA, Niewolny LA, Bolton JL, Haigh T, Incardona JP · 2022Chemical contaminant levels in edible seaweeds of the Salish Sea and implications for their consumptionPLOS ONERead source →} All seaweed types share one reference per basis, so a High rating for hijiki inorganic arsenic means high among seaweeds, not high against a safety threshold.^{1Rose M, Lewis J, Langford N, Baxter M, Origgi S, Barber M, MacBain H, Thomas K · 2007Arsenic in seaweed—Forms, concentration and dietary exposureFood and Chemical ToxicologyRead source →2World Health Organization · 2026GEMS/Food Contaminants database heavy-metal exportsGEMS/Food Contamination Monitoring and Assessment ProgrammeRead source →} Non-detects stay as intervals, and 54 rows whose values could not be reconciled with their own papers are shown with the reason and not compared.^{18Martín-León V, Paz S, D’Eufemia PA, Plasencia JJ, Sagratini G, Marcantoni G, Navarro-Romero M, Gutiérrez ÁJ, Hardisson A, Rubio-Armendáriz C · 2021Human Exposure to Toxic Metals (Cd, Pb, Hg) and Nitrates (NO3−) from Seaweed ConsumptionApplied SciencesRead source →19Sundhar S, Shakila RJ, Shalini R, Aanand S, Jayakumar N · 2023Effect of thermal processing on toxic heavy metals in edible seaweeds of Gulf of Mannar and their health risk assessmentRegional Studies in Marine ScienceRead source →20Khandaker MU, Chijioke NO, Heffny NAB, Bradley DA, Alsubaie A, Sulieman A, Faruque MRI, Sayyed MI, Al-mugren KS · 2021Elevated Concentrations of Metal(loids) in Seaweed and the Concomitant Exposure to HumansFoodsRead source →}