Guide · Published 26 October 2026

Iodine and heavy metals in seaweed, kelp and sea moss supplements

In short: Yes: kelp and sea moss can carry lead, cadmium, mercury and arsenic, but in the lab reports we checked all measured lead, cadmium and mercury were within EU limits. The bigger risk with kelp is iodine: many capsules declare 150–250 µg a day, above the 100 µg per daily dose that Germany’s BfR advises. Look for a batch report that gives metal values and, ideally, inorganic arsenic and measured iodine.

Seaweed supplements need two checks, not one. Like any supplement, they can carry lead, cadmium, mercury and arsenic. But seaweed also concentrates iodine, and the iodine dose itself can be the bigger risk. This guide explains both, using official limits and studies we re-read in full on 26 October 2026. It contains no shop or affiliate links.

Why seaweed is different

Seaweeds take up iodine and metals from seawater. Brown seaweeds such as kelp (Laminaria) and knotted wrack (Ascophyllum nodosum, often sold as “Norwegian kelp”) are the usual base of “natural iodine” capsules. Sea moss (Irish moss or “gold” sea moss) is a red seaweed, usually sold as a gel.

Iodine content varies hugely between species and batches, so a label dose is only as good as the batch testing behind it. Arsenic in seaweed is mostly in organic forms; the inorganic form is the more toxic one, so a total-arsenic figure on its own says little.

The numbers that matter for iodine

EU reference intake (NRV): 150 µg a day for adults, the figure behind “% NRV” on EU and UK labels.
Germany’s Federal Institute for Risk Assessment (BfR), December 2025: no more than 100 µg iodine per recommended daily dose in a supplement for everyone from age 15, and up to 150 µg for pregnant and breastfeeding women. This is a recommendation, not an EU law.
Tolerable upper intake for adults: 600 µg a day from all sources (EU Scientific Committee on Food, as cited by BfR); 500 µg for 15–17-year-olds.
Most kelp capsules sold in the EU and UK declare 100–250 µg per capsule. A product at 150 µg already covers the whole NRV before food and iodised salt are counted.

The numbers that matter for heavy metals

EU Regulation 2023/915 sets maximum levels for food supplements: lead 3.0, cadmium 1.0 and mercury 0.10 mg/kg. For supplements made mainly (at least 80%) of dried seaweed, the cadmium limit is 3.0 mg/kg. There is no EU limit for arsenic in supplements.

A good seaweed lab report therefore gives lead, cadmium, mercury, total arsenic and, ideally, inorganic arsenic as numbers for a named batch, plus a measured iodine value per daily dose.

What independent testing has found

ConsumerLab, kelp supplements (USA/Canada, April 2017) [1]

ConsumerLab bought six popular kelp supplements sold in the US and Canada. Half contained about twice the iodine on their labels: 530 to 960 µg per daily serving in the three that failed, against the US FDA’s position that a kelp supplement should give no more than 225 µg a day. One of those products was also contaminated with arsenic.

Its press release does not say which named products failed, so we don’t name any. These were North American products, not EU or UK ones.

Madrid public-health laboratory, 33 dried seaweed foods (2021) [2]

Scientists from the Laboratorio de Salud Pública de Madrid Salud analysed 33 dried seaweed products bought in Madrid markets. These were foods (kelp, kombu, hijiki, nori, wakame and others), not supplements. Metals and arsenic were measured by ICP-MS in an ISO/IEC 17025-accredited lab; iodine was not yet in its accredited scope.

Assuming a 5 g portion and a 60 kg adult: total arsenic ranged from 0.4 to 100 mg/kg, but inorganic arsenic was below 0.10 mg/kg in nori, kelp and sea spaghetti. Hijiki averaged 62 mg/kg inorganic arsenic, about 70% of its total. Some nori and wakame portions gave up to 70% of the tolerable cadmium intake, and some kelp, wakame and dulse 15–23% for lead.

Iodine was the biggest issue: a 5 g portion of kelp or kombu exceeded the tolerable daily iodine intake by about 3,000% and 4,000%. The authors suggest no more than 1 g a week of kelp or kombu and 2.5 g a week of hijiki. Sample numbers were small (two kelp, six kombu, three hijiki).

CVUA Stuttgart, 51 seaweed products (Germany, 2018–2023) [3]

Germany’s Stuttgart food-control lab tested iodine in 51 seaweed products, mostly dried nori sheets but also wakame, arame and others, mainly from Korea and China. Iodine ranged from 10 to 5,800 mg/kg; nearly 80% were above the 20 mg/kg level at which BfR advises that dried seaweed products carry a warning and a maximum daily amount.

For three products (Alaria, arame and kombu), one stated portion gave more than 500 µg iodine. 16 of the 51 had labelling faults, such as a missing iodine warning or daily amount.

How to check a seaweed product yourself

Is the lab named, independent and accredited (ISO/IEC 17025), and are the metals inside its accredited scope?
Does the report name the batch, and is it under about two years old?
Are lead, cadmium, mercury and arsenic given as numbers with units (mg/kg), not just “pass”? Is inorganic arsenic measured?
Was iodine measured, and how close is it to the label? Is the daily dose above 100 µg (BfR) or, with your diet, near 600 µg?
Does the label give an iodine amount and a maximum daily intake? Be wary of disease or “92 minerals” style claims: seaweed and sea moss have no authorised health claims of their own.

What the reports in our directory show

We list six seaweed products with a recent independent lab report: three with full numeric results, and three labelled “Partial report” (two pass/fail certificates and one sea moss gel report with no batch number). All results we could read were within the EU limits. Only one report measured iodine: 218 µg per capsule against 150 µg declared. Four of the five kelp products declare at least 150 µg per capsule, and two declare 200 µg or more, above BfR’s 100 µg advice.

See the full seaweed comparison: metals, iodine and price per day →

FAQ

Are kelp supplements safe?

Lab reports we checked show lead, cadmium and mercury within EU limits, but iodine is the main risk. BfR recommends no more than 100 µg iodine per daily supplement dose (150 µg in pregnancy and breastfeeding), and many kelp capsules declare 150–250 µg. If you have a thyroid condition, are pregnant or take thyroid medicine, ask a doctor or pharmacist first. This is not medical advice.

Is arsenic in seaweed dangerous?

Seaweed often has high total arsenic, but mostly in organic forms. Inorganic arsenic is the more toxic form. In the Madrid study it was below 0.10 mg/kg in kelp but averaged 62 mg/kg in hijiki. Look for a report that measures inorganic arsenic.

Does sea moss gel contain iodine?

Sea moss is a seaweed, so it contains some iodine, but the sea moss gel report we found did not measure iodine and the label gives no daily serving, so we can’t say how much.

Did the Madrid study test supplements?

No. It tested 33 dried seaweed foods bought in Madrid markets. We cite it for how much iodine, cadmium, lead and inorganic arsenic seaweeds can carry, not as a test of supplements.

Is there a lab-tested sea moss in the UK?

Only one product in our directory has a recent report we could read: Mighton’s Plain Sea Moss Gel (UK), with lead 0.07, cadmium 0.72, mercury 0.01 and total arsenic 5.78 mg/kg. It is a partial report: no batch number and the metal tests were subcontracted outside the lab’s UKAS scope. No sea moss is lab verified in our directory yet.

Do spirulina and chlorella have heavy metals?

They can. Spirulina and chlorella are microalgae, not seaweed, and are compared separately: six powders with batch reports, all within EU limits for lead, cadmium and mercury.

Sources

Not medical advice. CleanAssay does not test products; we read and link independent lab reports. This guide has no shop or affiliate links.