1Which seaweed is in the jar

"Sea moss" is a trade name, not a species. Three red seaweeds are sold under it.

• Chondrus crispus, also called Irish moss or carrageen moss. It grows on rocky shores on all British and Irish coasts and on both sides of the North Atlantic. In Ireland and Scotland it is gathered by hand for food: the Irish harvest of carrageen (Chondrus crispus and Mastocarpus stellatus together) was 37.5 wet tonnes in 2020, and Scottish hand-harvesting sites are recorded in Fife, Caithness, Loch Sunart, Ardnamurchan and Bute. The carrageenan industry takes its Chondrus mainly from Canada, the north-eastern United States and Brittany. It was the original source of carrageenan, the gelling substance that makes a sea moss gel set, and it is still used for that to a limited extent.
• Kappaphycus alvarezii, sold in the trade as "cottonii" and still labelled Eucheuma cottonii on many products. It is farmed in the Philippines, Indonesia and Tanzania. Its carrageenan is mainly the kappa type. Its relative Eucheuma denticulatum, "spinosum", is farmed in the same countries and gives mainly iota-carrageenan.
• Gracilaria. In St Lucia, "seamoss" has meant Gracilaria since at least the 1980s: six species were used, and four were farmed in trials. Gracilaria is a source of agar, not of carrageenan.

Which one is in a given jar is usually not stated on the label. The US Food and Drug Administration analysed 46 seaweed products bought in the United States and grouped them into 13 species by DNA. The red seaweeds in that set included "Elkhorne sea moss" (Kappaphycus alvarezii), a Eucheuma species and a Gracilaria species. We found no published study that identifies retail sea moss products species by species, so we cannot say how often a label and its contents agree.

The EU Novel Food Status Catalogue, read on 18 September 2026, records the three seaweeds differently:

• Chondrus crispus: "not novel in food".
• Eucheuma denticulatum and Gracilaria gracilis: "not novel in food supplements". For these entries the catalogue adds that other food uses "might be considered to be novel" and might need authorisation before the product can be sold as a food in the EU. Other Gracilaria entries carry other statuses, so for Gracilaria the species matters, not only the genus.
• Kappaphycus alvarezii, also searched as Eucheuma cottonii and by genus: no entry. The catalogue has no record either way.

Every entry ends with the same note: other legislation may restrict the placing on the market, and the competent authority of the Member State should be asked. Which seaweed is in the jar decides which entry applies, and a gel sold as a food is a different use from a capsule sold as a supplement.

The composition column shows how wide the ranges are. The ash of Kappaphycus, which is its mineral fraction, runs from 5 to 45 % of dry weight in the published literature. The mineral content of a sea moss product is a property of the batch, not of the species.

2Iodine: sea moss is not kelp

Iodine is the first number to check for any seaweed product. The red seaweeds sold as sea moss carry far less iodine than kelp. The figures below are from Nordic monitoring data and from published surveys, measured on the dried seaweed, in milligrams per kilogram of dry weight unless the table says otherwise.

The last column is arithmetic, not a measurement. One milligram per kilogram is one microgram per gram, so the iodine in one gram of dried seaweed is the concentration read as micrograms.

The safety threshold these figures are compared with is the tolerable upper intake level for iodine set by EFSA: 600 µg a day for adults. At 270 mg/kg, about 2.2 g of dried Chondrus crispus a day reaches that level. At 5100 mg/kg, one gram of Norwegian oarweed is eight and a half times that level. Sea moss is a low-iodine seaweed by kelp standards, and it still reaches the upper level at a few grams of dried seaweed a day.

A gel changes the arithmetic, not the seaweed. A gel is the dried seaweed with water added. The iodine in a serving of gel is the iodine in the dried seaweed divided by the dilution. We found no published serving size for sea moss gel, and the dilution is the maker's choice. The maker knows how many grams of dried seaweed go into a jar; the buyer usually does not. A measurement on the gel settles it.

The legal position on iodine is short. No EU or UK regulation sets a maximum level for iodine in seaweed or in supplements. Two other numbers exist. France has a recommended maximum of 2000 mg/kg dry matter for edible seaweed, set by its public health council and reproduced in a 2020 opinion of the food safety agency ANSES; it is a recommendation, not law. Germany's risk assessment institute, BfR, stated in August 2026 that an EU draft regulation under discussion would set 1 g of iodine per kg of dry matter for algae sold as food and for foods containing algae, and gave its opinion that this figure is not low enough for people who eat seaweed every day. The draft is not in force, and we did not find its text.

3The rows for a sea moss product, in plain words

The EU sets maximum levels for contaminants in a table in Annex I of Regulation (EU) 2023/915. Each line of that table names a food category and a limit. In this series we call one line a "row". Great Britain kept the earlier table, Regulation (EC) 1881/2006, with its own row numbers.

Whether a sea moss product is a "food supplement" or a "food" depends on how it is presented and sold, and that classification is the seller's. The rows differ, so both are given here.

Sold as a food supplement: capsules, or a powder in daily doses

The EU rows, in milligrams per kilogram of the product as sold:

• lead: 3.0 (row 3.1.28);
• cadmium: 1.0 (row 3.2.21.1), or 3.0 when the supplement consists of at least 80 % dried seaweed or products derived from seaweed (row 3.2.21.2);
• mercury: 0.10 (row 3.3.2);
• arsenic: no row. The inorganic-arsenic rows cover rice products, infant foods, juices and, since October 2025, fish, crustaceans, bivalves and cephalopods. None covers seaweed or supplements;
• nickel and iodine: no row for supplements.

The GB rows are the same numbers under different row numbers: lead 3.0 (3.1.22); cadmium 1.0 (3.2.21), or 3.0 for supplements "exclusively or mainly" of dried seaweed (3.2.22); mercury 0.1 (3.3.3); no arsenic, nickel or iodine row. The GB table states its levels on a wet weight basis.

The two seaweed wordings differ. A capsule of pure sea moss powder meets the 80 % test and the "exclusively or mainly" test alike. A blend that is 60 % sea moss with bladderwrack and burdock gets the 1.0 cadmium row in the EU, and in Great Britain the answer depends on how "mainly" is read.

The levels apply to the product as placed on the market. For a powder or a capsule that is the powder as sold, which is close to dry weight, so the survey figures in section 4 can be compared with the rows directly. For a gel it is the gel as sold, and section 6 gives the arithmetic.

Sold as a food: a gel eaten by the spoon, or dried whole seaweed

• lead, cadmium, mercury, arsenic: no row for seaweed as a food in the EU table;
• nickel: 30 mg/kg for seaweed (row 3.6.8.1) and 40 for wakame (row 3.6.8.2), in force since 1 July 2025. For dry seaweed the level applies to the product as sold; for fresh seaweed it applies on a dry-matter basis after washing. Whether a seaweed supplement falls under this row is not stated in the text;
• iodine: no row; the draft figure discussed in section 2 is not in force.

The EU did ask for data. Commission Recommendation (EU) 2018/464 asked Member States to monitor arsenic, cadmium, iodine, lead and mercury in seaweed and seaweed products during 2018 to 2020, and it names Irish moss among the species. That monitoring is the data collection behind any future row.

Where the EU has no row, one Member State's recommendations are a useful reference. The French recommended maxima for edible seaweed, in mg/kg dry matter, are: inorganic arsenic 3; cadmium 0.5; mercury 0.1; lead 5; tin 5; iodine 2000. They were set by the French public health council and are reproduced in the ANSES opinion of 2020. They are recommendations in France, not maximum levels in law.

One set of numbers is often quoted on sea moss pages and does not apply. The purity criteria for the food additives carrageenan (E 407) and processed Eucheuma seaweed (E 407a) are arsenic 3, lead 5, mercury 1 and cadmium 2 mg/kg. They apply to the additive as an ingredient, not to sea moss sold as a food or a supplement.

4What the surveys measured

Published measurements for the three seaweeds are uneven. Chondrus crispus has monitoring data from two Nordic countries. Kappaphycus has one small market study we could use. Gracilaria has data from farms, one of them in an urban estuary. The table gives what we found. Figures are in mg/kg dry weight unless the row says otherwise.

Two things stand out in the Chondrus crispus data. First, lead, cadmium and mercury are all well under the supplement rows: the highest lead is about a fifth of its row, the highest cadmium about a quarter of the 1.0 row, and mercury about a fourteenth of its row. Second, inorganic arsenic in Chondrus is roughly a hundred times lower than the Norwegian oarweed mean and about 250 times lower than hijiki. In the Nordic data, Chondrus crispus is not a heavy-metal problem.

For Kappaphycus the picture is thin. The one study we used puts lead at 2.05 mg/kg, about two thirds of the 3.0 row, on seaweed bought at a market and dried. The study does not state whether its figures are on a dry-weight basis, and it used flame atomic absorption, a method with higher reporting limits than ICP-MS; the figure is a reading of one dried lot, not a survey. We do not know whether it is typical of the Philippine or Indonesian harvest. We read two further small studies from the Philippines and India that reported mercury above 0.3 mg/kg, and we did not use them: single samples, values read from graphs or published in a journal we do not rely on, and figures fifty times the Nordic ones without any explanation. The FDA survey holds the per-species figures that would settle the question. We had access only to its abstract.

5Which number is most likely to be over the limit

For Chondrus crispus the answer is iodine, and the threshold it meets is a health figure, not a legal one. None of the four metals in the Nordic data came near its row, while 2.2 g a day of the dried seaweed reaches the iodine upper level. That is what the data show. It is not a statement about any particular product.

For Kappaphycus and Gracilaria the candidate is lead. The Philippine Kappaphycus sample sat at two thirds of the row. The Gracilaria farmed in New York estuaries reached 6.6 mg/kg, twice the row, in water that is not where sea moss is harvested; it shows what the seaweed can take up, not what a Caribbean lot carries. This rests on two studies. We say so, and we will revise it when better data are published.

6What a laboratory can measure in a sea moss product

A laboratory does not decide whether a product is legal. It reports numbers with their uncertainty, the method and the reporting limit. The list below says what each measurement establishes and which question it answers.

A wet gel is also a microbiological product. Yeasts, moulds and water activity are questions of their own and are outside this article.

Of the measurements in the table, the four metals, iodine, species identification by DNA, moisture and water activity, ash and protein are listed with their prices on our seaweed testing page, together with microbiology, pesticide residues and radioactivity. Arsenic speciation and the sugar and polysaccharide profile are quoted on request through the form on the analytical services page.

Which sample to test. The iodine and the metals come with the seaweed, and the gel adds water. The dried lot, at the point where it enters the business, is the sample that answers most questions at once, and a result on it can be divided into any gel recipe. A result on the gel is useful too: the gel is the product as sold, which is the basis the EU rows use, and it needs a moisture figure beside it so that it can be compared with the dry-weight surveys in section 4.

The arithmetic for a gel, with an assumed recipe: a gel made from one part dried seaweed and nine parts water is 10 % seaweed by mass. A dried lot at 270 mg/kg iodine gives a gel at about 27 mg/kg, and a 30 g spoon of that gel carries about 810 µg of iodine, the same as the 3 g of dried seaweed it was made from. The one-to-nine ratio is an assumption chosen to show the calculation. It is not a recipe found in any source.

7One table to keep

The working rule is this. Take one dried lot. Establish which seaweed it is, measure iodine and the four metals on it, and record the dry-matter content of the gel made from it. Every question in this article comes back to those numbers.

How to read the result itself, including the limit of quantification and the measurement uncertainty, is covered in Part 3 of the Heavy Metals series. The kelp figures that sea moss is compared with here are in Part 7. The tests themselves, with prices, are on the seaweed testing page, and the order form is on the analytical services page.