Heavy Metals and Seaweed: Arsenic, Cadmium and the Hijiki Advice

Seaweed takes up minerals from seawater, including some we would rather not eat: arsenic, cadmium, lead and a little mercury. Most of the worry has been about one seaweed, hijiki, and about kelp tablets. This page walks through what government agencies and published surveys actually say, in plain words, and what they do not say. For iodine, which is a separate seaweed concern, see Iodine in seaweed.

Read this first

• Four national food agencies we read advise people to avoid hijiki (a black, shredded seaweed) because of its inorganic arsenic: the UK, Canada, Hong Kong and Australia and New Zealand. [1][2][3][5][7]
• The same agencies say other common seaweeds (nori, wakame, kombu, arame) are not the concern. In the UK survey, inorganic arsenic was found only in hijiki. [1][2][5][8]
• Cadmium is the other metal to know about. EFSA (the European food safety agency) named seaweed among the foods with the highest cadmium levels, but also noted people eat it in small amounts. [15]
• Kelp tablets are not the same as seaweed eaten as food. Small surveys found arsenic and lead in some algae supplements, and the amounts and the arsenic form are not on the label. [12][13][14]
• If you are pregnant, feeding a baby, have kidney disease, or eat seaweed every day, ask your clinician or dietitian. This page is information, not medical advice.

Inorganic and organic arsenic: why the form matters

Arsenic is not one thing. Seaweeds hold it mostly as "organic" arsenic compounds (such as arsenosugars), which the Hong Kong Centre for Food Safety describes as the less toxic form. [5][9] "Inorganic" arsenic is the more toxic form; Hong Kong lists gastrointestinal effects, anemia and liver damage, and notes the International Agency for Research on Cancer classified arsenic in drinking water as a Group 1 carcinogen. [5] The UK study described inorganic arsenic as a form that can cause liver cancer. [1]

Hijiki is the exception. A 2019 study found about 60% of the arsenic in raw whole hijiki was arsenate (an inorganic form). [9]

These are different studies with different samples, so compare them loosely. The Spanish authors worked out that 1.7 g of hijiki a day would reach the WHO provisional weekly limit then in use for a 68 kg person. [11] Hong Kong notes that JECFA (the FAO/WHO expert committee) withdrew that older limit in 2010 and set a new reference point, so that figure is a historical illustration, not a current limit. [5]

What each agency said about hijiki

Japan: we could not open the Japanese health ministry's own statement, so we do not quote it. A search summary says it reported no arsenic poisoning cases from hijiki and judged normal diets low-risk, but we have not verified that wording. Proper evidence for Japan's current position was not found in the sources we read.

Note the difference in tone. The UK, Canada and Hong Kong say avoid. Australia and New Zealand's 2004 advice also says avoid, while its later page speaks of "higher potential risk" for regular high intake. Singapore describes a moderation route. [2][3][4][5][6][7] The agencies do agree that eating hijiki once in a while is unlikely to have caused harm. [4][5][8]

Cadmium, lead and mercury

Cadmium. In 2009 EFSA set a tolerable weekly intake of 2.5 micrograms per kilogram of body weight. EU law text that summarizes the opinion says average European intakes are close to or slightly above it, some groups reach about double, and the highest cadmium levels were found in seaweed, fish and seafood. [15] A Spanish survey of 112 seaweed products (packaged seaweed, tablets and concentrates, foods with seaweed, canned seaweed) found many samples above some national cadmium limits. Its authors estimated that 3 g a day of the products could reach up to 15% of the WHO tolerable daily intakes. [10] An earlier Spanish survey of 18 algae foods found cadmium from 0.03 to 1.9 mg/kg dry weight. [11]

Lead and mercury. That same 2002 survey found lead from under 0.05 to 1.33 mg/kg and mercury from 0.004 to 0.04 mg/kg dry weight. [11] In 15 algae supplements sold in Portugal, lead showed up in four samples (hijiki, arame, sea spaghetti and wakame), which the authors judged a considerable concern at the labeled or typical intake. This was a small, non-representative sample. [12]

We did not read the US FDA Total Diet Study tables for seaweed, so we make no claim about them. See the general pages for contaminant background: Heavy metals and contaminants in foods.

Kelp and algae supplements

Surveys of tablets are small. In Portugal, arsenic was found in all 15 algae supplements, highest in hijiki, arame and kombu products (about 60 to 66 micrograms per gram dry weight) and lowest in spirulina. [12] A 2007 case report in a medical journal linked a woman's illness to arsenic in a kelp tablet; an industry association letter replied that iodine overdose could explain her symptoms and noted that the woman took more than the labeled dose. [13] The case authors replied that most arsenic in kelp supplements is the more toxic inorganic form, but they cited unpublished data, so treat that claim as preliminary. [14] The industry letter says most arsenic in food seaweeds is organic, with hijiki the exception, and that the letter writers work for a herbal products trade association. [13]

The honest summary: a tablet gives you no way to know the arsenic form, and kelp tablets also carry iodine. See Kelp and seaweed supplements: iodine content and risks and Heavy metals in supplements and herbal products.

Can you cut exposure? What sources show

• Variety. The FDA says (on its general contaminant pages summarized in our food contaminants guide) that eating a variety of foods helps limit exposure to any one contaminant. The same idea fits seaweed: do not rely on one kind or one brand.
• Soaking and heating hijiki. A lab study found that heating raw hijiki at 90 degrees C for 5 minutes cut total arsenic by roughly 55 to 75%, and that adding salt water soaking cut it a little more. It also cites older work in which boiling at 100 degrees C for 20 minutes lowered inorganic arsenic by about 46 to 50%. [9] That means some arsenic remains. These are lab results on hijiki only; we found no source showing the same for other seaweeds, and the UK, Canada and Hong Kong advice is still to avoid hijiki. [2][5][7][9]
• Throw away the soaking water. Singapore's agency says do not consume it. [6]
• Check the label. Hong Kong advises reading labels because hijiki turns up in prepackaged salads and side dishes. [5]
• Choose other seaweeds, which the UK and Hong Kong describe as not having the inorganic arsenic problem. [5][8] For iodine, kombu is a separate concern; FSANZ notes some seaweed, particularly kombu, has very high iodine. [4]

Who should take extra care

• Regular, large eaters of hijiki, such as people on diets built around sea vegetables. The Canadian agency names macrobiotic diets as a group that may be at greater risk. [2][4]
• People with high arsenic from other sources (for example well water or a lot of rice). FSANZ says this group has a higher potential risk if they also eat hijiki often. [4]
• Pregnant women, infants and small children for iodine from some seaweed (FSANZ). [4] See Seaweed in pregnancy and breastfeeding.
• Anyone taking kelp tablets for long periods; ask your clinician. [12][13]
• People with kidney disease: proper evidence for a seaweed-specific cadmium or arsenic warning was not found in the sources we read; ask your clinician.

Related pages

• Hijiki
• Arame
• How to buy, prepare and store seaweed
• Iodine in seaweed: how much by type
• Heavy metals in natural ingredients
• Heavy metals in supplements and herbal products