Heavy metals in food databaseExplainerChecked 2026.10.03

Should You Avoid Hijiki? Inorganic Arsenic Numbers, Ways to Reduce It, and Views by Country

Inorganic arsenic in dried hijiki, MAFF’s cooking tests (soaking and boiling in fresh water), the “avoid it” advisories from the UK and other countries, and Japan’s position, laid out with figures from public sources.

Short answer

Inorganic arsenic in dried hijiki averages 67 mg/kg (4.5–130, 299 samples, 2006–08). All 100 samples each of kombu, wakame, and nori (dried) were below the limit of quantification (0.5), so hijiki stands apart even among seaweeds. Commercial rehydrated hijiki averaged 3.6 mg/kg (71 samples).

In MAFF’s tests, with 20 g of dried hijiki and 600 cc of water, inorganic arsenic fell by about 50% with soaking, about 80% with boiling from dry, and about 90% with soaking then boiling in fresh water. More than 70% of the iron, calcium, and dietary fiber remained.

Countries disagree. The UK, Canada, Hong Kong, Australia, and New Zealand say “avoid it”; Japan says “it is fine as part of a balanced diet, and use methods that reduce it.” We found no Japanese standard for inorganic arsenic in hijiki.

The numbers: dried and rehydrated differ by an order of magnitude

In MAFF’s survey (fiscal 2006–08), inorganic arsenic in hijiki was about 70% of total arsenic (67÷93). Dried products are measured by dry weight and rehydrated products in their rehydrated state, so the state has to be matched when comparing.

Food	Substance	Samples	Mean mg/kg	Range
Hijiki (dried)	Total arsenic	299	93	28–160
Hijiki (dried)	Inorganic arsenic	299	67	4.5–130
Hijiki (rehydrated)	Total arsenic	71	6.0	2.1–20
Hijiki (rehydrated)	Inorganic arsenic	71	3.6	<0.5–17
Kombu (dried)	Total arsenic	200	53	25–110
Wakame (dried)	Total arsenic	100	33	15–52
Nori (dried)	Total arsenic	100	25	13–51
Kombu, wakame, nori (dried)	Inorganic arsenic	100 each	All <0.5	—

For inorganic arsenic in kombu, wakame, and nori, the limit of quantification is high (0.5), so all that can be said is “below 0.5.” MAFF’s means of 0.15–0.19 are calculated by substituting values for the results below the limit; they are not measured means.
The Food Safety Commission of Japan (FSCJ, 2013) writes that arsenic concentrations in Japanese hijiki vary widely depending on factors such as the condition of the sample.
Inorganic arsenic in cooked hijiki (wet weight, 14 households × 3 meals’ worth from home kitchens) was 0.4–2.8 mg/kg (Nakamura 2008).
Even though total arsenic is high in seafood, inorganic arsenic is 0–4% of total arsenic (marbled sole, yellowtail, Japanese horse mackerel, chub mackerel, Pacific saury, Japanese sardine).

Reducing it at home: MAFF’s tests (fiscal 2014)

These tests cooked 20 g of dried hijiki in 600 cc of water at about 20°C. The reductions are given only as rounded figures such as “about 50%”; the source has no sample counts or table of means.

Method	Steps	Inorganic arsenic reduction	Time
Soaking	Soak in water for 30 minutes → discard the soaking water → rinse under running water for 20 seconds	About 50%	About 30 min
Boiling from dry	Start in cold water over high heat → after it boils, simmer 5 minutes → discard the water and rinse	About 80%	About 20 min
Soaking then boiling in fresh water (yudekoboshi)	Soak 30 minutes → discard the soaking water → boil 5 minutes in 600 cc of fresh water → discard the water and rinse	About 90%	About 40 min

More than 70% of the iron, dietary fiber, and calcium remained with every method (MAFF’s wording; calcium exceeds 100% because of calcium in the tap water).
MAFF recommends not using the soaking water for cooking. Cooking with the soaking water, or simmering dried hijiki as is, is not recommended from the standpoint of reducing arsenic.
MAFF reports that an average of 58% of total arsenic leached out during soaking (20–90%, 150 commercial samples, 40 times the volume of water, 20°C, 30 minutes).
Papers point the same way. One reported that heating at 90°C for 5 minutes reduced it by 33–80%, and that soaking in 2% salt water reduced it further (Park 2019); another reported that boiling after soaking reduced it much more than soaking alone (the National Institute of Health Sciences (NIHS), 2008).

Reduction in inorganic arsenic by cooking method for hijiki (MAFF fiscal 2014 home-cooking tests, 20 g dried hijiki)

Portion sizes against different yardsticks

This is a calculation applying several yardsticks to one serving of hijiki. The table is not meant to say either “don’t eat it” or “it’s fine”; it is meant to show that the same food reads differently when the standard moves.

Assumptions: 5 g of dried hijiki (MAFF’s materials give no serving size; we assumed a typical small side dish of simmered hijiki), inorganic arsenic at the dried average of 67 mg/kg, body weight 60 kg. For the rehydrated figures, we applied MAFF’s reduction rates as is (Urayomi’s calculation).

Method	Inorganic arsenic in 5 g
Dried, as is	About 335 μg
Soaking (about 50% less)	About 168 μg
Boiling from dry (about 80% less)	About 67 μg
Soaking then boiling in fresh water (about 90% less)	About 34 μg

Yardstick	Daily amount for 60 kg body weight	Note
JECFA PTWI (15 μg/kg per week, about 2.1 μg/kg/day)	About 129 μg	Value withdrawn in 2010
JECFA 2025 POD (0.3 μg/kg/day)	18 μg	Reference point for ischemic heart disease
EFSA 2024 reference point (0.06 μg/kg/day)	3.6 μg	Eating 1 g of rehydrated hijiki (3.6 mg/kg) gives 3.6 μg

EFSA’s reference point (BMDL05) is not a tolerable intake of the kind that means “anything below this is safe.” EFSA evaluates using the margin of exposure (MOE) and has not set an MOE value that counts as “low concern.”
MHLW’s Q&A (July 2004), based on the PTWI at the time (15 μg/kg per week, or 107 μg per day for a 50 kg person), calculated that even for the highest level measured by the UK FSA, 22.7 mg/kg (rehydrated), the limit would not be exceeded unless a person kept eating 4.7 g (33 g per week) or more every day. It estimated Japanese hijiki intake at about 0.9 g/day, from 14.6 g/day of seaweed × a 6.1% share for hijiki. That PTWI was withdrawn in 2010, so this has to be read as a 2004 calculation. We have not confirmed whether it has been updated.
MHLW (2004) writes that there have been no reports of poisoning from hijiki.

Views by country: “avoid it” versus “eat it, with the amount reduced”

Agency	Year	Content
Canada CFIA	2001 (FSANZ document)	Advises avoiding hijiki. Even a small amount can exceed the tolerable daily intake. Seaweeds other than hijiki, such as dulse, nori, and kombu, are low
UK FSA	Advised in July 2004, reconfirmed in 2010	Do not eat hijiki. Inorganic arsenic was 68.8–96.2 mg/kg as sold and 7.9–30.9 mg/kg after cooking. In 2010: “people who have eaten it occasionally need not worry”
Hong Kong CFS	2004	Do not eat hijiki or choose it as an ingredient. Occasional small amounts are unlikely to have an effect, but people who like it are at higher risk. The standard is 1.4 ppm as As₂O₃
Australia and New Zealand FSANZ	2004	Avoid it. The maximum limit for inorganic arsenic in seaweed is 1 mg/kg (since 1991). In import inspections, 6 of 28 samples failed (22–92 mg/kg)
Japan MHLW	July 30, 2004	As long as people do not eat extremely large amounts and keep a balanced diet, health risk does not rise. There is no international standard for hijiki
Japan MAFF	Fiscal 2014 tests, October 2015	No health effects from arsenic taken in through food have been recognized. To eat it more safely, soaking and soaking then boiling in fresh water are recommended. Manufacturers have been asked to ensure thorough rinsing and soaking
Japan FSCJ	December 2013	Does not consider the current situation to be a problem. Because intake may be high in some groups, eat a balanced diet without relying on particular foods

We found no document in which any national authority compared the reasons for its advice with the others’. It is a fact that only Japan has test data on cooking methods that reduce arsenic (MAFF).
We could not confirm the original UK FSA text; what is shown here was checked through a paper (Rose 2007), news reports, and citations in other countries’ documents. We have not confirmed whether the advice is the same today.

How much people actually eat

A survey of 14 households (Nakamura 2008): average intake was 6.5 g/day (1.1–14, median 5.5), and many households ate it 2–3 times a month. From the inorganic arsenic in cooked hijiki, the skin cancer risk calculated with a US EPA coefficient averaged 2.4×10⁻⁴, above 10⁻⁵. This is an estimate from 14 households’ data, and the Food Safety Commission of Japan has set neither a tolerable intake for inorganic arsenic nor a carcinogenic unit risk.
Distribution across the Japanese population (Oguri 2014, a probabilistic estimate from analyses of 56 rice and 32 hijiki samples): intake of the inorganic arsenic that dissolves in the body (bioaccessible) from rice and hijiki is a median of 19 μg/day and 59 μg/day at the 95th percentile (not intake from all foods). At the median, rice and hijiki contribute about equally, and the higher the intake, the larger hijiki’s share becomes.
For the whole diet, of 18.3 μg/day of inorganic arsenic (average for fiscal 2014–17), rice accounts for 12.2 (about 67%) and vegetables and seaweed for 2.9 (about 16%). A breakdown for hijiki alone has not been published.

What is not known

We found no public data showing whether the arsenic reduction rate differs between iron pots and stainless steel pots.
In what we checked, we found no specific national guidance for pregnancy or children. Canada’s CFIA says people who eat a lot of seaweed, such as on a macrobiotic diet, may be at higher risk.
We found no Japanese public data on other seaweeds in the same Sargassaceae family as hijiki, such as mozuku and arame. FSANZ writes that some other Sargassaceae species have more inorganic arsenic than hijiki.
On organic arsenic, MAFF writes that its effects are generally said to be small, but also that its effects in the human body are not well understood. It cannot be called “harmless.”
Some Japanese materials remain written with the 2004 yardsticks. We have not confirmed whether MHLW’s Q&A has been updated after JECFA’s 2010 withdrawal and the 2024 and 2025 reassessments.

Sources

Last checked 2026.10.03 (against the original government documents and papers, collected and cross-checked in Gemini Notebook).

Heavy metals occur naturally in soil and the sea, and every food contains small amounts. This page lists facts from government surveys, limits and assessments and from published papers; it does not judge any food as good or bad. Where there is official advice — such as on eating fish during pregnancy — follow it.

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