Heavy metals in food databaseSeaweedChecked 2026.10.03
Hijiki: arsenic
Dried hijiki is a food with especially high inorganic arsenic (the form of arsenic whose health effects are of greater concern) even among seaweeds. In a MAFF survey (fiscal 2006–08, 299 samples), inorganic arsenic in dried hijiki averaged 67 mg/kg. MAFF tests show that soaking and soaking then boiling in fresh water (yudekoboshi) greatly reduce it. Advice differs by country: the UK, Canada, Hong Kong, and Australia/New Zealand say to avoid it, while Japan says to eat it as part of a balanced diet and to reduce the arsenic first.
Levels measured in Japanese government surveys and studies
| Metal | Form / state | Mean | Range / max | Samples | Survey |
|---|---|---|---|---|---|
| Arsenic | Inorganic arsenic, dried | 67 (median 68) | 4.5–130 | 299 samples | MAFF, fiscal 2006–08 |
| Arsenic | Total arsenic, dried | 93 (median 92) | 28–160 | 299 samples | MAFF, fiscal 2006–08 |
| Arsenic | Inorganic arsenic, after soaking (the item name in MAFF’s table is “soaked product”) | 3.6 (median 3.2) | <0.5–17 | 71 samples | MAFF, fiscal 2006–08 |
| Arsenic | Total arsenic, after soaking (the item name in MAFF’s table is “soaked product”) | 6.0 (median 5.5) | 2.1–20 | 71 samples | MAFF, fiscal 2006–08 |
Units are mg/kg (= ppm). Dried values are per dry weight; values for rehydrated hijiki (the “soaked product” in MAFF’s table) are per weight after soaking. The items and sample counts differ, so these figures cannot be used to convert from dried to soaked hijiki (the FSCJ also writes that “results differ greatly depending on the state of the sample”).
How much to eat (official advice)
- MHLW (July 2004): as long as you do not eat extreme amounts and keep to a balanced diet, the health risk does not rise (a Q&A issued in response to the UK advice. The tolerable intake used for its calculation was later withdrawn)
- FSCJ (December 2013): does not consider the current intake of arsenic through food to be a problem. Do not rely on particular foods; eat a balanced diet
- MAFF: to eat hijiki more safely, soak it or soak then boil it in fresh water. Do not use the soaking water in cooking
Limits (Japan, international, EU)
| Set by | Limit | Note |
|---|---|---|
| Japan (Food Sanitation Act) | No standard | No limit for inorganic arsenic in hijiki or rice was found in the Food Sanitation Act (I did not check the text of the law itself). MAFF publishes cooking methods that reduce it |
| Codex (international standards) | No Codex standard for hijiki | MHLW’s Q&A (2004) says that “there is no international standard for hijiki” |
| EU (Regulation 2023/915) | No hijiki-specific limit confirmed | The seaweed category was not checked (unconfirmed) |
| Australia/New Zealand (FSANZ) | Inorganic arsenic in seaweed: maximum 1 mg/kg (since 1991) | Advised “avoid” in 2004. In import inspections, 6 of 28 samples failed (22–92 mg/kg) |
| Hong Kong (Food and Environmental Hygiene Department) | Arsenic (as As2O3) 1.4 ppm | A November 2004 survey found the statutory limit exceeded. Advice: “do not eat hijiki or choose it as an ingredient” |
Good to know
- Concentrations differ greatly between dried and soaked hijiki. Dried hijiki averages 67 mg/kg inorganic arsenic; soaked hijiki (the “soaked product” in MAFF’s table) averages 3.6 mg/kg (71 samples). How you read the numbers depends on whether a label or a recipe amount refers to the dried weight or the weight after soaking.
- About 70% of the total arsenic in hijiki is inorganic (67 ÷ 93), which is high for a seaweed. For kombu, wakame, and nori, inorganic arsenic in the dried product was below the limit of quantification (0.5 mg/kg) in all 100 samples tested of each. Not every seaweed is the same.
- Advice from different countries is split. The UK FSA (advised in 2004, reaffirmed in 2010), Canada’s CFIA (2001), Hong Kong (2004), and Australia/New Zealand (2004) say to avoid it; Japan’s MHLW, MAFF, and FSCJ say there is no problem if you eat it as part of a balanced diet, and advise reducing the arsenic by cooking. No document was found in which the authorities compare their reasons with one another.
- The MHLW Q&A (2004) calculation that you would not exceed the limit “unless you keep eating 4.7 g a day (33 g a week) or more” is based on a tolerable intake (15 μg/kg body weight/week) that JECFA withdrew in 2010. I could not confirm whether the Q&A has been updated, so these figures are better not read as a current assessment.
- No body currently issues a tolerable intake meaning “below this is safe.” In its 2025 re-evaluation JECFA used ischemic heart disease to set a reference point of 0.3 μg/kg body weight/day, and in 2024 EFSA used skin cancer to set 0.06 μg/kg body weight/day. Japan’s FSCJ did not set a tolerable intake in 2013, stating that “there are no data showing health effects in ordinary diets, and we do not consider the current situation to be a problem.” The picture changes depending on which benchmark you choose.
- Intake varies greatly with how often you eat it. A probabilistic estimate (Oguri et al., 2014) of bioaccessible inorganic arsenic (the portion that can be taken up by the body) from rice and hijiki gave a median of 19 μg/day and a 95th percentile of 59 μg/day; at the median, rice and hijiki contributed about equally, and the higher the intake, the larger the contribution from hijiki. In an analysis of cooked hijiki (wet weight) from 14 households (Nakamura et al., 2008), inorganic arsenic was 0.4–2.8 mg/kg and the average intake was 6.5 g/day (many households ate it two to three times a month).
- No public data were found on whether inorganic arsenic falls differently when cooking in an iron pot versus a stainless-steel pot. No Japanese public survey values were found for other seaweeds in the same family (Sargassaceae) as hijiki, such as mozuku and arame (FSANZ writes that “some other species of the Sargassaceae family have more inorganic arsenic than hijiki,” but I have not seen the original source).
Reducing it at home (shown in tests)
- Soaking: soak 20 g of dried hijiki in 600 cc of room-temperature water for 30 minutes, discard the soaking water, and rinse under running water for 20 seconds. Inorganic arsenic falls by about half (takes about 30 minutes)
- Boiling from dry: put 20 g of dried hijiki in 600 cc of water over high heat, simmer for 5 minutes after it boils, discard the water, and rinse. About an 80% reduction (takes about 20 minutes)
- Soaking then boiling in fresh water (yudekoboshi): soak for 30 minutes, discard the soaking water, boil for 5 minutes in 600 cc of fresh water after it boils, discard the water, and rinse. About a 90% reduction (takes about 40 minutes)
- Iron, calcium, and dietary fiber: more than 70% remained with every method (MAFF’s fiscal 2014 test. The published material has no table of sample counts or means, and the reduction rates are given as approximate)
- Do not use the soaking water in cooking (MAFF’s recommendation). In soaking, the average leaching of total arsenic was 58% (20–90%, 150 commercial samples, 40 times the volume of water, 20°C, 30 minutes)
Related explainers
- Should You Avoid Hijiki? Inorganic Arsenic Numbers, Ways to Reduce It, and Views by Country
- Can Home Cooking Reduce Heavy Metals? Figures Shown for Hijiki, Rice, Innards, Tap Water, and Canned Food
- Why Do Heavy Metal Standards Differ Between Japan and Other Countries? Japanese, Codex, and EU Standards and Tolerable Intakes Compared
Pages for each metal
Sources
- MAFF, “Survey of arsenic contained in food” (updated December 2025)
- MAFF, “Cooking methods to reduce inorganic arsenic in hijiki,” survey results (fiscal 2014)
- MHLW, “On arsenic in hijiki,” Q&A (July 30, 2004)
- FSCJ, “Food health impact assessment: arsenic” (December 2013)
- EFSA CONTAM, “Inorganic arsenic in food,” scientific opinion (2024)
- FSANZ, “Hijiki seaweed and inorganic arsenic”
- Nakamura Y et al. J Agric Food Chem 2008 (inorganic arsenic in cooked hijiki)（PMID 18324769）
- Oguri T et al. Nihon Eiseigaku Zasshi (Japanese Journal of Hygiene) 2014 (probabilistic estimate of inorganic arsenic intake)（PMID 25253519）
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.