Mercury in food

Forms

• Methylmercury (organic mercury): This is the form that makes up most of the mercury in seafood. In the 2024 total diet study, 74.4% of the mercury in seafood was methylmercury. It is this form that is thought to affect the fetus, and the MHLW abbreviates it to “mercury” in its materials for the general public.
• Inorganic mercury: This is total mercury minus methylmercury. The estimated 2024 intake was 0.024 μg per kg of body weight per day, which is 0.043 of the JECFA tolerable level (4 μg/kg per week).
• Total mercury: This is methylmercury plus inorganic mercury. Food concentration tables have two columns, total mercury and methylmercury, but they are not necessarily measured on the same samples, so for some species methylmercury comes out higher than total mercury (chub mackerel (masaba): total mercury 0.151, methylmercury 0.232 mg/kg).
• Units: Concentrations in this database are in mg/kg (= ppm = μg/g), as wet weight (the edible portion as sold).

Foods with mercury data

Sorted by mean, highest first. Values in different forms (total vs inorganic, dried vs soaked) cannot be compared directly — check each food page.

Effects on the body

• The fetus is the most vulnerable. The Food Safety Commission of Japan (FSCJ, 2005) judged that the fetus is the most susceptible to the effects of methylmercury. The MHLW describes the effect of concern as something like “a delay of less than one-thousandth of a second in the response to a sound,” and says that, if it occurs at all, it is not serious enough to interfere with future social life.
• The advisory applies only to women who are pregnant or may become pregnant. Children, women who are breastfeeding, and adults other than pregnant women are not covered. The MHLW gives these reasons: children excrete mercury the way adults do and the action on the brain is similar to that in adults, the Seychelles child development study did not demonstrate harmful effects on children, and little mercury passes into breast milk.
• The epidemiological studies disagree. In the Faroe Islands (1,022 births, mercury from pilot whale), prenatal exposure was associated with lower language, attention, and memory scores at age 7. In the Seychelles (779 mother-child pairs, mothers ate an average of 12 fish meals a week), the results did not support a risk to neurodevelopment up to age 9. The FSCJ calculated the tolerable level from both studies, and replied that it could not evaluate the influence of other components, such as nutrients in fish.
• Fish also has benefits. The FAO/WHO Joint Expert Consultation (2010) found that, in most of the situations it evaluated, the risk of poor neurodevelopment in a child is lower when a woman of childbearing age eats fish than when she does not. The MHLW hopes that by avoiding eating too much of the fish that are high in mercury, people can keep the benefits of eating fish.
• Mercury in the body declines over time. According to the MHLW, the time for mercury in the body to fall by half is about 2 months (some sources give 50–70 days). If you ate too much, it is enough to eat less the next week, and a single meal or a single week’s intake is not thought to change the concentration in the body much.
• This is a separate matter from Minamata disease. Minamata disease was a pollution disease caused by eating fish and shellfish contaminated by chemical factory wastewater containing methylmercury (Ministry of the Environment). The MHLW explains that the effect addressed by the current advisory is separate from serious health damage like Minamata disease.

Tolerable intake (the amount thought to cause no harm even if taken for a lifetime)

How much people in Japan take in

• Total mercury intake, in the total diet study, fell from 15.0 in 1977–78 to 5.04 μg/person/day in 2024 (of which methylmercury was 3.69). Excluding 1977–78, it has stayed between 5 and 12 and has declined gradually.
• Seafood accounts for 90.0% (2024). It was 87.6% in 1992–2001 and 88.1% in 1999–2008. The FSCJ hazard document gives 84.2%.
• The percentage of the tolerable level depends on the yardstick. Taking the 2024 total mercury of 5.04 μg/day as all methylmercury and assuming a body weight of 50 kg, it is 35% of the FSCJ value of 2.0 and 54% of the EFSA value of 1.3 (Urayomi’s calculation). The National Institute of Health Sciences (NIHS) reports a hazard ratio for methylmercury (intake ÷ reference value) of 0.24–0.36. The MHLW also wrote about a similar intake as “35% of the provisional tolerable level (3.4)” in 2003 and as “57% of the tolerable level for pregnant women (2.0)” in 2010.
• Fish consumption is falling. Annual per-person consumption of fish and shellfish (net food supply) went from 40.2 kg in fiscal 2001 to 21.4 kg (approximate) in fiscal 2023. Mercury intake in 2019 was 0.57 times that of 1997, a slightly larger drop than the decline in fish and shellfish consumption (0.65 times). The NIHS speculates that this may be because the 2010 advisory reduced occasions to eat fish high in mercury, but the cause has not been confirmed.
• Measurements in Japanese pregnant women: In the Japan Environment and Children’s Study (JECS, the Ministry of the Environment’s child health survey), methylmercury in the umbilical cord blood of 3,822 people had a median of 7.39 ng/mL (5th–95th percentile 2.95–18.1). In developmental tests at ages 2 and 4 (3,083 children), no significant association with mercury was found. Among fish, “katsuo and maguro” (bonito and tuna) correlated most strongly with mercury in cord blood.
• In an area where many people eat whale and dolphin, hair mercury in 1,017 residents of Taiji, Wakayama Prefecture (summer 2009) had a geometric mean of 11.0 ppm for men and 6.63 ppm for women. That is higher than in 14 areas in Japan (men 2.47, women 1.64), and 32 people (3.1%) were above 50 ppm. No one was found who raised suspicion of methylmercury poisoning, but the National Institute for Minamata Disease said it would consider continuing the survey.

History

• 1956: Minamata disease was officially confirmed. Niigata Minamata disease followed in 1965, and the government’s unified view was announced in 1968 (only the years could be confirmed in Ministry of the Environment materials).
• Relief and certification: The number of certified patients under the Pollution-Related Health Damage Compensation Law is 3,000 (as of May 2024). Those eligible for relief under the 1995 political settlement number 12,374, and those under the 2009 Special Measures Act number 38,320 (Ministry of the Environment).
• July 23, 1973: Provisional regulatory limit for fish and shellfish (total mercury 0.4 ppm, methylmercury 0.3 ppm; notice from the Director-General of the Environmental Health Bureau, Ministry of Health and Welfare). It does not apply to tuna-type fish (tuna, marlin, and skipjack), fish and shellfish from rivers (excluding lakes), deep-sea fish and shellfish, and so on. No primary source stating the reason for these exemptions has been found.
• June 2003: The first publication of the “Advice on eating fish and shellfish containing mercury” (covering shark, swordfish, kinmedai (splendid alfonsino), and some whales; tuna was not included). In the same month JECFA lowered its tolerable level from 3.3 to 1.6.
• 2005: In August the FSCJ issued its tolerable level of 2.0. In November the advice was revised to the “Advice on eating fish and shellfish and mercury for pregnant women,” adding tuna (Pacific bluefin, southern bluefin, and bigeye) to make 15 species.
• June 1, 2010: Recalculated with data from local governments, and kuro-mutsu (gnomefish) was added to make 16 species. The table here is the June 2010 revision.
• October 2013: The Minamata Convention on Mercury was adopted. It entered into force on August 16, 2017.

Good to know

• Concentrations by species are compiled through fiscal 2008 (453 kinds in Japan, 16,437 samples, May 2010) and may differ from what is on the market today. For fish with only 1–9 methylmercury samples (4–5 for whales), the average is unstable.
• For the same fish, concentrations are almost the same in Japan and the U.S., but the advice cutoffs differ. Swordfish is 1.003 in Japan (n=51) and 0.995 in the U.S. (n=636), yet Japan allows up to 80 g a week while the U.S. says “avoid.” For yellowfin tuna, Japan has no limit and the U.S. says once a week. We have not found any document from either country that explains why they differ.
• Pacific bluefin tuna (0.687) and swordfish (1.003), whose averages exceed Japan’s provisional regulatory limit (total mercury 0.4), are on the exempt side of that limit. In their place, there are serving amounts for pregnant women. That is the relationship.
• The FSCJ replied that it set the tolerable level without being able to evaluate the influence (confounding) of other components such as nutrients in fish. The tolerable level differs by about 3-fold depending on the agency (3.4, 2.0, 1.6, 1.3).
• The claim that “pregnant women must not eat tuna” does not match the MHLW’s explanation. The MHLW writes that it is not requesting people to avoid fish and shellfish with high mercury concentrations. On the other hand, the idea that selenium protects against mercury toxicity is at the stage of hypothesis and review articles, and it cannot be said that “fish with selenium are fine.”
• The table of serving amounts for pregnant women has not been revised since June 2010.

Related explainers

• How Much Fish Is OK During Pregnancy? Guidance for Tuna, Canned Tuna, and Kinmedai, with Mercury Numbers
• Why Do Heavy Metal Standards Differ Between Japan and Other Countries? Japanese, Codex, and EU Standards and Tolerable Intakes Compared
• How Much Heavy Metal Do Japanese People Take In? Fifty Years of Lead, Cadmium, Mercury, and Arsenic Intake
• Do Heavy Metal Detox and Chelation Work? What Public Agencies and Clinical Trials Show

Sources

• MHLW, “Advice on eating fish and shellfish and mercury for pregnant women”
• MHLW, “Advice on eating fish and shellfish and mercury for pregnant women,” Q&A (revised June 2010)
• MHLW, survey results on mercury in fish and shellfish (453 kinds in Japan, 16,437 samples, May 2010)
• FSCJ, notification of the tolerable weekly intake of methylmercury (August 2005)
• MAFF, “Provisional regulatory limit for mercury in fish and shellfish”
• MHLW Health and Labour Sciences Research Grants, total diet study, fiscal 2024 (National Institute of Health Sciences)
• Ministry of the Environment, “So that the tragedy of Minamata disease is not repeated,” and others
• Kuraoka S, et al. Sci Total Environ 2024 (methylmercury in cord blood, JECS)（PMID 39528210）

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

Pages for each metal

• Arsenic
• Cadmium
• Lead