Fish/shellfish and methylmercury｜Mercury Q&A｜National Institute for Minamata Disease

Q

What kind of element is mercury?

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A: Mercury is an element that can be found relatively often on Earth.

Mercury is a type of metal that exists on Earth in relatively large quantities and is the only metal that takes a liquid form at ordinary temperature. It gets emitted from the Earth's crust to come out onto the land surface and into the atmosphere as a result of volcanic eruptions and combustion of coal, etc. and exists in seawater as well. Throughout history, mercury has been used in various situations. For example, it has been used in Japan since ancient times as a material for pigments to paint things like Shinto shrine gates and lacquerware. Other usages of mercury include use for blood pressure gauges, thermometers and temperature indicators, and it has also served as an ingredient of some types of drug. Alloys of mercury with another metal, called mercury amalgams, have also been used, a major example being tin amalgam, which was used as fillings for dental cavity treatment in Japan and elsewhere up until around 2000.

Q

What kind of substance is methylmercury?

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A: Methylmercury gets generated from inorganic mercury in the natural environment.

When in the atmosphere or the soil, or in seawater, mercury is present in the form of inorganic mercury for the most part. Inorganic mercury contained in seawater can transform to methylmercury, a type of organic mercury, as a result of the actions of microbes, etc. While inorganic mercury seldom gets absorbed through the digestive tract, methylmercury, bound with a type of amino acid called cysteine, has a property of getting easily absorbed from the digestive tract and then penetrating into various parts of the body. Therefore, it can also make its way inside the brain and damage nerve cells, or cross the barrier of the placenta that connects a fetus with its mother, eventually entering the body of the fetus as well.

Q

Why does methylmercury get accumulated in fish and whales?

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A: It is through the aquatic food chain that methylmercury gets accumulated.

A very small amount of methylmercury that is generated from inorganic mercury by microbes in the ocean or lakes gets taken in by plankton. Small fish and other creatures feed on plankton, with methylmercury also taken in together. Small fish get eaten by small carnivorous fish, which are then eaten by larger carnivorous fish or marine mammals. This relationship is called a food chain. Thus, methylmercury gets accumulated more and more as this process progresses, as a result of which methylmercury ends up more concentrated in larger fish than in smaller fish, and in carnivorous fish and in whales, dolphins, etc. than in herbivorous fish. Likewise, deep-sea fish, which have a longer life-span, also tend to show higher levels of methylmercury. Finally, humans, situated at the top of the food chain, take in methylmercury by eating fish, shellfish and other aquatic creatures. This, however, normally has no impact on health because only a small quantity of methylmercury concentrates in the human body.

Q

In which body part of fish is methylmercury contained?

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A: Methylmercury is contained in a larger quantity in the lean part of the fish body.

Some harmful substances present in nature dissolve in fat and are contained more in the blubber, intestines, etc. as a result, in contrast to methylmercury, which gets bound with amino acids and are therefore contained in protein, which consists of amino acids. This means that it is largely contained in the lean part of the fish body. Note that since it is not possible to decompose or remove methylmercury by cooking, be it boiling, grilling or otherwise, there is no cooking or eating method that could reduce methylmercury in fish and other creatures.

Q

Has mercury been accumulated in fish as a result of ocean pollution caused by factories, etc.?

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A: Of the mercury found in the natural world, only a minor portion consists of human-caused emissions.

A large majority of mercury in the natural world is there as it was emitted from the Earth's crust through volcanic activities and other phenomena and has then accumulated in the ocean over the course of 4.6 billion years of the Earth's history. Considering that the history of the Earth is much longer, one can assume that only a minor portion of the mercury accumulating in fish, shellfish, etc. comes from human-caused emissions. As, however, combustion of petroleum, coal and other such resources does cause mercury to be emitted into the atmosphere, the quantities of mercury emitted into the environment have increased significantly since the industrial revolution. From this perspective, and also for the purpose of preventing human-caused mercury pollution, it is important for us to continue working on reducing mercury emissions into the environment.

Q

How have mercury emissions from factories been addressed?

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A: In Japan, there are strict regulations against mercury emissions.

A period in our history (up until the early 1970s) is marked with a tragedy of seeing large quantities of methylmercury emitted from factories pollute the ocean and fish, which caused Minamata disease to appear. As we now have strict regulations to control mercury contained in effluent from factories and other such sources, no oceans surrounding Japan will be polluted with mercury coming from factory wastewater.

Q

Is it possible to check how much methylmercury is accumulated in one's body?

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A: Methylmercury accumulation levels can be checked by reference to mercury concentration in hair.

We humans take in a very small amount of methylmercury contained in fish and other food that we consume in our everyday diet. Half the methylmercury that enters the body gets excreted out in 70 days. As a certain percentage of it gets excreted into hair as well, the quantity of methylmercury accumulated in the body can be estimated by measuring mercury concentration in hair. Since Japanese people eat a lot of fish and shellfish, their methylmercury intake is relatively larger, hence their methylmercury concentration in hair higher, compared to other groups of people, including those in the West. Still, these levels are not substantial enough to cause any immediate impact on health.

Distribution of mercury concentration levels
in hair in the Japanese population

Q

Why do pregnant mothers have to be careful about their fish and shellfish intake?

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A: Because the baby in the womb is particularly sensitive to methylmercury.

Methylmercury affects the nervous system and, being in the developing stage, the nervous system of the baby in the womb is particularly vulnerable to such effects, resulting in the baby having a higher concentration of mercury than the mother. For this reason, it is advisable that pregnant mothers should be particularly careful about the volume and types of fish and shellfish that they eat so as not to take in too much methylmercury. As babies already born have already passed the most vulnerable period, there is not much need for concern. As, on an additional note, it is now known that methylmercury, unlike dioxin, etc., is not likely to pass into breast milk, breastfeeding mothers do not have to worry. Naturally, there are no concerns at all for men and non-pregnant women.

Q

Given the knowledge that the baby in the womb is vulnerable to methylmercury exposure, how should pregnant mothers go about eating fish?

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A: They should be careful about the types and volume of fish they eat.

As fish and shellfish (including whales and dolphins) are rich in good protein as well as DHA (docosahexaenoic acid) and EPA (eicosapentaenoic acid), both of which work to prevent vascular disorders and restrain allergic reactions, and also are good sources of calcium, etc., they form an important food group for the purpose of leading a healthy diet. Meanwhile, fish and shellfish (including whales and dolphins) have a high concentration of mercury that has been taken in from nature via a food chain. Implications given by past research are that eating an extremely large volume of fish or other such unbalanced dietary habit could result in a mercury intake that potentially impacts the fetus.

Q

What is the current status of research on impacts of a small quantity of methylmercury on a fetus?

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A: Research is still ongoing to study the effects of a small quantity of methylmercury on a fetus.

According to research reports from the Faroe Islands, Denmark, children with a higher intake of mercury from eating a lot of whale (black whale) meat, etc. have been found to have slight delays in weight, height and nervous system development. On the other hand, there are findings from research done in the Republic of Seychelles in the Indian Ocean that show absolutely no such effect and rather suggest that the nervous system of children who eat a lot of fish, etc. has been better developed. Thus, much still remains unknown about the effects of methylmercury taken in through fish and other food sources on fetal development. Given that, in terms of dietary habit and environmental surroundings, Japan is different from the Faroe Islands or Seychelles, research is likewise underway in Japan by the Ministry of the Environment, among others, to study the effects of methylmercury on fetuses.