Mercury in fish is an important health issue for people who eat them, especially for women who are or may become pregnant, nursing mothers, and young children. Fish and shellfish take in mercury from their environment, often in the form of methylmercury, a type of mercury that is very harmful. Mercury builds up in the bodies of humans over time, so when people eat seafood that contains mercury, it can lead to mercury poisoning. Mercury is harmful to both natural environments and humans because it is a metal that can damage the nervous system.

In ecosystems where humans manage fish for food production, mercury moves up the food chain as fish eat small plankton and also from other sources like underwater sediment. Fish products can have different amounts of heavy metals, including mercury and pollutants from water pollution. Fish that live a long time and are high in the food chain, such as marlin, tuna, shark, swordfish, king mackerel, and tilefish, have higher levels of mercury than other species. Whales and dolphins also build up mercury and other pollutants. Groups that eat whale meat, such as people in Japan, Iceland, Norway, and the Faroe Islands, are also at risk of mercury exposure.

Biomagnification

The most important way humans and animals get mercury from food is by eating fish. Mercury and a form called methyl mercury are found in very small amounts in seawater. Algae, which are the first part of the food chain, take in methyl mercury from the water. These algae are then eaten by small fish and other sea creatures. Fish absorb methyl mercury quickly but remove it slowly from their bodies. Over time, the mercury builds up mainly in their internal organs, though some also stays in their muscle tissue. This causes mercury to increase in the fat tissue of animals at higher levels of the food chain, such as zooplankton, small fish, and larger fish. Older fish tend to have more mercury in their bodies. When animals eat these fish, they also take in the mercury that has built up. This explains why large fish like swordfish and sharks, and birds like osprey and eagles, have much higher mercury levels in their bodies than would be expected from direct exposure to mercury alone. Some animals can have mercury levels up to ten times higher than the animals they eat. This process is called biomagnification. For example, herring has mercury levels of about 0.1 parts per million, while shark meat has mercury levels greater than 1 part per million.

Origins of mercury pollution

There are three types of mercury emissions: human-caused, re-emission, and natural, such as from volcanoes and geothermal vents. Human-caused sources are responsible for 30% of all emissions, natural sources for 10%, and re-emission for 60%. Even though re-emission is the largest source, the mercury released from these sources likely originally came from human activities.

Human-caused sources include burning coal, producing cement, refining oil, small-scale gold mining, waste from consumer products, dental fillings, the chlor-alkali industry, vinyl chloride production, and mining and smelting of metals. In 2010, humans released about 1,960 metric tons of mercury. Most of this came from burning coal (24%) and gold mining (37%).

Re-emission is the largest source of mercury emissions and happens in many ways. For example, mercury deposited in soil can be released again into the environment during floods. Another example is forest fires, which release mercury absorbed by plants back into the air. It is difficult to measure exactly how much mercury is re-emitted, but studying this process helps scientists understand how long it takes for reduced human-caused emissions to affect the environment. Mercury released into the environment can also enter oceans. A 2008 study estimated that 3,700 metric tons of mercury were deposited into oceans that year, with rivers carrying about 2,420 metric tons. Much of the mercury in oceans is re-emitted, and up to 300 metric tons is converted into methyl mercury. Only 13% of this enters the food chain, which is still about 40 metric tons per year.

About 40% of mercury found in fish comes from coal-burning power plants and chlorine production plants. In the United States, coal-fired power plants are the largest source of mercury pollution. Chlorine plants once used mercury to extract chlorine from salt, but this practice has mostly been replaced by a safer method. Coal naturally contains mercury, which is released into the air when burned for electricity. Most of this pollution can be reduced with pollution-control devices.

In the United States, power plants release about half of the country’s mercury emissions. In other countries, like Ghana, gold mining often uses mercury compounds, exposing workers to high levels of mercury. Mercury from gold mines contributes to the buildup of mercury in aquatic food chains.

Elemental mercury often comes from coal power plants, and oxidized mercury often comes from incinerators. Oil-fired power plants also release mercury into the environment. The energy industry plays a major role in introducing mercury into the environment. To reduce mercury in seafood globally, it is important to identify major energy producers and consumers.

Aquaculture, or farming aquatic organisms, sometimes uses fish feed that contains mercury. A study by Jardine found no clear link between mercury in fish food and mercury levels in farmed or wild aquatic organisms. However, mercury from other sources can still affect aquaculture. In China, farmed fish species like bighead carp, mud carp, and Siniperca chuatsi carried 90% of mercury found in measured fish in a study by Cheng. This study showed that mercury builds up in food chains even in controlled aquaculture environments, mainly from sediments containing mercury, not from fish feed.

The Hawaii Institute of Marine Biology has noted that fish feed used in aquaculture often contains heavy metals like mercury, lead, and arsenic. These concerns have been shared with organizations like the United Nations’ Food and Agriculture Organization.

Mercury can enter freshwater systems through point sources, such as industrial spills, and through extended flooding. In Canada, mercury poisoning in Grassy Narrows was likely caused by a paper mill spill, a point source. Non-point sources, like floods, create conditions for bacteria to convert mercury into methyl mercury, the toxic form that builds up in aquatic food webs. Studies at the Experimental Lakes Area in Ontario, Canada, have examined these effects using whole-lake experiments and non-lethal fish muscle tests.

The U.S. Geological Survey (USGS) predicts a 50% increase in mercury levels over the next few decades, linked to industrial emissions rather than natural causes. Reducing emissions from industrial plants could lower mercury levels. Some countries, like South Korea, are starting to track mercury sources and use pollution-control systems such as air pollution control devices (APCDs), electrostatic precipitators (ESPs), and bag-based filters. Flue-gas desulfurization, typically used to remove sulfur dioxide, can also help remove mercury. However, efforts to control mercury emissions are still in early stages in many countries.

Health effects and outcomes

Mercury in fish affects different groups of people in different ways. Some ethnic groups and young children are more likely to experience harm from methyl mercury poisoning. In the United States, research by Wallace found that 16.9% of women who identify as Native American, Asian, Pacific Islander, or multiracial consume more mercury than is considered safe. A study of children in the Faroe Islands showed that mothers who ate pilot whale meat during pregnancy caused brain-related problems in their children. A 2020 study in coastal Colombia found that people born when fish had high mercury levels had worse educational and job outcomes than those born when mercury levels were low.

Although many studies show high mercury levels in fish, medical cases of poisoning are often not reported, making it hard to link mercury in fish to human health issues. Environmental problems include pollution from factories and construction, which can harm both the environment and human health. Some harmful substances may not cause symptoms over time, but mercury can cause physical symptoms quickly if the body accumulates it.

In the United States, the Environmental Protection Agency (EPA) sets limits for mercury in human blood that are not likely to cause serious harm. The EPA also enforces environmental rules. Studies show that pregnant women and women of childbearing age are most likely to be exposed to methyl mercury through eating fish. The U.S. Food and Drug Administration (FDA) advises pregnant women and young children not to eat raw fish because they have weaker immune systems and are more likely to get sick from food.

The EPA also provides guidelines for safe mercury levels in humans. High mercury exposure can cause problems with vision, hearing, speech, coordination, and muscle strength. Studies in the United States have shown that eating fish can affect child development. Long-term research confirms that human activities release mercury into the environment, which builds up in marine life. Health officials must address mercury exposure by identifying its sources in the human body. Some Native American tribes in the U.S. face higher mercury risks because fish is a major part of their diet. Studies show these groups experience more mercury poisoning than other groups. A medical study found that 20% of women in Sweden who eat a lot of fish had mercury levels above the EPA’s safety limit. This study also found that eating certain fish species could harm fetal brain development, meaning families planning to have children should avoid these fish.

Children are more likely to be harmed by mercury because they eat more food, water, and air relative to their body size, and they grow quickly, making them more vulnerable to mercury damage. Prenatal exposure to methyl mercury can lead to behavioral issues in babies and lower cognitive test scores. Research suggests 250,000 women in the U.S. may expose their unborn children to mercury levels higher than federal guidelines.

Economic status does not seem to affect mercury exposure, as studies found no differences in mercury levels in fish like tuna, bluefish, and flounder based on where people shop or live. In Ghana, people eat large amounts of fish, which increases their risk of mercury exposure.

In the Lower Amazon region, mercury in fish comes from human activities like gold mining and deforestation. Studies found mercury levels in fish muscle tissue ranging from 0.01 to 0.67 μg/g, with some species exceeding safety limits. People who rely on fish as a main food source face health risks. Mercury levels in fish and shrimp in the Amazon indicate environmental contamination, especially near mining areas. During the rainy season, herbivorous fish make up most of the diet for many women in an Amazonian village. People who eat fish daily in the Amazon also have higher mercury levels in their hair. Recent research in western Pará, Brazil, found that several fish species eaten by local communities have mercury levels above international safety standards. These findings highlight the risks of long-term mercury exposure in areas affected by mining.

The worst mercury poisoning case in modern history occurred in Minamata, Japan, in the 1950s. People exposed to high levels of methyl mercury before and after birth suffered serious brain-related problems. Victims also showed signs of mental health issues linked to brain damage.

A 2014 study by the U.S. Geological Survey found that methyl mercury levels in fish were highest in wetland areas, such as coastal streams in the Southeastern United States. Mercury levels were also high in the Western U.S., but only in streams that had been mined for mercury or gold.

Levels of contamination

The danger of eating fish depends on the type of fish and its size. Larger fish tend to have more mercury in their bodies. Sharks, like the mako shark, have very high mercury levels. A study of fish near the coast of New Jersey found that one-third of the fish tested had mercury levels above 0.5 parts per million, which could be harmful to people who eat this fish often. Another study of fish sold in markets near Southern Italy showed that heavier fish have more mercury in their bodies. Mercury levels, measured in milligrams per kilogram of fish, increase as the fish grow larger. Anglerfish near Italy were found to have mercury levels as high as 2.2 milligrams per kilogram, which is higher than the safety limit of 1 milligram per kilogram. Each year, Italy catches about one-third of its fish from the Adriatic Sea, where these anglerfish were found.

Fish that eat in certain ways may have higher mercury levels than others. Grass carp near China have much less mercury than bighead carp. This happens because bighead carp are filter feeders, which means they eat large amounts of small plankton and also suck up sediments that contain mercury. Grass carp do not eat this way.

Scientists from the US government tested fish in 291 streams across the country for mercury. They found mercury in all the fish tested, including those in remote, rural streams. Twenty-five percent of the fish had mercury levels above the safety limits set by the EPA for people who eat fish regularly.

Legislation

Since the Minamata disaster, Japan has improved its rules for controlling mercury. In the 1970s, Japan worked to reduce how much mercury was used and produced. A major effort was stopping the mining of inorganic mercury, which ended by 1974. Mercury use dropped from 2,500 tons per year in 1964 to about 10 tons per year in recent years. After these changes, Japan created rules that limit how much mercury can be in many materials.

These rules help reduce mercury pollution in the environment, which lowers the amount of mercury in fish and, through biomagnification, in humans. Japan also set rules for how much mercury is allowed in the environment. Japan wants to help other countries avoid similar disasters by supporting international rules about mercury. However, the public in Japan still has limited information about mercury risks. Recommendations from Japan’s fish advisory are less strict than those in the United States.

In the United States, the Environmental Protection Agency (EPA) has regulated mercury emissions under the Clean Air Act. In 2005, the EPA tried to control mercury from power plants using a cap-and-trade system, but the rule was canceled in 2008 because the court said the EPA had not properly classified power plants as sources of hazardous air pollution. Later, the EPA classified mercury emissions from power plants as hazardous, leading to the 2012 Mercury and Air Toxics Standards (MATS) rule. This rule targets mercury from power plants and other sources. Mercury in the air dissolves in oceans, where microbes change it into methyl mercury, which enters the food chain and builds up in fish.

The EPA estimated that the MATS rule would prevent about 90% of mercury emissions from power plants. It also predicted health benefits worth $37 billion to $90 billion by 2016, with an estimated annual cost of $9.6 billion. In 2020, the Trump administration weakened the MATS rule by questioning the EPA’s previous calculations, making the rule vulnerable to legal challenges.

The EPA also created rules for mercury in industrial wastewater, including battery manufacturing, chemical production, oil and gas drilling, and metal smelting.

In the European Union, the regulation (EU) 2017/852 covers all stages of mercury use. It bans the production and trade of many mercury-containing products, stops the use of mercury in industrial processes, and limits mercury use in dental fillings. The EU recently measured mercury levels in topsoils using the LUCAS survey. The average mercury content in EU topsoils is 38 micrograms per kilogram, with about 45,000 tons of mercury in the top 20 centimeters of soil.

Some people believe global rules are needed because mercury pollution spreads across borders. Pollution from one country can affect others. International rules began in the 1970s with agreements about shared water bodies. The 1972 Oslo Convention and 1974 Paris Convention reduced ocean pollution by banning ship dumping and limiting land-based pollution near coasts. The first global mercury rule was the Basel Convention of 1989, which limits the movement of mercury across borders. The 1998 Convention on Long-Range Transboundary Air Pollution, adopted by many countries, aims to reduce emissions of heavy metals. In the early 2000s, voluntary programs became more common, but the Minamata Convention, named after the Japanese city affected by mercury pollution, represents a global effort. After four years of talks, the Minamata Convention was adopted by over 140 countries and ratified after 50 countries signed it. The convention requires countries to stop mercury releases from small-scale gold mining and reduce emissions from coal burning.

Current advice

The EPA described the complicated ways mercury moves through the environment and ends up in living things in its 1997 Mercury Study Report to Congress. Because methyl mercury and high amounts of elemental mercury can harm a fetus or young children, government groups like the EPA and FDA advise pregnant women, women planning to become pregnant within one or two years, and young children to eat no more than 6 ounces (170g, one average meal) of fish per week.

In the United States, the FDA set a limit of 1.0 parts per million (ppm) for methylmercury in commercial fish from the ocean and freshwater. In Canada, the total mercury limit in fish is 0.5 ppm. The "Got Mercury?" website, created by Turtle Island Restoration Network, a non-profit group, has a tool to help people check mercury levels in fish.

Fish with naturally low mercury levels include shrimp, tilapia, salmon, pollock, and catfish (FDA, March 2004). The FDA lists shrimp, catfish, pollock, salmon, sardines, and canned light tuna as low-mercury seafood. However, some tests show that up to 6% of canned light tuna may have high mercury levels. A 2008 study found that mercury levels in tuna meat are lower when the fish has more fat, which may help reduce mercury intake by diluting it. Many fish used in sushi also have high mercury levels.

The FDA says that eating fish and shellfish is generally safe for most people. However, some seafood may have mercury levels that could harm an unborn baby, especially the baby’s brain and nervous system. In young children, high mercury levels can harm nervous system development. The FDA gives three guidelines for young children, pregnant women, and women who may become pregnant:

• Avoid eating shark, swordfish, king mackerel, or tilefish from the Gulf of Mexico because these fish may have high mercury levels.
• Eat up to 12 ounces (two average meals of 170g each) per week of a variety of fish and shellfish that are low in mercury. Common low-mercury fish include shrimp, canned light tuna, salmon, pollock, and catfish. Albacore or "white" tuna may have more mercury than canned light tuna, so it should be limited to no more than 6 ounces (one average meal) per week.
• Check local fish safety advisories for fish caught by family and friends in local lakes, rivers, or coastal areas. If no advisories are available, eat up to 6 ounces (one average meal of 170g) of fish caught locally each week, but eat no other fish that week.

Research shows that selenium in fish can help protect against the harmful effects of methylmercury. Fish with higher amounts of selenium compared to methylmercury are better choices because selenium can bind to methylmercury and help the body remove it without absorbing it.

In 2012, the European Food Safety Authority (EFSA) reported on chemical contaminants found in food from over 20 European countries. They found that fish meat and fish products were the main sources of methylmercury in people’s diets across all age groups. Fish like swordfish, tuna, cod, pike, whiting, and hake were especially linked to high mercury levels. EFSA recommended a safe weekly intake of 1.3 micrograms of methylmercury per kilogram of body weight.

Additional sources

• Kidd, K. and Batchelar, K. (2011). "Mercury." Part of the book Fish Physiology: Homeostasis and Toxicology of Non-Essential Metals, edited by Wood, C. M., Farrell, A. P., and Brauner, C. J., pages 238 to 297. Published by Academic Press. ISBN 9780123786340.
• Lubick, N. (2009). "Mercury levels in the ocean are rising." Nature. doi: 10.1038/news.2009.218.
• Rasmussen, R. S.; Nettleton, J.; and Morrissey, M. T. (2005). "A Review of Mercury in Seafood: with a focus on tuna" (PDF). Journal of Aquatic Food Product Technology. 14 (4): pages 71 to 100. doi: 10.1300/J030v14n04_06. S2CID 94568585. The original PDF was saved on April 15, 2016, and accessed on January 9, 2012.