Not all metals are essential micronutrients. Mercury, notably, does not play a role in promoting human health. Rather, it is considered the most toxic heavy metal in the environment and, according to the World Health Organization, one of the top 10 chemicals of public health concern.
Mercury exists in three forms: organic (e.g. methylmercury in fish), inorganic (e.g. batteries and disinfectants) and elemental (e.g. dental amalgam and thermometers). All types of mercury accumulate in the body over time.
Found naturally in the environment, soil and as a byproduct of pollution in the air, mercury is transformed by bacteria into the harmful organic compound methylmercury. In the United States, more than 3,000 lakes have been closed to fishing due to mercury contamination.
Seafood recommendations and safety
Fish and shellfish contain protein, omega-3 fatty acids and other essential nutrients. The 2015-2020 Dietary Guidelines for Americans recommend adults eat a variety of protein foods, including two servings of seafood per week. Unfortunately, consuming seafood is the most common way people in the U.S. are exposed to mercury.
While small amounts of mercury from seafood do not pose serious health concerns for most people, pregnant women and young children are more susceptible to potential neurological development issues. Pregnant and lactating women should eat eight to 12 ounces of low-mercury seafood per week, and children should be served smaller portions one to two times per week based on their age and calorie needs.
Nearly all fish and shellfish absorb and accumulate methylmercury. Larger, long living fish such as swordfish and bigeye tuna accumulate the highest levels and pose the greatest risk.
Mercury levels range from 0.003 parts per million in scallops to 1.123 parts per million in tilefish from the Gulf of Mexico. Methylmercury in fish is 95 to 100 percent absorbed in the intestines. Centers for Disease Control and Prevention data shows most people have blood mercury levels below 10 micrograms/liter; levels under 20 micrograms/liter are normal and not associated with negative health effects.
| Level | Mercury per serving | Examples |
| Best Choices | ≤0.15mcg/g | cod, flounder, haddock, scallop, tilapia, shrimp, canned light tuna, salmon, catfish |
| Good Choices | 0.15mcg/g up to 0.46 mcg/g | yellowfin and white albacore tuna, Chilean sea bass, grouper, halibut, Mahi Mahi, monkfish, snapper |
| Choices to Avoid | >0.46 mcg/g | shark, swordfish, king mackerel, marlin, orange roughy, bigeye tuna |
The U.S. Environmental Protection Agency, or EPA, recommends eating low-mercury fish and limiting higher-mercury fish. Limit locally caught fish to one serving per week, if there is no advisory.
The EPA and U.S. Food and Drug Administration categorize fish in three levels for women of childbearing age (16 to 49), especially those who are pregnant or breast-feeding, and children older than 6 months. Adults should eat two or three servings per week of “Best Choices” or one serving per week of “Good Choices.”
Mercury poisoning
Mercury poisoning takes months or years to develop with excess exposure. Skin exposure, inhalation or ingestion of mercury can have harmful effects on the nervous, digestive and immune systems, lungs, kidneys, brain, heart and eyes.
Toxicity depends on dose, exposure, frequency and health of the person. Symptoms vary by type of mercury and may include tremors, insomnia, memory loss, neuromuscular effects, headaches, and cognitive and motor dysfunction.
There are no consensus criteria for the diagnosis of mercury overload. Treatment includes removal of the source and, in some cases, chelation therapy, which binds toxins in the bloodstream.
Populations affected
Pregnant and breast-feeding women, infants and young children are most susceptible to the effects of mercury. Of greatest concern is how methylmercury crosses the blood-brain barrier and placenta. High levels of regular exposure can result in serious neurological effects to a fetus and may lead to mental retardation in children. There may be an association between elevated methylmercury and risk of cardiovascular disease in adults, but evidence is mixed.
Bottom line
Evidence shows the benefits of consuming low-mercury fish outweigh the risks of mercury overload. Young children and women who are of childbearing age, pregnant or breast-feeding should consume the recommended weekly servings of low-mercury fish and avoid eating high-mercury fish.
References
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