Love Eating Fish? What You Should Know About Mercury Poisoning

Updated: Aug. 24, 2021

Fish are a good source of health-promoting nutrients like omega-3 fatty acids. But some types of fish also contain mercury, a neurotoxin, so you should eat them in moderation.

The health benefits of eating fish

There are many health benefits of eating fish.

They tend to be rich in omega-3 fatty acids, which are good for the brain and the heart, notes Felicia Wu, PhD, the John A. Hannah distinguished professor in food safety, toxicology and risk assessment in the department of agricultural, food, and resource economics at Michigan State University in East Lansing.

Fish that contain high amounts of omega-3 are what Wu calls the “SMASH” fish, including, salmon, mackerel, anchovies, sardines, and herring.

Fish can also be a good source of calcium and other nutrients. That’s part of the reason why the American Heart Association recommends eating fish at least two times per week as part of a healthy diet.

However, some fish can have high levels of mercury, a neurotoxin, which is something that you definitely don’t want to over-consume.

Here’s what people who eat a lot of fish need to know about mercury poisoning.

What is mercury poisoning?

Mercury is a naturally occurring element found in soil, water, and air.

Although small amounts in food may not affect your health, too much could be toxic.

Most people associate mercury poisoning with fish consumption, but too much exposure is also possible in other ways.

“People can also be exposed to mercury that is released from containers or defective devices containing mercury, such as fever thermometers and dental fillings,” says Malina Malkani, a registered dietitian nutritionist and author. “Mercury is also sometimes found in pesticides, fungicides, and preservatives.”

It’s also important to understand that there are two main types of mercury—methylmercury as well as elemental (metallic) mercury.

The former, methylmercury, can be found in the tissues of fish and shellfish, according to the U.S. Environmental Protection Agency (EPA).

Exposure to the latter usually occurs after after a metallic mercury spillage or after a product that contains metallic mercury breaks, according to the EPA.

Neither is good for the body.

Mercury poisoning symptoms

Mercury poisoning in people of all ages can cause neurological effects. According to the World Health Organization and the EPA, some of the issues and symptoms include:

  • memory problems
  • tremors
  • anxiety, irritability, and depression
  • insomnia
  • headaches
  • vision loss
  • impaired speech
  • muscle weakness

Intake of mercury is especially dangerous during pregnancy and infancy, when sensitivity to the effects of mercury is increased, according to Malkani.

“High levels of mercury during pregnancy can cause stillbirths, craniofacial malformations, neural tube defects, brain damage, and infantile cerebral palsy,” she says.

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A list of high-mercury fish

The types of fish that have the most mercury are large fish that are high on the seafood chain, according to Wu.

This includes:

  • Larger tuna
  • Shark
  • Swordfish
  • Marlin
  • King mackerel

“The reason they contain more is that mercury bioaccumulates in the seafood chain, starting with plankton that take up mercury from the environment, which are then consumed by smaller fish, which are in turn consumed by larger fish,” she says.

A list of low-mercury fish

Other fish that tend to be low in mercury include:

  • Salmon
  • Trout
  • Sardines
  • Anchovies
  • Herring
  • Cod
  • Pollack
  • Shellfish, such as shrimp or scallops

The levels of mercury in fish and shellfish depend on the food sources of the fish, how long they live, and how high up they are in the food chain, Malkani.

High-mercury fish can have 10 to 20 times as much mercury as those fish with low levels.

Why is there mercury in fish?

Mercury is a naturally occurring element of the earth, so some will be naturally occurring in ocean and sea environments, Wu explains.

“However, there is also runoff into water from human activities that emit mercury in the environment, such as coal burning, gold mining, and various manufacturing activities,” she says.

Fish and shellfish absorb and accumulate mercury, and when larger fish eat smaller mercury-containing fish, it tends to accumulate in their body, says Malkani.

How much do fish-eaters need to worry about this?

In general, the benefits of eating fish outweigh the potential risks associated with mercury, Malkani says.

“Being strategic about smart seafood choices can help reduce the risk of high mercury intake,” she says.

Rarely is mercury intoxication from seafood consumption so high that it could cause acute health damage or death, Wu adds. But even low-dose mercury exposure is undesirable.

While deaths or symptomatic mercury poisoning cases are very rare, mercury exposure in general might affect the heart and other parts of the body (in addition to the brain) if levels are high enough.

“If you are only eating seafood once or twice a week, you should not be too worried about mercury,” she says. “It is only if you are consuming large amounts of the larger fish (tuna, shark, swordfish) that you should be concerned.”

The new 2020-2025 Dietary Guidelines for Americans recommends at least two servings of seafood per week for people of all ages.

Note that mercury poisoning builds up over time. So constantly eating high-mercury fish, in more-than-recommended amounts, could be an issue.

Again, Wu notes that mercury intoxication is linked to neurological effects and cognitive deficits in infants and young children.

For pregnant or breastfeeding women, in particular, the Dietary Guidelines for Americans recommends consuming eight to 12 ounces per week of low-mercury fish, and two to three ounces per week for toddlers ages one through two.

“Fish offers high-quality protein, iron, and vitamin D, which are critical nutrients during infancy,” Malkani says.

“Fish is also an excellent source of omega-3 fatty acids which are important for both brain development and eye health.”

(These are the best fish to eat.)

How to prevent mercury poisoning from fish

Choosing small, sustainably sourced fish helps ensure higher levels of omega-3 fatty acids and lower levels of mercury, according to Malkani.

“Great options include salmon (preferably wild Alaskan or canned in BPA-free packaging), steelhead trout, sardines, herring, arctic char, and North American mackerel,” she says.

Remember, infants and pregnant and or breastfeeding women should avoid eating high-mercury fish such as shark, swordfish, tilefish, king mackerel, and bigeye tuna.

If you keep all this in mind, and don’t over-eat the high-mercury fish, you should be in the clear.