Know Mercury Levels to Choose Safer Seafood

Fish may be an anti-aging superfood, but all too often, eating the wrong kinds of fish can raise the level of mercury in your body. This is especially dangerous for pregnant and breastfeeding women because fetuses and newborns are very sensitive to mercury. Learn about the mercury levels in fish, the best types to eat, and how much is considered safe.

Risks of Mercury

All fish contain trace amounts of mercury. For most people, the small amounts in fish do not pose a health problem. Some fish, however, contain high amounts of mercury—enough to cause damage to a developing fetus or newborn. That's why pregnant and nursing mothers must be very careful about the amounts and types of fish they eat. Young children should also avoid eating fish high in mercury.

There are three types of mercury: organic, inorganic, and elemental (metallic). Methylmercury is an organic form of mercury that is highly toxic to humans. It's a metal that turns to liquid at room temperature, and over time, can slowly accumulate in the bodies of humans, fish, and other animals that eat fish. Those with industrial occupations such as coal miners may also be subjected to methylmercury exposure and poisoning.

Methylmercury is toxic to the central nervous system—the brain and spinal cord. It causes irreversible damage. The brains of unborn babies and infants are especially susceptible. How much damage is done depends on how much you are exposed to the chemical.

The effects of methylmercury poisoning include cerebral palsy, blindness, deafness, impaired mental functioning, impaired lung function, growth problems, and having a small head.

Mercury levels can gradually build up in adults, too—eventually becoming harmful to health. High mercury levels can cause permanent damage to the kidneys and brain. Mercury is naturally eliminated from your body, but it can take several months for it to pass through. For that reason, women who are planning to become pregnant may want to begin to avoid fish that are higher in mercury before they become pregnant.

Mercury Levels in Fish

The mercury found in fish is methylmercury—but how does it get there? First, mercury is released into the air from environmental events such as volcanic eruptions or forest fires, or from human-made activities like burning coal, oil, and wood. After mercury hits the air it eventually settles onto the land and into the water, where bacteria and other microorganisms convert mercury into methylmercury. Then, fish and shellfish in the water begin to absorb it.

Fish that eat other fish tend to have higher levels of methylmercury. Large fish have more mercury for the simple reason that they usually live longer. They have more time to build up higher levels of mercury in their bodies.

See the lists below for general mercury levels of many common types of fish and how often to safely consume each type, according to the National Resource Defense Council (NRDC) and the FDA.

Low-Mercury Fish

According to the FDA, there are many types of fish that tend to have low levels of mercury. You can safely eat two to three servings a week of the following fish—but pregnant and nursing women and small children should not eat more than 12 ounces (or two servings) a week:

• Anchovies
• Catfish
• Clam
• Crab
• Crayfish
• Croaker (Atlantic)
• Flounder
• Haddock
• Hake
• Herring
• Mackerel (North Atlantic, Chub)
• Mullet
• Oyster
• Perch
• Pollock
• Salmon
• Sardine
• Scallop
• Shrimp
• Sole
• Squid
• Tilapia
• Trout
• Whitefish

Moderate-Mercury Fish

Other fish have moderate levels of mercury, which makes them safe to eat in moderation—six servings or fewer per month. However, pregnant and nursing women and small children should avoid eating the following fish:

• Bass (saltwater, striped, black)
• Buffalofish
• Carp
• Cod (Alaskan)
• Halibut
• Lobster
• Mahi Mahi
• Monkfish
• Perch (freshwater)
• Snapper
• Skate
• Tilefish (Atlantic)
• Tuna (canned chunk light)

High-Mercury Fish

Several larger fish contain higher levels of mercury, which could contribute to potentially dangerous levels of mercury levels in the body if consumed in excess. It is recommended to eat just three servings or less of these types of fish per month, but pregnant and nursing women and small children should avoid them entirely:

• Bluefish
• Grouper
• Sea Bass (Chilean)
• Mackerel (Spanish, Gulf)
• Croaker (White, Pacific)
• Sablefish
• Perch (ocean)
• Tuna (canned albacore, yellowfin)

Fish to Avoid

There are some fish that are very high and mercury and should be avoided altogether, particularly bluefish and grouper, according to the NRDC.

Additionally, the FDA recommends that adults and children should avoid eating the following large fish:

• King mackerel
• Marlin
• Orange Roughy
• Shark
• Swordfish
• Tilefish (from the Gulf of Mexico)
• Tuna (Bigeye, Ahi)

A Word From Verywell

Fish can be part of a healthy diet, but it's important to pay close attention to the types of fish your eating. Choose low-mercury fish whenever possible, and consume fish with moderate levels of mercury only in moderation. As a rule of thumb, it's best to avoid large fish with high levels of mercury entirely. If you're pregnant or are trying to become pregnant, keep a list of the different types of contaminated fish handy to stay safe and protect your developing baby.

Fortunately, there are many types of low-mercury fish you can safely enjoy. Explore recipes for fish, like these:

• Crusted Pesto Salmon
• Greek Shrimp with Feta and Olives
• Sweet Pan-Seared Tilapia Over Golden Rice

Remember, fish is a great source of protein and omega-3 fatty acids. In fact, many people around the world include fish and seafood as part of a balanced diet for overall health and longevity. Stay informed about the mercury levels in different types of fish so you can stay healthy and continue to enjoy this versatile addition to any meal.