Fish is one of the healthiest foods you can eat.
That's because it's a great source of protein, micronutrients, and healthy fats.
However, some types of fish can contain high levels of mercury, which is toxic.
In fact, mercury exposure has been linked to serious health problems.
This article tells you whether you should avoid fish over potential mercury contamination.
Mercury is a heavy metal found naturally in air, water, and soil.
It's released into the environment in several ways, including through industrial processes like burning coal or natural events like eruptions.
Three main forms exist — elemental (metallic), inorganic, and organic (1).
People can be exposed to this toxin in a number of ways, such as breathing in mercury vapors during mining and industrial work.
You can also be exposed by eating fish and shellfish because these animals absorb low concentrations of mercury due to water pollution.
Over time, methylmercury — the organic form — can concentrate in their bodies.
Methylmercury is highly toxic, causing serious health problems when it reaches certain levels in your body.
SUMMARY Mercury is a naturally occurring heavy metal. It can build up in the bodies of fish in the form of methylmercury, which is highly toxic.
The amount of mercury in fish and other seafood depends on the species and the levels of pollution in its environment.
Another study discovered that one-third of fish caught on the New Jersey shore had mercury levels higher than 0.5 parts per million (ppm) — a level that could cause health problems for people who eat this fish regularly (3).
Overall, larger and longer-lived fish tend to contain the most mercury (4).
Larger fish tend to eat many smaller fish, which contain small amounts of mercury. As it’s not easily excreted from their bodies, levels accumulate over time. This process is known as bioaccumulation (6).
Mercury levels in fish are measured as parts per million (ppm). Here are the average levels in different types of fish and seafood, from highest to lowest (5):
- Swordfish: 0.995 ppm
- Shark: 0.979 ppm
- King mackerel: 0.730 ppm
- Bigeye tuna: 0.689 ppm
- Marlin: 0.485 ppm
- Canned tuna: 0.128 ppm
- Cod: 0.111 ppm
- American lobster: 0.107 ppm
- Whitefish: 0.089 ppm
- Herring: 0.084 ppm
- Hake: 0.079 ppm
- Trout: 0.071 ppm
- Crab: 0.065 ppm
- Haddock: 0.055 ppm
- Whiting: 0.051 ppm
- Atlantic mackerel: 0.050 ppm
- Crayfish: 0.035 ppm
- Pollock: 0.031 ppm
- Catfish: 0.025 ppm
- Squid: 0.023 ppm
- Salmon: 0.022 ppm
- Anchovies: 0.017 ppm
- Sardines: 0.013 ppm
- Oysters: 0.012 ppm
- Scallops: 0.003 ppm
- Shrimp: 0.001 ppm
SUMMARY Different types of fish and other seafood contain varying amounts of mercury. Larger and longer-lived fish usually contain higher levels.
Interestingly, seawater contains only small concentrations of methylmercury.
In fact, larger, predatory fish may contain mercury concentrations up to 10 times higher than the fish they consume. This process is called biomagnification (11).
U.S. government agencies recommend keeping your blood mercury levels below 5.0 mcg per liter (12).
One U.S. study in 89 people found that mercury levels ranged from 2.0–89.5 mcg per liter, on average. A whopping 89% had levels higher than the maximum limit (13).
Additionally, the study noted that higher fish intake was linked to higher mercury levels.
SUMMARY Eating a lot of fish — especially larger species — is linked to higher levels of mercury in the body.
Exposure to mercury can cause serious health problems (16).
In both humans and animals, higher levels of mercury are associated with brain problems.
A study in 129 Brazilian adults found that higher levels of mercury in hair were associated with a decrease in fine motor skills, dexterity, memory, and attention (17).
However, more studies are needed to confirm this link.
One study in 1,800 men found that those with the highest levels of mercury were twice as likely to die from heart-related problems than men with lower levels (24).
Nevertheless, the nutritional benefits of fish likely outweigh the risks from mercury exposure — as long as you moderate your consumption of high-mercury fish (25).
SUMMARY Higher levels of mercury can harm brain function and heart health. However, the health benefits of eating fish may outweigh these risks as long as you limit your intake of high-mercury fish.
Mercury in fish does not affect everyone in the same way. Therefore, certain people should take extra care.
At-risk populations include women who are or may become pregnant, breastfeeding mothers, and young children.
Fetuses and children are more vulnerable to mercury toxicity, and mercury can easily be passed to a pregnant mother's fetus or a breastfeeding mother's infant.
One animal study revealed that exposure to even low doses of methylmercury during the first 10 days of conception impaired brain function in adult mice (26).
Additionally, some studies suggest that certain ethnic groups — including Native Americans, Asians, and Pacific Islanders — have a greater risk of mercury exposure due to diets traditionally high in fish (29).
SUMMARY Pregnant women, breastfeeding mothers, young children, and those who regularly consume large amounts of fish have a higher risk of problems related to mercury exposure.
Overall, you shouldn't be afraid of eating fish.
Fish are an important source of omega-3 fatty acids and provide multiple other benefits.
In fact, it is generally recommended that most people eat at least two servings of fish per week.
However, the Food and Drug Administration (FDA) advises people at high risk of mercury toxicity — such as pregnant or breastfeeding women — to keep the following recommendations in mind (30):
- Eat 2–3 servings (227–340 grams) of a variety of fish every week.
- Choose lower-mercury fish and seafood, such as salmon, shrimp, cod, and sardines.
- Avoid higher-mercury fish, such as tilefish from the Gulf of Mexico, shark, swordfish, and king mackerel.
- When choosing fresh fish, look out for fish advisories for those particular streams or lakes.
Following these tips will help you maximize the benefits of eating fish while minimizing your risks of mercury exposure.