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Q& A - Mercury in Fish

What is mercury and how does it get into the environment?

Mercury is a metal that occurs naturally at low levels in rock, soil and water throughout North Carolina. Mercury is also released into the air, water and land when fossil fuels (coal, oil and natural gas) are burned; when municipal solid waste or medical waste is incinerated; during forest fires; and during some manufacturing processes.

How does mercury get into fish?

Most mercury pollution is released into the air and then falls directly into water bodies or onto land, where it can be washed into waterways. When mercury gets into water, bacteria can change it into a form called methylmercury, which is absorbed by tiny aquatic organisms. When fresh water and ocean fish eat those organisms, the mercury begins to build up in their bodies. When larger fish eat smaller fish, mercury can build up to high levels in the tissues of the big fish. Because it binds to the protein in fish muscles — the "meat" of the fish — mercury cannot be removed by cooking or cleaning the fish.

How can mercury affect people's health?

Mercury mostly affects nerve cells in the brain and spinal cord, especially in unborn babies and young children. The more mercury that gets into a person's body, the longer the exposure time, and the younger the person, the more severe the effects are likely to be.

Mercury is most harmful to the developing brains of unborn children and young children. Mercury can interfere with the way nerve cells move into position as the brain develops, resulting in abnormal brain development. Prenatal exposure to mercury can affect the way children think, learn, and problem-solve later in life. Effects can also occur in adults at much higher doses. The earliest obvious signs of mercury poisoning in adults are tingling or numbness of the lips, tongue, fingers, or toes; fatigue; and blurred vision.

How much fish can people safely eat?

Fish is an excellent, low-fat source of protein and other nutrients and an important part of a balanced diet. But some fish also contain unsafe levels of mercury. The amount of mercury in fish varies depending on the type of fish; their size, weight and age; what they eat; and where they live. Smaller, non-predatory fish with shorter life spans tend to have lower levels of mercury. Larger, older fish that eat smaller fish tend to have the highest levels. Fish with an average level of less than 0.4 milligram of mercury per kilogram of body weight are considered safe for eating.

North Carolina encourages people to eat fish low in mercury because of the health benefits to the heart as well as to the developing brains of children. While most freshwater fish in North Carolina contain very low levels of mercury and are safe to eat, some ocean fish and freshwater fish may contain high levels of mercury and may be unsafe. Click here for a list of fish high in mercury in the fact sheet, North Carolina's Advice on Eating Fish [English (PDF)] [Spanish (PDF)].

Current fish advisories in effect for North Carolina are available on the Occupational and Environmental Epidemiology Current Fish Advisories page as well as through the U.S. Environmental Protection Agency.

For Additional Information

  • Agency for Toxic Substances and Disease Registry: Mercury
  • U.S. Environmental Protection Agency: Fish Advisories External link
  • U.S. Food and Drug Administration: Popular Topics - Seafood External link
  • Gateway to Government Food Safety Information ( Consumer advice on seafood External link
  • Contact the Occupational and Environmental Epidemiology Branch of the Division of Public Health at (919) 707-5900.