Mercury in Fish and Shellfish

Mercury is a highly poisonous metal that poses a serious health risk to developing fetuses, babies and children, who may suffer brain damage and learning disabilities from prolonged or repeated exposure to small amounts of mercury. Although the metal occurs naturally in the environment, mercury levels in our air, land and water have increased dramatically since the rise of industrialization in the late 19th century.

Mercury is released into the air primarily from industrial sources, falls to the ground in rain or snow and is deposited into water bodies, where it is converted into another highly toxic form of mercury (called methylmercury). Methlymercury then builds up in the tissues of fish and other animals, and in high concentrations can pose serious health risks to people who frequently eat contaminated fish. Based on the available data on mercury concentrations in fish tissue, Environmental Defense recommends limited consumption of certain fish (see chart).

What is Mercury and Where Does It Come From?

Mercury is a naturally occurring toxic metal that exists at low levels throughout the environment, and as an element it never breaks down or disappears. Mercury cycles through the environment, passing between the air, land and water, and affects plants and animals.

According to the Environmental Defense report Out of Control and Close to Home [PDF], local sources of mercury (i.e., coal-fired power plants, waste incinerators, and certain factories and mining operations) can create hot spots of pollution that can impact local communities. Mercury emissions are a global problem as well, since mercury can be transported through the atmosphere over large distances.

Mercury enters streams, rivers, lakes and oceans primarily through rain and surface water runoff. Bacteria can then convert it to an organic form called methylmercury -- the form that is dangerous to people. Although mercury levels are almost always low in water bodies, methylmercury biomagnifies up the food chain. When small fish with low mercury levels get eaten by bigger fish, the amount of mercury biomagnifies. For this reason, long-lived fish and top-level predators like swordfish and shark often have the highest mercury levels. According to EPA, mercury concentrations in fish can be 1 to 10 million times the mercury concentration in the water.

The problem of mercury-contaminated fish is widespread. According to the EPA's National Listing of Fish and Wildlife Advisories, mercury advisories increased 163% between 1993 and 2003 (from 899 to 2,362). The number of states that have issued mercury advisories has risen steadily from 27 in 1993 to 45 in 2003. As of 2003, more than 13 million lake acres and almost 800,000 river miles were covered by some type of mercury advisory. Currently, 21 states have statewide mercury advisories in freshwater lakes or rivers, and 11 states have statewide advisories for mercury in their coastal waters. Statewide advisories urge people to limit their consumption of all fish and shellfish from freshwater or coastal areas.

What Are the Health Risks Associated With Consuming Mercury-Contaminated Fish?

Mercury targets the nervous system and kidneys. Developing fetuses, infants and young children are at the highest risk from mercury exposure, since their brains and nervous systems are still forming. Fetuses can absorb mercury directly across the placenta, and nursing infants can get it from their mother's breast milk. This is why it is so important for women of childbearing age to minimize their consumption of fish with high mercury levels. It can take 12-18 months for women in their childbearing years to significantly rid their body burden of methylmercury.

Children exposed to mercury before birth may exhibit problems with mental development and coordination, including how they think, learn and problem-solve later in life. These neurological symptoms may appear similar to cerebral palsy. Developmental and neurological damage can be irreversible for fetuses and young children, but as children get older, the risk associated with mercury exposure decreases.

Mercury exposure can also harm adults. Symptoms can include numbness, burning or tingling of the extremities (lips, fingers, toes); fatigue; weakness; irritability; shyness; loss of memory and coordination; tremors; and changes in hearing and blurred vision. Extremely high mercury levels can permanently damage an adult's brain and kidneys, or even lead to circulatory failure.

How Can I Reduce the Risks of Eating Seafood Contaminated With Mercury?

Since methylmercury binds to proteins, it is found throughout fish tissue, including muscle tissue that makes up fish steaks and fillets. Therefore, cleaning and cooking methods that can reduce amounts of other contaminants (like trimming fat and removing skin and organs) are not successful in reducing mercury levels in fish.

The best way to reduce exposure to methylmercury is to moderate or eliminate your consumption of predatory or long-lived fish (such as swordfish, shark, orange roughy and tuna). Refer to Health Alerts to see which species are known to have high levels of mercury. Be especially conservative if you are a woman of childbearing age or are feeding young children.

What Fish Should I Avoid?

Fish low in contaminants are an important part of a healthy diet. That's why Environmental Defense recommends limited consumption of certain fish because of their elevated mercury levels.

Posted: 01-Jan-1900; Updated: 19-Jan-2008