• What is mercury? Mercury occurs naturally and can also be released through industrial pollution. It falls from the air and can accumulate in streams and oceans, turning into methylmercury, a neurotoxin. Excessive amounts of mercury can cause brain and kidney damage, but most of the concern is focused on unborn babies and children under 6 because smaller amounts of mercury can damage developing nervous systems.

Fish absorb the methylmercury as they feed. It builds up more in some types of fish and shellfish than others, depending on what they eat. If you regularly eat types of fish that are high in methylmercury, it can accumulate in your bloodstream over time. The body purges it naturally, but it may take more than a year, which is why women who are pregnant or trying to conceive are warned about eating too much fish.

• The bad news: Nearly all fish and shellfish contain traces of mercury. In 2002, the Food and Drug Administration and Environmental Protection Agency issued a joint advisory that pregnant women, nursing women and children younger than 6 should limit seafood consumption to no more than 12 ounces, or two average meals, a week. In addition, moms and young children should avoid larger fish -- shark, swordfish, king mackerel and tilefish (found mostly in Hawaii) -- because they have the highest levels of methylmercury since they've lived longer.

• The good news: Most adults need not worry about the risks unless they consume large amounts of high-mercury fish. A number of public health groups now fear the government advisory was too cautious and that too many people cut fish, which is extremely healthy, from their diet. The FDA now says that for most kids and adults the benefits of eating fish outweigh the risks.

• The best fish: Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock and catfish. A variety of small fish is best. Don't fret over wild vs. farmed fish. A Harvard School of Public Health report shows that farm-raised fish contain as much, if not more, healthy Omega-3 fatty acids as wild species do. Fish sticks and fast-food sandwiches are commonly made from fish that are low in mercury, the EPA says.

• What about tuna? Albacore or "white" tuna has more mercury than canned light tuna. The EPA says it's OK to eat up to six ounces (one average meal) of albacore tuna per week.

• Learn more: Mercury levels in various types of fish, including a geographical breakdown on fish from local waters, is at www.epa.gov/waterscience/fish .

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