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Good Fish, Bad Fish
As with many things in science, there is controversy about what levels of mercury are safe.
"There is evidence that mercury taken out of a bottle or out of a smokestack is toxic," Lands says. "But there is no evidence that methylmercury in seafood causes a problem."
Both the National Academy of Sciences and the FDA have convened expert groups to study the risks and benefits of seafood consumption. Their findings are expected later this year.
Mercury isn't the only concern. Polychlorinated biphenyls (PCBs) -- substances linked to skin problems, reproductive disorders, liver disease and neurological problems and suspected of causing cancer -- also accumulate in both wild and farm-raised seafood.
So what should consumers do?
Whether fish is farm-raised or wild, "it would be unfortunate if people cut their consumption," Willett says. Neither the mercury concern nor the PCB contamination levels are "enough for people to reduce their fish intake."
Also lost in much reporting is the fact that any potential problems of mercury contamination appear to be limited to children and to women of childbearing age.
"Other adults should not be concerned about mercury at all," notes Joshua Cohen, author of a recent analysis of mercury exposure conducted for the Harvard School of Public Health's Center of Risk Analysis.
Some of the environmental groups that see dangers in mercury-tainted seafood also urge consumers to eat at least the federally recommended minimum of two meals a week. "Even the higher-mercury-containing fish, if they are not eaten frequently, are not a big concern," says physician Gina Solomon, senior scientist at the Natural Resources Defense Council (NRDC), which tracks mercury levels in seafood.
There's also reason to think that even fish laced with mercury has more benefits than risks for a fetus. Omega-3s are so crucial for brain and nervous system development "that limiting fish consumption during pregnancy may cause the very harms that everyone involved has been working to prevent," says Nicholas Ralston, who studies mercury at the University of North Dakota's Energy & Environmental Research Center.
Those worried about mercury's effects in pregnant women often point to a study recently conducted in Denmark's Faroe Islands. The study found that children born to mothers with the highest levels of mercury had a very slight decrease -- just a millionth of a second -- in the time it took for a sound to pass from their ears to their brains. Recent findings show that the children who are now 14 years old have persistent attention deficits and score lower on tests that measure motor skills and verbal ability. But often overlooked is that the major source of mercury in the Faroe Islanders' diet was not fish, but rather pilot whales, which have very high concentrations of the chemical.
Other recent research, including an ongoing 20-year study among residents of the Seychelles Islands in the Indian Ocean, has not linked adverse effects with increased fish consumption.