Mercury levels have been steadily rising in fish and a new study suggests the mercury is coming from coal-burning power plants as far away as China and India.
The levels are especially high in shark, swordfish, king mackerel, and tilefish. Shrimp, canned light tuna, salmon, pollock and catfish have much lower levels.
And, unfortunately, more mercury is falling into the Pacific Ocean, source of much of the seafood eaten by humans, because of the concentration of coal-fired plants in Asia and India.
The rising contamination prompts health officials to warn that pregnant women and young children should avoid eating more than an occasional small serving of the affected fish.
A new study in Nature Geoscience traces the path of the toxins. When coal is burned, tiny amounts of mercury are emitted into the atmosphere, where they can travel thousands of miles before falling back to earth, either on land or sea.
On land, the mercury is not very hazardous but in the sea, microorganisms like planton convert it into another form of mercury, methyl mercury, which health experts say is very hazardous.
Small fish then eat the plankton, bigger fish eat the smaller fish and so on. Eventually, the toxic mercury winds up in the seafood that is eaten by humans.
Methyl mercury is especially harmful when eaten by pregnant women. Scientists say it kills neurons in the developing infant's brain, resulting in babgies that have three to eight points lower IQ than they would otherwise have had. They may also have shorter attention spans and behavioral issues.