Billions of tons of metallic mercury are being poured into the Pacific Ocean from over ten thousand coal-fired powerplants primarily in China and India, with the result that mercury levels in some common Pacific fish are rising far beyond safe levels for human or animal consumption. The primary problem is among larger fish, which concentrate the mercury when they eat smaller varieties which are also contaminated. Seaborne bacteria convert metallic mercury that reaches the ocean in polluted rain and wind to methyl mercury, which is absorbed into the cells of any animal–or human–who ingests it. Mercury is a brain poison, in that it kills neurons. Mercury poisoning, or hydrargyria, causes impairment of speech, vision and the sense of smell, and problems with sensation and co-ordination. It damages the brain, kidneys and lungs. In extreme cases, Minimata Disease can result. This illness was identified in 1956 after fishermen from Minamata, Japan suffered from severe mercury poisoning due to massive releases of methyl mercury from the Chisso corporation.
The riskiest fish are shark, swordfish, king mackerel, and tilefish. The least risky are canned light tuna, shrimp, pollock, salmon, and catfish.
Children are particularly at risk because mercury poisoning occurring at subclinical level can result in lowered intelligence and impaired attention spans.