Human exposure to mercury
When mercury is deposited in waterways, bacteria convert it to methylmercury. Methylmercury builds up in the tissue of fish, which may then be eaten by wildlife (e.g., eagles, osprey, common loons, river otters, minks) and by people. Because mercury is tightly bound to the fish muscle tissue, there is no method of cooking or preparation that will remove or reduce mercury once it's in fish.
This doesn't mean you should stop eating fish. It's a good source of protein and is low in saturated fat. You can still get the benefits of eating fish by using moderation in how much you eat.
The two organ systems most likely affected by methylmercury are the central nervous system and the kidneys. The groups most vulnerable to the effects of mercury toxicity are women who are pregnant or may become pregnant, nursing mothers and young children. The most significant concerns about chronic exposure to low concentrations of methylmercury in fish are for neurological effects in children and developing fetuses.
Although human exposure to mercury occurs most frequently through eating contaminated fish, other human exposures to mercury can occur. People have been exposed to mercury from inhaling mercury vapors from broken fluorescent lamps, gas regulators or even home fever thermometers. There have been cases of mercury exposures from accidental swallowing, but these cases are rare.