Mercury poses danger for those at the top of the food chain
Mercury (Hg on the periodic table) is a dense heavy metal that is liquid in pure elemental form at room temperature. When ingested by humans, it affects the nervous system, brain, kidneys, heart, lungs and immune system, causing developmental problems in children and, at higher doses, various health problems in adults.
Sometimes called quicksilver, mercury has long been used to make a variety of products, including light bulbs, switches, thermometers and even hats -- the term "mad as a hatter" is believed to have originated from neurological damage suffered by craftsmen using mercury to cure felt headgear.
Mercury occurs naturally in rocks, soil, air and coal, and it is emitted from volcanoes. When coal is burned to create electricity, mercury is released. Coal-burning power plants are the country's largest source of mercury emissions caused by man, at about 40 percent.
The amount of mercury on Earth can never change, but when it is pushed into water or the atmosphere by human activities or natural phenomena, it can pose a health risk. Mercury emitted into the air from a factory or coal-burning plant falls back to earth mostly in the surrounding region, but it can also be transported thousands of miles in the atmosphere. In the United States, the emissions aren't usually concentrated enough to be inhaled in toxic quantities.
Most mercury that affects Americans comes from fish. When airborne mercury falls into the ocean or bodies of water, microorganisms turn it into a soluble form called methylmercury, which is consumed by organisms low on the food chain and passed on in increasingly higher concentrations to predators. Fish that are highest on the food chain, such as tuna, are the most likely to contain a lot of mercury.
-- Kari Lydersen