Mercury climbs the food chain, concentrating most in the biggest predatory fish
Small organisms absorb a little mercury, the fish that eat them store more, and the hunters at the top carry the most. This build-up, called biomagnification, is why tuna and swordfish hold far higher levels than little fish, and why eating fish is now the main way people take in the metal.
Mercury comes in several forms: liquid metal, vapour, salts and organic compounds, and they behave very differently. Organic forms are the most toxic. Methylmercury, the chief organic type people encounter, accumulates through the food web, while dimethylmercury is so dangerous that a few microlitres on skin, or even on a latex glove, can kill. Salts mostly harm the gut and kidneys because they struggle to cross into the brain, and the more water-soluble mercuric salts are usually more acutely toxic than mercurous ones.
Methylmercury also works slowly. It gradually ties up selenium the body needs for certain enzymes, so symptoms can lag well behind exposure; the longest recorded delay after a single exposure is five months. Damage appears to track the peak level reached rather than how long exposure lasted. Symptoms reported include numbness, tingling, poor coordination, and trouble seeing, hearing or speaking. High-level methylmercury exposure is known as Minamata disease, and affected children can develop pink, peeling skin.
Medicine itself was once a source. Calomel, a mercury compound popular in Victorian times, left Scottish seamstress Elizabeth Storie disabled after a doctor dosed her for a mild childhood illness, as she recounted in an 1859 autobiography. Civil War soldier Carleton Burgan was disfigured by calomel given for an infection in 1862. Mercury compounds in indoor latex paints, used against mildew, were removed in 1990 after toxicity cases.
Coal plants and other human activity emit roughly half of atmospheric mercury, volcanoes much of the rest. Small-scale gold miners, including an estimated 20,000 to 50,000 galamseys in Ghana, face high risk from crude processing.
Source: Mercury poisoning
