Acute mercury poisoning (which today is less common) is associated with ulcerations of the stomach and intestine and with toxic changes in the renal (kidney) tubules. Anuria (failure to form urine) and anemia may occur.
Death and illness from mercury poisoning have long been recognized. The expression "mad as a hatter" came from the occupational hazard of hat makers in the 1800's who were poisoned by mercury salts used in the making of felt hats. Consumption of food contaminated with methyl mercury in the second half of the 20th century resulted in death or illness for thousands of people around Minamata Bay, Japan.
Minamata disease is, in fact, the name of a disorder caused by methyl mercury poisoning that was first described in the inhabitants of Minamata Bay and resulted from their eating fish contaminated with mercury industrial waste. The disease is characterized by peripheral sensory loss, tremors, dysarthria, ataxia, and both hearing and visual loss.
Even the unborn child is at risk from Minamata disease. Methyl mercury readily crosses the placenta from mother to fetus and is teratogenic, particularly to the developing brain. Children born with Minamata disease can have growth deficiency, microcephaly (an abnormally small head), severe mental retardation and be deaf and blind.
Minamata disease has not been confined to Minamata where the source of the mercury was primarily from eating fish caught in the contaminated Bay. Other sources of maternal exposure to methyl mercury have included flour made from seed grain treated with methyl mercury (which affected at least 6,500 people in Iraq) and meat from animals raised on mercury-tainted grain (in New Mexico, USA).
Mercury is discharged into the air by such sources as coal-burning power plants, incinerators, and mining. It eventually contaminates waterways where it is converted by bacteria into methylmercury. This molecule collects in the fatty tissues of fish and the animals that eat fish. Fish are the major dietary source of mercury, but it can also enter the body in other ways including diet, dental fillings, pharmaceuticals and contact with mercury metal or its compounds.
The U.S. Food and Drug Administration (FDA) as of 1999 recommends limiting consumption to 1 kilogram (2.2 pounds) of fish and shellfish a week and considers 1 part per million of mercury in seafood to be safe. However, top predator fish such as shark and swordfish can have mercury concentrations over 1 part per million. The FDA recommends no more than one serving a week of these two fish for most people and no more than one serving a month for pregnant or nursing women or those who may become pregnant.