The severity of mercury's toxic effects depends on the form and concentration of mercury and the route of exposure. Exposure to elemental mercury can result in effects on the nervous system, including tremor, memory loss and headaches. Other symptoms include bronchitis, weight loss, fatigue, gastro-intestinal problems, gingivitis, excitability, thyroid enlargement, unstable pulse, and toxicity to the kidneys.
Exposure to inorganic mercury can affect the kidneys, causing immune-mediated kidney toxicity. Effects may also include tremors, loss of co-ordination, slower physical and mental responses, gastric pain, vomiting, bloody diarrhea and gingivitis.
Symptoms of methylmercury toxicity, also known as Minamata disease, range from tingling of the skin, numbness, lack of muscle coordination, tremor, tunnel vision, loss of hearing, slurred speech, skin rashes, abnormal behaviour (such as fits of laughter), intellectual impairment, to cerebral palsy, coma and death, depending on the level of exposure. In addition, methylmercury has been classified as a possible human carcinogen by the U.S. Environmental Protection Agency. More recently, additional findings have described adverse cardiovascular and immune system effects at very low exposure levels.
Prenatal exposure to organic mercury, even at levels that do not appear to affect the mother, may depress the development of the central nervous system and may cause psychomotor retardation for affected children. Mild neurological and developmental delays may occur in infants ingesting methylmercury in breast milk. Affected children may exhibit reduced coordination and growth, lower intelligence, poor hearing and verbal development, cerebral palsy and behavioural problems.
Much of our knowledge of mercury toxicity comes from studying cases of occupational or acute community poisonings. In 1956, Minamata disease (later found to be methylmercury poisoning) was officially "discovered" in a Japanese community near a polyvinyl chloride plastic plant which was discharging untreated effluent containing methyl mercury chloride into Minamata Bay and the Shiranui Sea. Once in the bay's sediments, the mercury was readily absorbed by marine species, which resulted in the contamination of the entire ecosystem. Many of the local residents consumed fish and seafood from the mercury-contaminated waters as a staple part of their diet. Thirty-nine years later, 2,252 patients had been officially diagnosed with Minamata disease, and over 1,000 had died.
The largest ever outbreak of mercury poisoning occurred in Iraq in 1971 and 1972 when the population accidentally ate bread made from seed-grain treated with a mercury-based seed treatment pesticide. About 6000 cases of mercury poisoning were diagnosed. Over 500 cases were fatal. Instances such as these highlight the critical need for governments to implement mercury management strategies and to educate the public on the risks of mercury exposure.