Infamous Occurrences
The most noted occurrence of clinical mercury poisoning was in Minamata, Japan. Starting before World War II and continuing until 1968, industrial mercury-containing waste was dumped into the sea near Minamata. The townspeople’s diet consisted of locally caught fish; the fish were heavily contaminated with mercury. Many people suffered permanent disability, and more than a thousand died.8 The effect of mercury poisoning was so great the disease was called Minamata’s disease.
Another epidemic occurred in the early 1970s in Iraq. Grain intended for use as seed for planting was treated with a mercury-containing fungicide. Accidentally, the local population used the grain for making bread, resulting in many cases of clinical mercury poisoning.8
One of the most historically interesting stories of mercury poisoning involves the first emperor of the Qin Dynasty in China, Qin Shi Huang Di, who lived from 259 to 210 B.C. At age 12 when Qin Shi Huang ascended to the throne, China was divided into warring feudal states. By asserting ruthless power, he succeeded in conquering the last warring province and unified China in 221 B.C. Although his rule was brutal and tyrannical, his accomplishments included standardization of the systems of measurement and weights, currency, and Chinese script. He was also a builder, conscripting labor for the development of roads, canals, and—most famously—his own mausoleum. The tomb of Qin Shi Huang at Xian is famous for its size and contents, including the army of more than 8,000 life-size, terracotta warriors.9
Qin Shi Huang Di (Di means supreme god) set out on a tour in 210 B.C. seeking eternal life in the “Islands of the Immortals.” It is not clear if his entourage found the site, but he was given a potion or pills, made by his court scientists, to make him immortal. The medication contained a toxic compound of mercury, and he died.9 The mausoleum was completed just in time to accommodate the emperor.
Exposure and Toxicity
Other than occupational exposure to mercury, the most important mercury exposures in the United States are dietary, the use of mercury amalgam in dentistry, and mercury used as a stabilizing agent in vaccines. The risk of toxicity depends on the form of mercury involved. Elemental mercury is poorly absorbed through the gastrointestinal tract; methylmercury, on the other hand, is readily absorbed if ingested. As previously noted, inhaled elemental mercury vapor is easily absorbed in the lungs.
Consensus supports the fact that dietary ingestion of methylmercury as found in large game fish—tuna, swordfish, shark, king mackerel, and tilefish—is the most important exposure to organic mercury. Small fish eat the mercury-contaminated plant life on the ocean bottom and are consumed by larger fish, and those fish are consumed by larger fish. Thus, the large game fish in the food chain accumulate high concentrations of mercury. It is recommended that children and pregnant women limit consumption of these types of fish.8
Dental silver for tooth fillings is a mixture of 50% mercury and 50% metal powder, such as silver, copper, or zinc. Because dental amalgam uses elemental mercury, gastrointestinal absorption is limited. Two studies examining more than 500 children each followed the neuro-psychiatric development of children with mercury amalgam fillings with a control group. No statistical differences were identified in IQ, memory, visual/motor, and renal function between the two groups over five- and seven-year follow-ups. Yet, doubts linger concerning long-term follow-up and questions raised with respect to the incidence of multiple sclerosis and Alzheimer’s disease.10 Overall, dental mercury amalgam is believed to be relatively safe with no evidence of acute toxicity.