Mercury: A toxic poison
No other metal better illustrates the diversity of effects caused by different chemical species than does mercury. On the basis of chemical speciation, there are three forms of mercury: elemental, inorganic, and organic compounds.
The major source of mercury is the natural degassing of the earth's crust, including land areas, rivers, and the ocean, and this source is estimated to produce on the order of 2700 to 6000 tons per year. The total man made release into the atmosphere is about 2000 to 3000 tons, and it is difficult to assess what quantities of mercury come from human activities and what quantities from natural resources. Run-off into natural bodies of water may contain mercury from both anthropogenic and natural sources, so it is difficult to assess how much released into the atmosphere is from man made or natural sources. Nevertheless, mining, smelting, and industrial discharge have been factors in the environmental contamination in the past. For instance, it is estimated that loss in water effluent from chloralkali plants, one of the largest users of mercury, has been reduced to 99% in recent years. Industrial activities not directly employing mercury or mercury products give rise to substantial quantities of this metal. Fossil fuel may contain as much as 1 ppm of mercury, and it is estimated that about 5000 tons of mercury per year may be emitted from burning coal, natural gas, and from the refining of petroleum products. Calculations based on the mercury content of the Greenland ice cap show an increase from the year 1900 to the present and suggest that the increment is related both to an increase in background levels of mercury in rainwater and to man made release. Regardless of source, both organic and inorganic forms of mercury may undergo environmental transformation. Metallic mercury may be oxidized to inorganic divalent mercury, particularly in the presence of organic material such as in the aquatic environment. Divalent inorganic mercury may, in turn be reduced to metallic mercury when conditions are appropriate for reducing reactions to occur. A potential source of alkyaltion of divalent mercury is methylation to dimethyl mercury by anaerobic bacteria. Methyl mercury is of major toxicology significance. If it is taken up into the food chain by fish, it may eventually cycle through humans or it may diffuse into the atmosphere and return to the earth's crust or to bodies of water as methyl mercury in rainfall1.
The Japanese Tragedy
The steadily mounting environmental contamination by mercury was ignored until a tragic series of events occurred in Japan. In the beginning many cats were seen to dance in the small fishing villages along Minamata Bay on Kyushu Island. They clearly were mad, because they screamed incessantly and often ended their dance and their lives by flinging themselves into the sea. This activity was first observed in...