Mercury is a basic element forming a significant portion of the earth's crust. The amount present today is identical to that present at the time the earth was formed, but the distribution of mercury has been drastically changed as a result of continued offgassing from the earth and increasingly wide industrial use and subsequent release of fumes from burning coal and oil, and the disposal of mercury-containing products. When mercury ore, or mercury-containing coal, is dug out of the earth vaporization and oxidation occur, creating elemental mercury (Hg0), ionic mercury (Hg++) and particles containing inorganic mercury (Hgpart). In the course of time the mercury compounds become a component of the atmosphere, and begin global distribution. Mercury is unique in that 1) it is the only heavy metal that is a liquid at room temperature, 2) it is the heaviest liquid in existence, 3) it readily vaporizes into the atmosphere, 4) it can form stable covalent bonds with carbon, 5) most of its compounds are toxic, depending primarily upon their solubility in water and lipids and 6) it has no known physiological benefit to living organisms.
In earlier geologic times mercury, as a component of highly insoluble mercury ore (cinnabar), was buried along with other minerals and only released into the atmosphere during earth movements such as volcanoes and similar thermal situations. The natural background levels of mercury have been greatly enhanced in recent times by the release of thousands of tons of mercury and its compounds into the environment as a result of anthropogenic activities such as mining, combustion of contaminated fossil fuels, and disposal of mercury-containing wastes. Consequently we are facing dramatic increases in the levels of mercury spread over the surface of the planet - in the atmosphere, in the oceans, over the land and eventually in living organisms. The industrial revolution has exacerbated a massive transformation from underground (essentially inert) deposits of cinnabar, coal and crude oil to widely dispersed mercury vapor with its capacity for transformation into ionic mercury. In the presence of bacteria in soil and sediment methylation can occur transforming the ionic inorganic mercury into organic mercury potentially increasing its eventual uptake and accumulation in living organisms.