Occurrence and Characterization of Mercury
Mercury is a chemical element that occurs naturally in the environment and exists in several forms. These forms can be organized under three headings: metallic mercury (also known as elemental mercury), inorganic mercury, and organic mercury. All forms of mercury can enter the body and are potentially toxic.
Metallic mercury is a shiny, silver-white metal that is a liquid at room temperature and traditionally called "quick silver". Metallic mercury is the elemental or pure form of mercury (i.e., it is not combined with other elements). Metallic mercury metal is the familiar liquid metal used in thermometers and some electrical switches. At room temperature, some of the metallic mercury will evaporate and form mercury vapors. Mercury vapors are colorless and odorless. The higher the temperature, the more vapors will be released from liquid metallic mercury. Some people who have breathed mercury vapors report a metallic taste in their mouths. Metallic mercury has been found at 714 hazardous waste sites nationwide.
Inorganic mercury compounds occur when mercury combines with elements such as chlorine, sulfur, or oxygen. These mercury compounds are also called mercury salts. Most inorganic mercury compounds are white powders or crystals, except for mercuric sulfide (also known as cinnabar) which is red and turns black after exposure to light.
When mercury combines with carbon, the compounds formed are called "organic" mercury compounds or organomercurials. There is a potentially large number of organic mercury compounds; however, by far the most common organic mercury compound in the environment is methylmercury (also known as monomethylmercury). In the past, an organic mercury compound called phenylmercury was used in some commercial products. Another organic mercury compound called dimethylmercury is also used in small amounts as a reference standard for some chemical tests. Dimethylmercury is the only organic mercury compound that has been identified at hazardous waste sites. It was only found in extremely small amounts at two hazardous waste sites nationwide, but it is very harmful to people and animals. Like the inorganic mercury compounds, both methylmercury and phenylmercury exist as "salts" (for example, methylmercuric chloride or phenylmercuric acetate). When pure, most forms of methylmercury and phenylmercury are white crystalline solids. Dimethylmercury, however, is a colorless liquid.
Several forms of mercury occur naturally in the environment. The most common natural
forms of mercury found in the environment are metallic mercury, mercuric sulfide (cinnabar
ore), mercuric chloride, and methylmercury. Some microorganisms (bacteria and fungi) and
natural processes can change the mercury in the environment from one form to another.
The most common organic mercury compound that microorganisms and natural processes
generate from other forms is methylmercury. Methylmercury is of particular concern
because it can build up or biomagnify in certain edible freshwater and saltwater fish and
marine mammals to levels that are many times greater than levels in the surrounding water.