The most common forms of mercury that occur naturally in the environment are metallic mercury; the inorganic salts, mercuric sulfide and mercuric chloride; and methylmercury. Microorganisms and various natural processes can change these forms of mercury, from one form to the other. Methylmercury is the most common form created by these natural processes. This compound is of particular concern because it can bioaccumulate through the food chain. That is, small organisms and plants take up methylmercury as they feed, animals higher up the food chain eat these plants and organisms, and the process continues with methylmercury levels increasing up the food chain. Of particular concern is seafood, such as fish and shellfish which had bioaccumulated mercury. This is so as there may be harmful effects on humans when they consume seafood containing mercury.
Mercury in the environment is formed by either natural or anthropogenic processes. Natural mercury arises from the degassing of the earth’s crust via volcanic emissions or by evaporation from oceans. Anthropogenic causes are combustion of fossil fuels (specially coal), gold and mercury mining, chlorine,caustic soda, mirrors and medical equipment, industrial leaks, dentistry, waste and corpse incineration.