Mercury can exist in many different chemical and physical forms in the environment (see Box 2). Those different forms are called “species.” In this document, we will use the general term “mercury” to refer to all forms of the element, while “methylmercury” will be used to distinguish organometallic mercury compounds (essentially those that contain carbon) from inorganic forms of mercury. Pure mercury is known as elemental mercury or metallic mercury. Ionic mercury (both mercuric and mercurous) com- bines with other substances to form different chemical compounds (such as salts) in water, soil and rock. Mercury is present in the atmosphere mainly as a metallic vapour. It occurs in various organic forms, mainly in.
