Metals are defined chemically as elements which conduct electricity, have a metallic luster, are
malleable and ductile, form cations, and have basic oxides. However, this term has been loosely
used by biologists to refer to both the element and compounds. Heavy metals are by definition
applied to metals of density 5gcm-3 and greater, and “heaviness” does not necessarily relate to
“toxicity” (Duffus, 2002). Trace metals are metals found in low concentration, in parts per million
or less, in matrices like water, tissue or sediments. It has to be understood that all metals may
become toxic, remembering the fundamental rule of toxicology that all substances are toxic if
exposed to a high enough concentration. However, essential metals are toxic at higher doses than
“toxic” metals as the essential metals are those which are required for some biological function, an
absence of which causes deficiency symptoms. “Toxic” metals are those which have no known
biological function as yet and an increase in bioaccumulation may lead to deleterious effect and
ultimately death if exposure is to a high enough concentration or dose or for a long period of time.
Examples of essential metals include copper (Cu), manganese (Mn) and zinc (Zn) while “toxic”
metals include cadmium (Cd), lead (Pb) and mercury (Hg)

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