Typically the term refers to elements of atomic
number 21 or higher (e.g. Scandium or above) The term
heavy metal chiefly arose with discussions of pollutants
discharged to the environment in the form of air, water
or soil contaminants, while many heavy metals have
considerable toxicity, others are considered not deemed to
possess significant toxic properties, and, in fact, several
of these elements including zinc, iron, copper, chromium
and cobalt are necessary for metabolic function for a large
class of organisms (Hogan 2011).