Heavy metal is a poorly defined term that refers to a metallic or metalloid element with a high density and high atomic weight. There is considerable disagreement about whether both or only one of the parameters must be high and about where to set the cutoff values for the parameter(s). Values of 4, 5, and 6 g/cm3 (grams per cubic centimeter) have been used to define high density; 23, 40, and 63 have been used as cutoff values for atomic weight. To remedy these inconsistencies, another parameter, atomic number, has occasionally been used to define heavy metal, but this sometimes resulted in the inclusion of obviously light metals in the list of heavy metals. The absence of a consensus about the definition of the term ...