Bjerrum's definition of heavy metals is based upon the density of the elemental form of the metal, and he classifies heavy metals as those metals with
elemental densities above 7 g/cm3 . Over the years, this definition has been modified by various authors, and there is no consistency. In 1964, the editors of Van Nostrand's International Encyclopedia of Chemical Science and in 1987, the editors of Grant and Hackh's Chemical Dictionary included metals with a
specific gravity greater than 4. A little later, in 1989, 1991, and 1992, Parker, Lozet and Mathieu, and Morris chose a defining
specific gravity "greater than 5". However, Streit used a
specific gravity of 4.5 as his reference point, and Thornton chose
6. The Roempp Chemical Dictionary gives
3.5 as a possible defining specific gravity. However you work with these definitions, it is impossible to come up with a consensus. Accordingly, this basis for
defining heavy metals must be abandoned as yielding nothing but confusion.
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