ABSTRACT
Absolute deœnitions of “heavy metals” are elusive in modern science. Many different deœnitions have been proposed. Some are based on density, some on atomic number or atomic weight, and some on chemical properties or toxicity [1]. One deœnition holds that they are elements with a speciœc weight higher than 6 g/cm3 [2] (see Figure 23.1). But no single deœnition œts well in modern usage. The term “toxic metals” has become to some extent synonymous with heavy metals, but that term is equally problematic because levels of toxicity are highly variable between different metals and vegetation species. At best, heavy metals can be classiœed as a poorly deœned subset of elements that exhibit metallic properties and are toxic to living organisms at some level of concentration or exposure. The term “heavy metals” itself has been criticized as functionally meaningless [1,3].
