Definition of “Heavy Metals” and Their Role in Biological Systems

At first glance, it would appear to be a rather simple matter to define a “heavy metal” – it is a metal that is “heavy”. Unfortunately, a more in-depth consideration reveals a huge amount of problems with this simple definition. This definition is meant to suggest that the density of a heavy metal is high, but this physical property is quite meaningless in the context of plants and other living organisms. Plants do not deal with metals in their elemental (valence state of 0) forms; they are not accessible to plants. Metals are only available to them in solution, and it is necessary for metals to react with other elements and form compounds before they can be solubilised. Once such a chemical compound is formed (e.g. a salt), the density of the metal does not play any role. We do not know of any correlation between the density of a metal and its physiological or toxicological effects, or even the chemical properties of its compounds. Therefore, let us leave the question of how to define a “heavy metal” until later, and first consider the definition of a “metal”.