Elements are the fundamental materials of which all matter is composed; they can be classified as metals, metalloids, and non-metals. Approximately, three-quarters of all known chemical elements are metals. They are characterized by high electrical and thermal conductivity as well as by malleability, ductility, and high reflectivity of light [1]. Examples of metals and metalloids are lead (Pb), mercury (Hg), cadmium (Cd), and arsenic (As): they have relatively high densities, atomic weights, or atomic numbers, and they are toxic even at low concentrations. They can be the cause of environmental degradation and generate health effects in humans. Generally, they are referred to as heavy metals (HMs), although the use of this terminology is regularly questioned by the scientific community [2]. In 2002, an IUPAC report argued that the term ‘heavy metal’ is meaningless and misleading, suggesting that a more precise classification based on the periodic table and the chemical basis of toxicity would be more useful [3]. Nevertheless, the term HM continues to be used worldwide. Other metals, such as zinc (Zn), selenium (Se), copper (Cu), and nickel (Ni), are considered essential, at low concentrations, for metabolic/ biological functions in humans and are therefore required in the diet in a certain defined amount, above which they may become toxic [4].