Metals may be divided into four groups: (1) those with greatest toxicological signicance that are widespread in the human environment [arsenic (As), cadmium (Cd), lead (Pb), mercury (Hg), chromium (Cr), and uranium (U)], (2) essential trace metals [cobalt (Co), copper (Cu), iron (Fe), manganese (Mn), selenium (Se), and zinc (Zn)], (3) other metals with evident biological interest (nickel and vanadium), and (4) metals of pharmacological interest (aluminum, gallium, and lithium) (Domingo 1994).