Heavy metals are chemical elements with a specific gravity greater than 5.0. Among the 90 naturally occurring elements, 21 are non-metals, 16 are light metals, and the remaining 53 are heavy metals. The majority of naturally occurring elements, which from a biological perspective is not very helpful. However, only a limited number of heavy metals are soluble under physiological conditions and bio-available to living organisms. Some of these are considered essential for life, including Fe, Co, Mo, Mn, Zn, V, Ni, Cr, Cu, and W. They are required as micronutrients or trace elements because they often act as co factors in biochemical reactions, but they are toxic when present in excess. Other heavy metals such as Cd, Hg, Ag, Pb, and U have no known biological function and are toxic even at very low concentrations. Heavy metals are often present as naturally occurring in environment, but the prevalence of heavy metals in the environment has increased more recently as a result of human activity. Metal processing facilities, mines, dumps, sewage sludge, and traffic are all sources of heavy metals. In addition, the intense use of phosphate fertilizers and municipal sewage sludge in agriculture contributes to the accumulation of heavy metals in soils. The increasing concentrations are potentially toxic to biological systems.