Heavy metals include basic metal elements such as lead,
mercury, cadmium silver nickel, vanadium, chromium and
manganese. They are natural components of the earth’s crust;
they cannot be degraded or destroyed, and can be transported
by air, and enter water and human food supply. In addition,
they enter the environment through a wide variety of sources,
including combustion, waste water discharges and manufacturing
facilities. To a small extent they enter human bodies
where, as trace elements, they are essential to maintain the
normal metabolic reactions. However, at higher (although relatively
low) concentrations they can become toxic (Jarup,
2003).