The term "heavy metal" refers to any naturally occurring metallic chemical element that has a relatively high density, at least five times greater than that of water. At low concentrations, heavy metals can be toxic or poisonous.
Mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), thallium (Tl), and lead (Pb) are some examples of heavy metals.
Heavy metals are significant health hazards because of their characteristic of bio-accumulating, which is the gradual buildup of chemicals over time in a biological organism. Cadmium, mercury, lead and arsenic are included in the World Health Organization’s list of 10 chemicals of major public concern.
An example of an environmental disaster involving heavy metals occurred in 1932 in Japan. Sewage containing mercury was released into Minamata Bay. The mercury accumulated in the marine life, eventually leading to mercury poisoning in the population. The poisoning of the population by mercury was so severe that it caused the population to develop a neurological syndrome also known as Minamata disease.