ABSTRACT
Pollution caused by metals is a worldwide phenomenon. Among the many metals, lead (Pb), mercury (Hg), cadmium (Cd), arsenic (As), chromium (Cr), zinc (Zn), and copper (Cu) are of most concern for toxicity, although the last three metals are also essential nutrients in animal and human nutrition. Lead is one of the ancient metals and has been used by humans for several thousand years. Lead plays an important role in the economy of all industrialized countries in the world. Cadmium is a transition metal in Group IIb along with Zn and Hg. It is frequently associated with Zn ores. Excluding the United States, China is the world's largest producer of primary Cd, with 7600 metric tons refined in 2015. Mercury is the only common metal that is liquid at room temperature. It is rare in the Earth's crust. Although several forms occur, the principal ore is cinnabar.
