The element arsenic exists in three allotropes: grey arsenic, density 5.73 g/ mL; yellow arsenic, density 1.93 g/ mL; and non stable black amorphous arsenic, density 4.73 g/mL. Arsenic (atomic weight 74.92) shows metallic as well as non metallic properties. In its inorganic compound it presents different oxidation states: -3, 0, +3, +5. It is released into the air by volcanoes and is a natural contaminant of some deep-water wells. Arsenic is used to preserve wood, as a pesticide, to produce glass, in copper and other metal manufacturing, in the electronics industry and in medicine.

Occupational exposure to arsenic is common in the smelting industry (in which arsenic is a by-product) and in the microelectronics industry. Low-level arsenic exposure takes place in the general population through the use of inorganic arsenic compounds in common products such as wood preservatives, pesticides, herbicides, fungicides, and paints; through the consumption of foods treated with arsenic-containing pesticides; and through the burning of fossil fuels in which arsenic is a contaminant. The toxicity depends on its valence oxidation state and on its form inorganic or organic. In general, inorganic arsenic is more toxic than organic arsenic, and trivalent arsenite is more toxic than pentavalent and zero- valent arsenic. Arsenic, particularly in its trivalent form, inhibits critical sulphydryl — containing enzymes. In the pentavalent form, the competitive substitution of arsenic for phosphate can lead to rapid hydrolysis of the high-energy bonds in compounds such as ATP. The normal intake of arsenic by adults primarily occurs through ingestion and averages around 50 gg/ d. After absorption, inorganic arsenic accumulates in the liver, spleen, kidneys, lungs, and gastrointestinal tract. It is then rapidly cleared from these sites but leaves a residue in keratin-rich tissues such as skin, hair, and nails.

Guide line value for drinking water is 0.01 mg/L. It is a provisional value, as there is evidence of a hazard, but the available information on heath effects is limited (WHO, 2008).