The most common oxidation numbers of arsenic are +5, +3, and −3, in which the element is able to form both inorganic and organic compounds in the environment and within the human body (Hei and Filipic, 2004). In combination with other elements such as oxygen, sulphur and chlorine the element is referred to as inorganic
arsenic and as combined with hydrogen and carbon as organic arsenic. Since most arsenic compounds are colourless and/or do not smell, the presence of arsenic in food, water or air, is a serious human health risk. Arsenic is toxic to the majority of organ systems; inorganic arsenic being more toxic than methylated organic arsenic (Mandal and Suzuki, 2002). The trivalent forms are the most toxic and react with thiol groups of proteins.
Arsenic toxicity involve genetic changes, the involvement of increased oxidative stress, enhanced cell proliferation and altered gene expression. Arsenic is known to induce hypoxia signalling pathways. Arsenic is a well-documented carcinogen in a number of studies (Waalkes et al., 2004). Chronic exposure to inorganic arsenic from contaminated water is responsible for various adverse health effects such as developing tumours of the lung, skin, liver, bladder and kidney. Skin lesions, peripheral neuropathy and anemia are hallmarks of chronic arsenic exposure. Arsenic is also a potential risk factor for atherosclerosis.