Humans can be exposed to arsenic from a variety of environmental, occupational and medicinal sources. Exposure from drinking water is the most common source nowadays. Inorganic arsenic is metabolized by two main biochemical reactions: reduction and oxidative methylation. Recent studies have confirmed a reactivation of the toxic effects of arsenic associated with such methylation process, because the methylated metabolites, especially the trivalent forms, are more toxic than the inorganic arsenicals. Chronic exposure to arsenic can cause a variety of cancerous and noncancerous health hazards in humans. The main non-cancerous health hazards include cardiovascular disease, skin lesions, neurological problems, diabetes mellitus and hypertension.