Arsenic occurs naturally throughout the environment, generally at low levels. Certain areas, such as parts of Taiwan, Mexico, Chile, Argentina, India, and the U.S. (Borum and Abernathy 1994; Cantor 1996) contain natural mineral deposits from which relatively high levels of inorganic arsenic may leach into well water. In addition to natural sources, application of arsenical pesticides, herbicides, and defoliants, burning of coal, and smelting of certain ores can introduce arsenic into the environment (Hood 1985), and arsenic is a contaminant at several sites on the U.S. National Priorities List of waste disposal sites (ATSDR 1993). Nearly 32 million pounds of arsenicals, principally arsenic trioxide, were imported into the U.S. in 1989, with the greatest use in the production of wood preservatives (ATSDR 1993). Human exposures tend to be oral and chronic, largely from the diet and drinking water (ATSDR 1993).