Arsenic (As)  is a semi-metal element that has no taste, odor, or color, and is not easily dissolved. It occurs naturally in the environment, as a by-product of mining, or agricultural areas when materials containing arsenic are used as industrial poisons. Arsenic is considered highly toxic and a carcinogen. Arsenic is not easily dissolved in water. Therefore, if it is found in a water supply, it usually comes from mining or metallurgical operations or from runoff from agricultural areas where materials containing arsenic were used as industrial poisons. Arsenic in water has no color, taste or odor. Arsenic and phosphate easily substitute for one another chemically, therefore commercial grade phosphate can have some arsenic in it. Arsenic is highly toxic and has been classified by the US EPA as a carcinogen, which was derived from toxicity considerations rather than carcinogenicity.