Fluoride, the 13th most abundant element on the earth's crust, is a chemical ion of the element fluorine - fluoride has one extra electron that gives it a negative charge. Fluoride is found naturally in soil, water, foods, and several minerals, such as fluorapatite and fluorite. Fluoride concentration in seawater averages 1.3 ppm (parts per million), while in fresh water supplies the natural range is typically between 0.01 to 0.3 ppm. In some parts of the world, fresh water contains fluoride levels which are dangerous and can lead to health problems. Fluoride is also synthesized in laboratories. Synthesized fluoride is commonly added to drinking water, toothpaste, mouthwashes and various chemical products. In the following article we will discuss why it is added to drinking water, what its effects are - both positive and negative - and the controversy that surrounds it.