Fluoride (F-) is the mildly toxic ionic form of the element fluorine. It is a common additive to toothpaste, mouthwash, and city water throughout the developed world. It is commonly believed to prevent tooth decay, since it tends to bond with the calcium in teeth. It is believed by many to help harden teeth, although some research has shown the opposite effect. Unfortunately, fluoride also strongly attracts aluminum, which some studies suggest causes Alzheimer's disease.
Human teeth (and those of many other animals as well) are covered with dental enamel, which is the hardest substance in the human body. It consists primarily of calcium phosphate, in the form of hydroxyapatite crystals. As fluoride is very reactive substance, it chemically bonds to this hydroxyapatite, resulting in fluoridated hydroxyapatite and fluorapatite. This resulting material seems to be more resistant to acids than the hydroxyapatite itself.
In 1978, Michigan became the first state to repeal mandatory fluoridation laws, leaving the decision to local jurisdictions. Many municipalities have since mounted successful challenges against mandatory fluoridation laws.