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.
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.