As we've discussed, fluoride, like many other minerals or naturally occurring substances, can be harmful in high doses. Over-exposure to fluoride can lead to dental or skeletal fluorosis.
Dental fluorosis is caused by moderate amounts of excessive fluoride in early childhood. According to the World Health Organization, ingesting fluoride after the age of six will not cause dental (sometimes called enamel) fluorosis, which is marked by a staining or pitting of the teeth.
However, long-term exposure to high levels of fluoride can cause skeletal fluorosis to develop in later years. Skeletal fluorosis can cause a change in the actual structure of the bones, and a bending of the limbs. Ligaments can calcify, resulting in painful muscle impairment and stiffening.
This kind of change to the body is seen mostly in places with an extremely high level of naturally-occurring fluoride in drinking water. While developed countries like the United States and Japan (which both have so-called "fluoride belts") can adjust the fluoride in the water to prevent harm, many of these belts are in impoverished regions like Sudan, Kenya, India, and Afghanistan.
We'll come back to how fluorosis and fluoride treatment is addressed globally in an upcoming section. But first, let's take a look at why fluoride has become such a hot-button issue both politically and socially.