Fluoride is not fluorine.
If you're scanning your handy chemical table for fluoride, don't bother. Fluorides are actually compounds of fluorine -- that's F, on your periodic table -- combined with a metal. When we refer to the fluoride in our drinking water, toothpaste, or food additives in this article, we're mostly referring to sodium fluoride, although we'll see later on that for large-scale water distribution, other forms of fluororide are sometimes used.
Fluorine is a naturally occurring gas that is quite irritating to the eyes, skin, and respiratory system. When it is combined with sodium, however, it's easily dissolvable in water, making it an ideal way to add fluorides to drinking water, toothpaste, mouthwashes, and a variety of other dental products.
Fluoride does accumulate in plants and animals. Animals (and that includes you, mammal!) see fluoride accumulation in their bones or soft shell. That means that with a high level of fluoride accumulation, bones can become brittle and be at risk for skeletal damage. In extremely high doses, reproductive organs and fertility can be affected [source: ATSDR].
We'll explore some of the risks of over-exposure to fluoride later, but first we'll explore how poisonous fluoride really is.