Yes, I know fluoride can be toxic
. And yes, I can geek out on chemistry
.
Low levels of fluoride is toxic to bacteria. The recommended level of fluoride in water is 0.5 to 1.0 mg/L (milligrams per liter). This is significantly lower than the amount toxic to humans.
Fluoride is toxic to humans if 5-10 g of NaF is consumed, 32-64 mg fluoride/kg body weight. That is quite a lot compared to what is added to drinking water.
Fluoride, when present as tooth enamel (made largely of calcium phosphate) is forming in children, creates teeth with much harder enamel that resists bacterial erosion.
The mechanism of fluoride action is quite different than the mechanism of chlorine's toxicity. Fluoride is a negative anion that inhibits an enzyme in glucose metabolism. Most bacteria are significantly more sensitive to low levels of fluoride than we are. Chlorine (Cl2) is a gas that is a potent oxidizer. In water, it generate hypochlorous acid, bleach. It destroys nearly all bacteria, fungus, or virus particles at low levels. Animal tissues can also be damaged by chlorine, if exposed to the chlorine gas or high levels dissolved in water.
If you boil water, or pass it through an activated charcoal filter, chlorine is removed. Dissolved fluoride, cannot be so easily removed. That requires distilling the water, reverse osmosis, or passing the water through a deionizing column.