Almost 100 years ago, people discovered that communities that had fluoride naturally in the water supply had fewer cavities and tooth decay and that communities that had higher amounts of it had almost no tooth decay and kids with brown teeth. It's since been discovered that people who grew up with fluoride but move to an area without it, will get more cavities in the new area, and that those who grew up without it but move to it, will have less cavities in the new area. It is a temporary effect that's mostly environmental.
This is the reason that dentists promote fluoride: (Not for the brown teeth) It's good for stronger teeth in small amounts. After that, the benefits are pretty minimal, and they're not shy about that in class. In at least three different courses they've taught us the advantages and disadvantages of fluoride. Fluoride tends to bind to and soak up free calcium in your blood (which is valuable for enzyme activity and crucial to proper nerve conduction) making it unusable by the body. It can also interfere in cellular processes to some degree if taken into cells. This makes fluoride a weak anti-microbial as well, but it's a small effect.
Fortunately, as a water soluble ion/mineral, it is easily filtered by kidneys and urinated out. Since the kidneys can filter an entire person's blood supply several times a day, fluoride is quickly passed into urine in the small amounts we encounter it in nature and daily living.
This amount is small. In most communities, fluoride is added to water to be 1 part per million (ppm), that's one miligram per liter of water. Studies have shown that between .7 ppm and 1.2 ppm, the maximum benefit can be had per risk. Above this, there is very little extra advantage and increasing risk of undesirable effects. The effect is a topical one. For a while, it was taught that fluoridated water worked by getting fluoride into a person's system and becoming incorporated into growing and developing teeth, but this has been refuted a lot recently and most researchers agree the effect works on the surface more than anything. Small concentrations in water and higher concentrations on topical pastes and things provide the needed fluoride.
Fluoride is considered poisonous above a few grams. The least amount of fluoride known to kill someone was 4 grams eaten by a little kid within an hour. Generally, 5-10 grams is fatal. It is dangerous. But very few people have access to that much fluoride. The FDA allows water at around 1 ppm, and readily available toothpastes at 1000 ppm, which aren't supposed to be swallowed. That comes to about .15% in almost every toothpaste since that is the cap set by the FDA. Some fudge numbers due to rounding and marketing tactics, but it's all the same. A small child should be okay if they happen to eat a whole tube of toothpaste, that's why it's available over-the-counter. Prescription toothpastes can have up to 5000 ppm and would have a good chance of making a child sick were they to eat the whole tube, but they'd probably survive the encounter.
University of the Pacific's Dental School promotes some mouth rinses with calciums, fluorides, and other things to make teeth strong as well as varnishes smeared onto teeth following a polishing. These can have up to 22,500 ppm. The high concentrations are compensated by the very small amounts needed and the monitoring of the use within the clinic. A very very small amount of varnish goes a very long way and is released/dissolved slowly over several hours. Fluoride Foams are looked down on in the clinic here since children tend to swallow them, most of it is wasted not being on the surface of the teeth, and they're pretty messy. Also, they teach us that until children learn to spit out toothpaste, don't bother using a fluoridated toothpaste. They'll lose those teeth anyway and don't need to be swallowing something that's not going to help them in the long run.
Recommendations of the school based off of research and study go loosely as follows: if a person has healthy teeth, great. If they live in a fluoridated community, that's okay/good. If teeth are only okay, fluoridated water is recommended. Bad teeth, fluoridated water and fluoride treatments strongly encouraged. They encourage fluoridated toothpase for anyone (just don't eat it).
There are various forms of fluoride. Most common in toothpaste and mouthrinses is sodium fluoride, NaF. It's a salt that dissolves in water, is pretty tasteless, and inert (this is not to be confused with sodium fluoroacetate, which is a natural plant poison that's been mass produced and used in Australia as a pest poison). Sodium fluoride is most commonly and most abundantly produced as a by-product of making fertilizer. Some people see this fact as proving that fluoride is bad. Being a by-product of another process doesn't make anything bad by itself, as long as the product is purified and used appropriately. Biologically it doesn't matter how it was made.
Other forms of fluoride are Stannous fluoride, SnF2. Stannous is tin. Tin fluoride, literally. A few toothpastes use it and try to market off the fact that there are two fluorides in it (literally, the Colgate saleswoman presented it as "Polyfluorite Technology"). The concentration of fluoride ion in the toothpaste is still the same, 1000 ppm. The tin can help with tooth sensitivity for some people, although it causes surface stains and leaves a metallic aftertaste.
There are also acid-fluorides for very specific cases to be used very carefully. The idea is to partially dissolve the hydroxy groups by the fluoridated toothpaste so that fluoride can step right into the empty spots. Not used very often.
For water supplies, some communities use hexafluorosilicic acid (H2SiF6, it's an acid because in water, one or both of the H's drop off, leaving fluoride ions and Silicon ions), others use sodium fluoride.
An argument that deserves thought is that fluoridating water is forced medication of the public by the government. In at least 4 states who have had the issue go to the supreme court, the courts have ruled that, at least legally, this is not true and that mineral content of water does not constitute a medication. Also, since most communities vote whether to fluoridate water or not, leave it to the community to decide. It is of biggest advantage to the 20% of people in of lower socio-economic status, who tend to have poorer diets, hygeine, and less access to dentistry, and who have 80% of tooth decay in this country.
A few people bring up the relationship of fluorine (a deadly gas) to fluoride. Fluoride is the ionic form of the element Fluorine. Fluoride occurs naturally, Fluorine does not (it is too reactive and too quickly becomes Fluoride by taking an electron from whatever it can). Thus said, fluoride is not the same as fluorine and the notable similarities end with the first 5 letters of the name. Someone once skeptically told me, "Yeah, I've heard that argument before...." I kinda wanted to recommend he take a basic chemistry class, but I bit my tongue. I'm much meaner in my head than I am in person.
Lastly, there was a theory that floated around about fluoridated water being a communist plot to subvert the US government, and that it was used by Nazi's to force the countries they conquered into submission. Someone told me once that a study was done in China once that showed fluoridated water led to lower IQ's in children, but there was no source, and I've never heard this one anywhere else. Nothing valid has come of any of these ideas/theories.
So, that's fluoride as I know it. Consider yourselves edumacated.
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