In the 1940s, the Cold War was starting. Completely unrelated to that, President Truman's administration began pushing water fluoridation. Somehow, healthy teeth became a communist plot to instill socialist values in every American from birth.
If you've ever seen the movie Dr Strangelove, you may be reminded of General Ripper being terrified of having his "precious bodily fluids" diluted by the communists. What seems to be a parody to us now was the genuine opinion of some. This conspiracy theory has become somewhat mainstream with the invention of the Internet. While we're not afraid of communists anymore (well, maybe some people are), some people are still terrified of this water additive.
My favorite conspiracy theory is that fluoride weakens our willpower so the global Illuminati can take over. Go figure.
Fluoride is typically ingested at a rate of around 0.5-5 mg in a given day. The USDA's rule for MAXIMUM long-term consumption of Fluoride is 10 mg which means you are drinking about 10 liters of water every day to reach that amount. Drinking 2.6 gallons of water in a single day might cause physical harm.
Cellular rupturing due to over-hydration as the kidneys become overwhelmed. Your kidneys process about 1 liter of water per hour. If you keep chugging water, eventually, your body becomes overwhelmed. Anyway, you'll never hit that amount unless you're going for some terrible world record.
Another interesting fact: If you drink nothing but de-ionized, de-mineralized water, you're harming yourself. DI water will leech minerals out of your body due to osmosis. The reverse is true for normal tap water. You will absorb important minerals from the water into your body.
Now let's talk about the highly toxic item that fluoridated water is always conflated with Fluorine!
I couldn't find toxicity levels in humans, but death from acute fluorine poisoning has been documented in injuries equivalent to spilling some on your fingers of one hand. That's not to say that Fluorine is the worst thing ever. Remember, everything in moderation! Your body contains about 2 grams of fluorine which is pretty much concentrated in your bones. Without fluorine, you'd have osteoporosis and other problems related to skeletal defects.
You might be asking yourself what the difference between fluorine and fluoride is now. One electron. That's all. Fluoride is the negatively charged ion of fluorine. That means it has one more electron in its valence electron shell. That changes the bond type that occurs, which results in different compound formations. Lucky for us, fluoride forms compounds that tend to help us have a stronger skeletal structure and are especially good at preventing bacterial attacks on our teeth.
That's why we put it in our water. Think of it like hiding medicine in peanut butter to get your dog to eat it. People are going to drink water. People need fluoride to prevent tooth decay. Why not put it in water to make sure they get enough? This has reduced tooth decay by over 50% since its implementation in America. Similarly, that's why we have iodized salt. People need iodine too! Why not put it in the most used food additive? That way, everyone is better off in the long run.
So no, fluoride in your water isn't going to kill you. It's also not part of a communist plot. You could say that it's part of the shadowy cabal of secretive dentists trying to bring about world domination by preventing tooth decay. I don't think that'll sell any books, though.