1.  Yes, "the dose makes the poison".  According to the US EPA, the safe dose of lead, arsenic, and mercury, is that obtained from water with a respective concentration of 15 ppb, 10 ppb, and 2 ppb, respectively, of those substances.  Yes, these substances are poison above a certain level.  However, such poisoning is dose dependent, as it is for every other substance known to man.
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2.  Toxicity is toxicity.  A certain dose of water consumed over a certain amount of time is toxic.   A certain dose of fluoride consumed over a certain amount of time is toxic.  No difference.  Both are dose dependent.
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to be cont…….
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Steven D. Slott, DDS