Comment by somenameforme

Yeah, note the measurement is in urine. So there are two separate issues. Determining whether fluoride is damaging to IQ, and then whether the levels in water can drive this. The former is way easier to evaluate than the latter. The reason comes from that study's intro pargraphs:

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"Since 1945, the use of fluoride has been a successful public health initiative for reducing dental cavities and improving general oral health of adults and children. There is a concern, however, that some pregnant women and children may be getting more fluoride than they need because they now get fluoride from many sources including treated public water, water-added foods and beverages, teas, toothpaste, floss, and mouthwash, and the combined total intake of fluoride may exceed safe amounts."

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So the issue is trying to isolate the exact amount and source of fluoride people are getting. And that probably has no answer because it's going to vary dependent on how much fluoridated water somebody drinks, the rest of their diet, their other dental hygiene composition, and more. So levels that would be safe for one percent of the population, will be dangerous for another percent of it.