NaF is an archetypal neurotoxin well known in the literature. The arguments "debunking" concerns for fluoride are twofold:
i) it's worth the risk because of the alleged benefits on dental health, and reducing dental health problems is important because dental disease raises the risk of cardiovascular incident and other dangers enormously.
ii) it's at such low doses that it doesn't have negative effects.
i) May be true, but it's somewhat beside the point. The benefits supposedly come from topical treatment not ingestion. There's a reason why toothpastes often have poison control information on them in case of ingestion - because something which may be of benefit to teeth may be harmful to other parts of the body when systemically metabolised, so you limit the body's exposure to it. The next question is the ease of delivery to a large amount of people, which is another point in itself. But this is not a counter-argument.
ii) Anybody who has done even Pharmacology 101 would know why to question this line of reasoning. Dose-response curves aren't magical relationships that are static and fixed in stone. D-R curves can be biased by a variety of factors. Repeated doses, for example, can change the D-R curve. Treatment regimes (intermittent vs. continuous; fluoride intake is intermittent unless you're gulping from the faucet constantly your entirely life) bias the D-R curve. As InternetPropagandist points out, fluoride intake is highly variable (as is the response to fluoride, this is just a basic fact of pharmacology - responses are variable) and often just looking at the concentration in water is an incomplete model because it excludes other sources. Another point is the following: you don't just come into contact with fluoride as an adult drinking water for the first time. It's been systematically introduced since you were in utero. Another basic fact of biology is that the growing babby shares a blood supply with the mother and is particularly sensitive to environmental insults. So the threshold for risk is lowered to a smaller dose.
A final note: what IS known about fluoride's biological effects? Well, we know it can seriously interfere with Ca2+ metabolism. Anybody who has done basic cell physiology or biochemistry would begin to see the significance of this. Ca2+ metabolism is vital to the regulation of myriad different enzymes and proteins that determine cell function, and Ca2+ must always be highly regulated because raising the ICF conc. of Ca2+ beyond certain limits causes toxicity. Furthermore, this doesn't even take into account excitable cells or cellular communication. Excitable cells are what your nervous system and musculature are composed of. This means the membrane are electrochemically excitable and the processes underlying it rely on highly regulated and co-ordinated cation flux across the membrane (including Ca2+). It's a basic principle of physiology that altering membrane excitability (e.g. by altering cation concentration or metabolism, which changes the electrochemical driving force plus protein function) changes the behaviour of communities of cells. And so on.
What's my point? Well, people who jump to the conclusion fluoride is fun "BECAUSE SCIENZNE!!!!!!11111" are acting like fucking mouthbreathers who clearly don't grasp what science is or what it is about and clearly lack a basic understanding of contemporary physiology. Maybe fluoride isn't that harmful. But being dogmatic (either way, for or against) is just a fucking shit attitude to have and betrays deep ignorance and reactionism. Look at the evidence and use reasoning. Explicitly stating to be critical of appeals to authority and the Idol of the Tribe were among the founding practices of Francis Bacon's formulation of empirical scientific method... which underlies so much of Western scientific method.
Really sick of people who treat science as an unchanging dogma and feel qualified in understanding science because they watch tedtalks or w/e.... if you treat science as a dogma without actually practicing method, then what you're doing is dogmatically practicing the religious worship of an Idol of science... which is profoundly anathema to science. Like I said, I don't know either way, but I know bad reasoning, bad arguments, and bad science when I see it. There's people on both sides of the arguments guilty of the the aforementioned. So let's be reasonable adults and look at the evidence, use reasoning, and draw cautious conclusions which are open to honest criticism. Fluoride use should at least raise questions and there are good reasons to question it, like there's good reasons to question any practice which may have a detriment to public health.