Comments on the Harvard Meta-Data Study:
The second significant concern is that fluoride interferes with neurodevelopment and lowers children’s IQ. This is not quite as easy to dismiss. A 2012 National Research Council meta-analysis of studies that compared children in high-fluoride and low-fluoride areas concluded that fluoride exposure significantly lowered IQ. The effect in this study is reasonably large. The National Research Council gives some credence to these studies, arguing that more research should be done.
These studies, however, must be taken with a grain — maybe even a whole box — of salt. They largely rely on data from China, and the variation in fluoride levels there arises not from community water fluoridation but from natural variation in the water’s fluoride levels. The biggest issue is that the “high” fluoride levels are much, much higher than you’d see in the United States. In fact, the low fluoride level group in these studies has a level similar to what is typical in the U.S. for tap water. So even if we take the conclusions of these studies at face value, it is not at all clear whether they apply to the U.S.