It's factually correct that too much fluoride is correlated with decreased average IQ.

By "too much" a factor of > 10x western safe levels is meant and by "correlated with" is meant a slew of other heavy metals are generally present.

This comes from studies that look at places in China, in Africa, and elsewhere that have unusally high levels of fluoride and other elements naturally occurring in water or as a by product of other industrial processing going on.

Where the problem lies is in the "fill in the missing line" extrapolations that the anti-fluoride folk do to "conclude" that if really high levels of stuff in water makes you stupid and affects your health then it surely must follow that small amounts make you a bit stupid and a bit unhealthy.

This is despite no such evidence existing even given large western populations with meticulously kept water quality and health records in the UK, Canada, Australia, US, etc.

The G20 recommended fluoride levels are safe by all the evidence to date and work to decrease cavity rates.