it’s recently been found that relatively low levels of fluoride in the water have an adverse effect on IQ.

Yeah, but the levels where it starts being noticeable (1.5mg/L) are double the level that fluoride treatment targets (0.6-0.8mg/L). It’s something to be worried about if you’re drinking well water and it’s got a natural high fluoride content, not from anything added by the water treatment processes.