I can see that fluoride could cause brain damage in very high concentrations. I don't know about high, medium, or low concentrations.
I think the real problem here is quantifying "very high concentrations." If you look at Table 1 in the study, it shows some very basic information about different studies. A number of those studies note lower IQ scores with the "high" or "exposed" group compared to the "reference" group, and many of the levels specified as "high" or "exposed" are below the 4.0mg/L maximum specified by the EPA.
Ultimately, the study concluded "more research is needed." I think we can both agree with that statement.
On the other hand, fluoride only benefits you when it's in your mouth and in contact with your teeth. Once you swallow it, it can no longer help... but that's when it can hurt. Given this information, fluoridating water doesn't make any sense. Fluoride should be used in rinses and toothpastes where they're not swallowed, and left out of the water supply.
