The study, published in PNAS, examined Wisconsin state testing records, archival information about when Wisconsin cities began to fluoridate their water, and data from the Wisconsin Longitudinal Study, which has followed a random sample of 10,317 high school seniors from 1957 through 2026. Key findings include:
- There is no evidence supporting a connection between community water fluoridation and children’s IQ.
- There is also no evidence supporting a connection between community water fluoridation and cognitive functioning at various points later in life.
- Findings confirm evidence published in previous research which also used a national sample, but considered school achievement test scores instead of actual IQ scores.
According to Wikipedia (I know, not the most accurate source. I just needed a quick statistic) the U.S. has a fluoride level of 0.7 mg / L since 2015. Poking around for other countries, it looks like 0.7-1.2 mg / L is the range that most countries add to their water supply.
For reference, the WHO recommends 1.5 mg / L as the upper limit. Additionally, in many places, the groundwater has fluoride levels a bit higher than that.