A long-term study by Rob Warren, a sociologist and population health expert at the University of Minnesota, has found that fluoride in drinking water has no effect on IQ or brain function. The study, published in the Proceedings of the National Academy of Sciences, is the first to measure community water fluoridation exposure during childhood in the U.S. and any potential impact on cognition up to age 80. This study contradicts claims by Health Secretary Robert F. Kennedy Jr. that fluoride is linked to IQ loss. Fear over a link to lower IQ scores has led to bans on the addition of fluoride to drinking water in several states. The optimal level of fluoride in water is 0.7 milligrams per liter.