Adding fluoride to public drinking water for dental purposes has been controversial since the practice first began in 1945. A new study suggests that prenatal exposure to this chemical may affect cognitive abilities and that children born to mothers exposed to high amounts of fluoride could have lower IQs. The study ... found an association between lower intelligence and prenatal fluoride exposure in 299 mother-child pairs in Mexico. Even when other possible factors were taken into account, such as exposure to other chemicals, results continually showed that higher prenatal fluoride exposure was linked to lower scores on tests of cognitive function in children at age 4 and then again between 6 and 12. The mothers in this study did not have fluoride added to their water. In Mexico, fluoridated salt is the main way that women get salt into their diet, says Hu, unlike in the U.S., where fluoridated water is the main avenue. The data could renew the debate about the safety of adding fluoride to tap water, in part because experts have not been quick to dismiss the findings. "This is a very well-conducted study, and it raises serious concerns about fluoride supplementation in water," says Dr. Leonardo Trasande, a pediatrician who studies potential links between environmental exposures and health problems at New York University. Trasande ... also explains that fluoride is known to disrupt thyroid function, which in turn is crucial for brain development.