Increased levels of prenatal fluoride exposure may be associated with lower cognitive function in children, a new study says. The study, published ... in the journal Environmental Health Perspectives, evaluated nearly 300 sets of mothers and children in Mexico and tested the children twice for cognitive development over the course of 12 years. The study found a drop in scores on intelligence tests for every 0.5 milligram-per-liter increase in fluoride exposure beyond 0.8 milligrams per liter found in urine. Although the researchers found a potential connection to a child's exposure to fluoride in utero, they found no significant influence from fluoride exposure on brain development once a child was born. "Childhood exposure to fluoride is safer than prenatal. The fetal system tends to be more sensitive to environmental toxicants than once the child is born," said the study's lead author, Howard Hu, founding dean of the Dalla Lana School of Public Health at the University of Toronto. Fluoride is commonly added to drinking water in the United States in order to improve dental health, though a number of communities including Portland, Oregon, and Tucson, Arizona, have rejected water fluoridation. What the new research means for pregnant women in the United States is up in the air. Previous studies have found fluoride to be a potential neurotoxin at extremely high levels.