An analysis of data from studies conducted in Denmark, Mexico, and Canada found that prenatal exposure to high concentrations of fluoride is associated with lower IQ scores in children at age 7. However, this association depended on the dose. In the low-exposure Danish sample, no association was found. A concentration of 0.3 mg/L of fluoride ions in maternal urine appears to be the critical threshold above which fluoride exposure begins to have toxic effects on children’s cognitive development. The research was published in the European Journal of Public Health.
Fluorides are naturally occurring minerals found in water, soil, plants, and air, containing the element fluorine. These include compounds such as sodium fluoride, fluorosilicic acid, and sodium fluorosilicate. When dissolved in water, these minerals release fluoride ions that inhibit bacterial growth. They also help prevent tooth decay by strengthening tooth enamel and promoting remineralization. For this reason, fluorides are commonly used in toothpastes and mouth rinses to prevent cavities. In some countries, they are also added to public water supplies through a process called fluoridation.
Common dietary sources of fluoride, include tea and seafood. The fluoride concentration in tea can range from 1 to 6 mg/L depending on the type of tea and brewing method. Seafood, particularly fish eaten with bones (like sardines) and shellfish, also contains notable amounts of fluoride. Fruits and vegetables grown in regions with fluoride-rich groundwater can also contribute to fluoride intake.
At appropriate levels, fluoride is beneficial for oral health and significantly reduces the risk of tooth decay. However, excessive fluoride exposure, particularly during early childhood, can lead to dental fluorosis, a condition that causes discoloration or pitting of tooth enamel. Prolonged high intake may result in skeletal fluorosis, a condition that weakens bones and joints. Emerging research also suggests that exposure to high concentrations of fluoride can be toxic to children’s neurons during development. Elemental fluorine itself is highly toxic and chemically reactive, but it is rarely found in nature.
Study author Philippe Grandjean and his colleagues sought to examine the association between prenatal fluoride exposure and cognitive performance at school age. They analyzed data from a Danish study called the Odense Child Cohort (OCC) and combined it with similar datasets from Mexico and Canada.
The Odense Child Cohort is a longitudinal research study in Denmark investigating the health, development, and environmental exposures of children from prenatal stages through childhood to understand factors influencing long-term well-being. This cohort included pregnant women residing in the Odense municipality in Denmark between 2010 and 2012. For this analysis, the researchers focused on 837 singleton mother-child pairs, where data on the child’s cognitive performance at age 7 and maternal urine fluoride levels during pregnancy were available.
Water fluoridation is not legal in Denmark, but elevated fluoride levels naturally occur in groundwater in some parts of the country. In Odense, the fluoride concentration in drinking water is generally around 0.2–0.3 mg/L, which is considered low. The study authors combined the Odense data with data from similar studies in Mexico and Canada, where water fluoridation is common and exposure to fluoride during pregnancy is generally higher.
Fluoride exposure in this study was assessed by measuring fluoride ions in maternal urine samples collected during pregnancy. In the Odense study, children’s cognitive performance was assessed at age 7 using the Wechsler Intelligence Scales for Children. In the Mexican and Canadian datasets, cognitive assessments were conducted at comparable ages using different, yet equivalent, scales that generate IQ scores.
Results showed that in the Odense cohort, urine fluoride concentrations ranged from 0.08 to 3.04 mg/L, with a median of 0.52 mg/L. These values reflect generally low exposure levels compared to the North American cohorts, where fluoride concentrations were higher due to water fluoridation practices. In the Odense group, no statistically significant association was found between maternal fluoride concentrations during pregnancy and children’s cognitive performance at age 7, as measured by IQ tests. This suggests that at lower fluoride exposure levels, as observed in Denmark, the potential neurotoxic effects may not manifest.
However, when data from Mexico and Canada were combined with the Odense data, a different pattern emerged. The combined results demonstrated a weak but statistically significant negative association between higher prenatal fluoride exposure and children’s cognitive performance. Specifically, a maternal urine fluoride level increase of 1 mg/L was associated with a 2.06-point IQ decrease in children.
Further analysis revealed a potential threshold effect, with maternal urine fluoride concentrations of approximately 0.3 mg/L representing a critical level. Above this threshold, the likelihood of fluoride exposure having neurotoxic effects on children’s developing brains increased.
“The present study contributes new information on the weak association between fairly low levels of prenatal fluoride exposure and cognitive function at school age in a Danish birth cohort. A possible negative association could not be confirmed within the exposures measured in the OCC [Odense Child Cohort]. When merged with data from two previous prospective studies at higher exposures, a revised BMCL [benchmark concentration lower confidence limit] fluoride concentration of about 0.3 mg/l in maternal pregnancy urine suggests that elevated fluoride intakes, whether from drinking water, black tea, or other sources, during pregnancy may require public health attention,” the study authors concluded.
The study contributes to the scientific understanding of the effects of fluorides on brain development. However, fluoride exposure was measured at a single point during pregnancy, which may not accurately represent cumulative exposure over the entire gestation period. While the findings demonstrate an association between higher prenatal fluoride exposure and lower IQ in children, they do not establish a direct causal relationship.
The analysis adjusted for many control variables (e.g., parental education, gestational age, breastfeeding), but the observational nature of the study means that other unmeasured factors, such as nutritional deficiencies or exposure to other environmental neurotoxicants, could contribute to the observed cognitive effects.
The paper, “Dose dependence of prenatal fluoride exposure associations with cognitive performance at school age in three prospective studies,” was authored by Philippe Grandjean, Alessandra Meddis, Flemming Nielsen, Iben H. Beck, Niels Bilenberg, Carly V. Goodman, Howard Hu, Christine Till, and Esben Budtz-Jørgensen.
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