Fluoride's safety under scrutiny as study finds prenatal exposure may impair cognition

A Swedish study suggests that even fluoride levels below international safety guidelines may impair children’s cognitive development, highlighting a need to reconsider current fluoride exposure standards.

Study: Prenatal and childhood exposure to fluoride and cognitive development: findings from the longitudinal MINIMat cohort in rural Bangladesh. Image Credit: greenaperture / Shutterstock

Elevated fluoride concentrations can occur in well water; in some countries, it is added to drinking water to counteract caries in the population. A study from Karolinska Institutet in Sweden now supports a few previous studies indicating that exposure to fluoride during the fetal stage or early childhood may impair cognition in children. The study is published in the journal Environmental Health Perspectives.

Fluoride occurs naturally as fluoride ions in drinking water, but the concentrations are generally low in public water supplies. In some countries, such as the USA, Canada, Chile, Australia, and Ireland, fluoride is commonly added to the municipal water supply at around 0.7 mg per liter to prevent caries.

"Given the concern about health risks, the addition of fluoride to drinking water is controversial and has been widely debated in the USA and Canada," says Maria Kippler, associate professor at the Institute of Environmental Medicine, Karolinska Institutet. "Our results support the hypothesis that even relatively low fluoride concentrations can impact children's early development."

500 mothers and children

The researchers followed 500 mothers and their children in rural Bangladesh, where fluoride occurs naturally in the drinking water, to investigate the association between early exposure to fluoride and children's cognitive abilities. The concentrations are similar to those found in many other countries worldwide.

Trained psychologists evaluated the children's cognitive abilities at five and ten years of age using well-established tests. Exposure to fluoride in the mothers and children was determined by measuring the concentrations in urine samples, which reflects ongoing exposure to all sources, primarily from drinking water, food, and dental care products.

"I'd like to stress that dental care products such as toothpaste are not normally a significant source of exposure since they are not intended for ingestion," says Dr Kippler. "Fluoride in toothpaste is important for preventing caries, but it's important to encourage small children not to swallow the toothpaste during brushing."

Decreased cognitive abilities

The median concentration of fluoride in the urine of the pregnant Bangladeshi women was 0.63 mg/L. Increasing fluoride concentrations in pregnant women could be linked to decreasing cognitive abilities in their children at five and ten years of age.

Children who had more than 0.72 mg/L fluoride in their urine by age ten also had lower cognitive abilities than children with less fluoride in their urine, with the most pronounced associations for verbal reasoning skills and the ability to interpret and process sensory input.

The exposures that were associated with impaired cognitive development are lower than those obtained at the existing WHO and EU threshold for fluoride in drinking water, which is 1.5 mg/L. The researchers found no statistically significant link between fluoride concentrations in the urine of the five-year-olds and their cognitive abilities.

"This may be due to the shorter exposure time," Dr. Kippler speculates, "but also to the fact that urinary fluoride concentrations aren't as reliable in younger children owing to greater variations in how much fluoride is taken up and stored in the body, particularly in the bones."

The researchers found that prenatal fluoride exposure had an inverse association with cognitive abilities at five and ten years, with no apparent threshold below which fluoride had no impact.

The impact of prenatal fluoride exposure on cognitive outcomes appeared somewhat stronger in girls than in boys at age ten, although this difference was not statistically significant.

More studies needed

Since it was an observational study, no firm conclusions can be drawn about causalities. The authors note that the use of single-spot urine samples might also introduce misclassification of fluoride exposure. According to the researchers, It is important to assess the overall results of several similar longitudinal studies.

They will now investigate the associations in other populations and establish experimental models to determine the possible molecular mechanisms driving it. "There is a need for more research to create a robust basis for reviewing fluoride health risks and thresholds for drinking water, foods, and dental care products, especially for children," she continues. "Even small changes in cognition at a population level can have serious public health consequences."

National grants from the Swedish Research Council and the Swedish Research Council for Environment, Agricultural Sciences, and Spatial Planning mainly financed the study. The researchers have declared no conflicts of interest. https://doi.org/10.1289/EHP14534