The World Health Organization recommends fluoride levels below 1.5 mg/L.
Researchers from Tulane University in the US recruited 74 school-aged children and rated their ability to draw familiar objects such as a donkey or a house, with scores reflecting any missing details.
They used a standard computerised memory test which is language and culture neutral as another tool to measure cognitive ability.
The study found that higher exposure to fluoride in drinking water was linked to more errors on the drawing and memory tests.
“The causal relationship between fluoride exposure and neurotoxicity remains unclear,” said lead author Tewodros Godebo, assistant professor at Tulane University School of Public Health and Tropical Medicine.
However, Godebo hopes these preliminary findings will spur more research into the potential cognitive impacts of fluoride exposure.
“Though further epidemiological studies are needed to validate the findings, these results add to the growing concern about the potential neurotoxic effects of fluoride, especially during early brain development and childhood,” Godebo said.
“These tests affirmed a clear association between high fluoride and cognitive impairment,” he added.
Fluoride is essential for preventing tooth decay. However, its excess intake has been linked to lower IQs in past epidemiological studies in rural communities in China and India, the researchers said.
Past animal research has shown that fluoride can cross the placenta and blood-brain barriers.
In regions with no alternative water sources, this means excess fluoride exposure could be a chronic issue that begins at conception, they said.
Over 200 million people worldwide are estimated to be exposed to high fluoride levels in their drinking water, according to the researchers.
The Ethiopian Rift Valley, where this study was conducted, is an ideal research area for investigations of potential impacts because those raised in the area have consistent exposure to stable, naturally occurring fluoride levels and share similar lifestyles with surrounding villages, limiting the risk of confounding factors.
Godebo hopes to replicate the results in Ethiopia with a larger group of children and study the cognition of children in low-fluoride Ethiopian communities for potential signs of cognitive impact.