Pregnant women who drink fluoride-treated water may give birth to children with lower IQs, a new study suggests.
Researchers from York University in Toronto, Canada, found that the more fluoride that was present in a mother's urine, the lower her child's IQ score was.
Surprisingly, boys seemed to have much lower scores when their mother's had high fluoride levels than girls did.
About two-thirds of Americans and one-third of Canadians drink fluoridated water from community water systems that add the mineral to prevent tooth decay.
While the study raised questions about the possible negative neurological effects of fluoride on young children, some scientists are skeptical about the findings.
Fluoride is a naturally occurring mineral found in soil, rocks and water.
It's added to toothpastes, mouthwashes and drinking water because studies have shown water with optimal fluoride levels can lower the prevalence of tooth decay.
According to Centers for Disease Control and Prevention, water fluoridation can reduce the amount of decay in children's teeth by up to 60 percent.
However, experts say fluoride can be dangerous in high concentrations.
It can cause dental fluorosis, which is when faint white streaks appear on the teeth when younger children consume too much fluoride.
Excess levels of the chemical can also cause skeletal fluorosis.
As the bones become hardened and less elastic, the risk of pain and fractures increases and can eventual lead to loss of mobility.
But health officials say these aren't their only concerns.
A 2012 analysis led by the Harvard TH Chan School of Public Health looked at studies in which children had been exposed to high fluoride content in water.
The researchers found that the children lost an average of seven IQ points on tests.
And a 2017 study from the Dalla Lana School of Public Health in Toronto, Ontario, Canada, found that higher exposure to fluoride before birth led to lower scores on cognitive function tests.
For the new study, published in JAMA Pediatrics, the team looked at 601 Canadian mother-child pairs.
Researchers examined the measures of fluoride women were exposed to during pregnancy through urinary analysis or the women self-reporting the tap water or water-based beverages they drank.
The IQ scores were measured when the children were between ages three and four.
They found that for every increase of 1 milligram per liter (mg/L) of fluoride in a mother's urine, there was about a 3.7-point lower IQ score.
For boys, in particular, there was a 4.5-point lower IQ score for every 1mg/L increase.
Interestingly, there was not the same association with girls.
However, the authors note that the study has limitations, including the fact that the women could have consumed fluoride right before a urine sample was taken.
Additionally, the researchers didn't measure the children's fluoride exposure during infancy.
Scientists say they're skeptical about the findings.
'A curious finding is that the link between maternal urine fluoride and IQ decrements is only seen in boys and not girls,' said Dr Alastair Hay, a professor emeritus of environmental toxicology at the University of Leeds in the UK, who was not involved in the study.
'Whilst the authors are just reporting what they found, I find these sex differences difficult to explain. With a neurotoxicant you might expect both sexes to be affected.'