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Adding fluoride to public drinking water for dental purposes has been controversial since the practice first began in 1945, and the latest findings are sure to stir that pot yet again. A new study suggests that prenatal exposure to this chemical may affect cognitive abilities and that children born to mothers exposed to high amounts of fluoride could have lower IQs.

The study, published Tuesday in Environmental Health Perspectives, found an association between lower intelligence and prenatal fluoride exposure in 299 mother-child pairs in Mexico. The team measured fluoride levels from mothers via urine samples and followed up on their children until they were between 6 and 12 years old. Even when other possible factors were taken into account, such as exposure to other chemicals, results continually showed that higher prenatal fluoride exposure was linked to lower scores on tests of cognitive function in children at age 4 and then again between 6 and 12.

Although the study accurately measured how much fluoride was in each mother's urine samples, it could not pin down the exact amount of fluoride the children had been exposed to. That's because pregnancy can change how certain substances are secreted in urine. However, the team estimates that these levels of exposure are not exceptionally high.

"If you just assume for the moment that fluoride in the urine of pregnant women is the same as it is in nonpregnant women, then these levels are a bit higher—but not hugely higher—than that seen in general population samples in North America," lead study author Dr. Howard Hu, who studies environmental health at the University of Toronto, tells Newsweek.

The mothers in this study did not have fluoride added to their water. Rather, they ingested fluoride from natural sources or through fluoridated salt and supplements. In Mexico, fluoridated salt is the main way that women get salt into their diet, says Hu, unlike in the U.S., where fluoridated water is the main avenue.

Related: Fluoridation may not prevent cavities, scientific review shows

The data could renew the debate about the safety of adding fluoride to tap water, in part because experts have not been quick to dismiss the findings. "This is a very well-conducted study, and it raises serious concerns about fluoride supplementation in water," says Dr. Leonardo Trasande, a pediatrician who studies potential links between environmental exposures and health problems at New York University Langone Health. (He was not involved in the new study.)

Trasande emphasizes that the levels of fluoride seen among the mothers in this study are slightly higher than what would be expected in U.S., based on current fluoride supplementation levels. However, he also explains that fluoride is known to disrupt thyroid function, which in turn is crucial for brain development.

"These new insights raise concerns that the prenatal period may be highly vulnerable and may require additional reconsideration," Trasande says.

Related: Water fluoridation linked to higher ADHD rates

Fluoride is a naturally occurring chemical that is noted for its ability to help prevent tooth decay and is often added to public drinking water for this reason. According to the Centers for Disease Control and Prevention, drinking fluoridated water reduces cavity occurrence by 25 percent in areas where it is practiced. In 2014, about 74 percent of the U.S. population was on fluoridated public water systems, and by 2020 the CDC hopes to increase this number to 79.6 percent.

A large body of research has confirmed the safety of fluoride in water. One review of more than 70 studies concluded that the practice was safe. A 2009 review of more than 50 studies deemed the practice safe. A study in 1996 called the evidence confirming the safety of water fluoridation "compelling."

But this new study is not the first time that research has raised alarm about toxic levels of the element. One critical review stated that fluoride ingestion or inhalation "constitutes an unacceptable risk with virtually no proven benefit." A review of studies conducted in China, where naturally occurring levels of fluoride in water can be dangerously high, found a connection between exposure to fluoride and children’s IQ.

However, these investigations were examining a water supply with fluoride levels reaching 30 milligrams per liter. In the U.S., the Environmental Protection Agency sets the limit at 4 mg per liter, and most public water supplies contain about 0.7 mg per liter. In other words, the findings in China are most likely inapplicable to the U.S. population. (But an extensive report by the National Research Council, part of the National Academies of Sciences, Engineering and Medicine, did call for a reduction in the legally allowed limit of 4 mg/liter.)

This occasion is not the first time that Environmental Health Perspectives has waded into the fluoride debate waters. The same journal also published a controversial investigation, known as "the Harvard Study," which reported a link between exposure and neurological development in children. But that study was heavily criticized, in part for its reliance on data from China.

Dr. James Betoni, an ob/gyn who specializes in high-risk maternal fetal medicine at Saint Alphonsus Medical Group in Boise, Idaho, tells Newsweek that he would advise patients not to be too fearful of the findings.

"There are so many variables and so many factors for cognitive development, I think it's such a nebulous thing," says Betoni. "The fluoride may be high, but there may be something in the water that we don't even know."

He says he would tell patients concerned about these new findings that, although they are interesting, this is only one study and they shouldn't make any lifestyle changes based on the results of a single study.

Still, many groups oppose the practice, and the American Academy of Pediatrics has a campaign to quell fears. And the new study, which found a link between fluoride and lower IQ starting at 0.5 mg per liter in the mother's urine (lower than the levels in tap water), may be viewed as supporting their argument.

Hu and colleagues note that their results need to be reproduced by other researchers—and in different, larger populations—before any conclusion can be made about the effects of this finding on fluoride levels in public drinking water. They would also like to explore how other factors, such as different nutrients and genetics, may play a role in a link between f