A new study has found a clear relationship between high fluoride exposure and decreased I.Q. in children.

The findings, published this week in JAMA Pediatrics, add fuel to the ongoing national debate around whether or not fluoride should be banned from U.S. drinking water—a discussion most recently pushed by Donald Trump’s highly contested health secretary pick, Robert F. Kennedy Jr.

The study has also prompted division among medical experts, with the journal publishing two opposing editorials alongside it—one of which says the findings “underscore the need to reassess the potential risks of fluoride during early brain development,” with the other cautioning against the study’s various methodological issues.

The meta analysis of 74 previous epidemiological studies, conducted by the National Institute of Environmental Health Sciences, looked only at past studies from 10 foreign nations—including China, Mexico, Canada, and India, which use higher concentrations of fluoride than the U.S. Still, while it cannot prove cause and effect, the analysis found evidence of a statistically significant “dose-response” relationship, with I.Q. scores going down as fluoride exposure went up.

“Fluoridated water has been used for decades to reduce dental cavities and improve general oral health,” the study’s lead researcher, Kyla Taylor, tells Fortune in an email. “However, there is concern that pregnant women and children are getting fluoride from many sources including drinking water, water-added foods and beverages, teas, toothpaste, floss, and mouthwash and that their total fluoride exposure is too high and may affect fetal, infant, and child neurodevelopment.”

What is fluoride and why is it in the water?

Fluoride, the chemical ion of the mineral fluorine, is naturally present in trace amounts, according to the Centers for Disease Control and Prevention, in soil, water, plants, and some food sources including shellfish, raisins, yogurt, and potatoes. It can also be released from volcanic emissions or as a byproduct of aluminum, fertilizer, and iron ore manufacture.

Since 1962, the U.S. Public Health Service has recommended it be added to drinking water to prevent tooth decay—although it is not enforceable and a decision made on a local, not federal, level. And local fights about removing fluoride from water systems have been popping up nationwide; most recently, in Washington, the city of Aberdeen announced it would cease the practice following a survey of residents.

How do the study’s findings relate to the U.S.?

The new meta-analysis found that for every 1 mg/L increase in urinary fluoride, which is an estimate of total fluoride exposure, there is a decrease of 1.63 IQ points in children.

The U.S. Public Health Service’s recommendation is a fluoride concentration of 0.7 mg/L of drinking water—and there were not enough data to determine if 0.7 mg/L of fluoride exposure in drinking water affected children’s IQ. The same was true for fluoride levels of less than 1.5 mg/L, which is the upper safe limit of fluoride in drinking water as established by the World Health Organization.

But a fluoride level of above 1.5 mg/L is found in wells and community water systems that serve over 2.9 million people in the U.S., according to the study authors.

Further, the study notes, “the U.S. Geological Survey estimates that 172,000 U.S. residents are served by domestic wells that exceed [the Environmental Protection Agency’s] enforceable standard of 4.0 mg/L fluoride in drinking water, and 522,000 are served by domestic wells that exceed EPA’s nonenforceable standard of 2.0 mg/L fluoride in drinking water. This is particularly true for Hispanic and Latino communities, according to the study. (To find the amount of fluoride in your water, it’s best to contact your local water provider; the CDC also offers a search tool, though incomplete.)

The CDC recommends that parents “use an alternative source of water for children aged 8 years or younger and for bottle-fed infants if their primary drinking water contains greater than 2 mg/L of fluoride,” the study authors point out. But that is to reduce risk of dental fluorosis—the discoloration of tooth enamel that can come from too much fluoride.

“Currently,” the study notes, “there are no recommendations or restrictions on fluoride levels in drinking water based on cognitive neurodevelopmental outcomes.” The authors added that, to their knowledge, no studies of fluoride exposure and children’s IQ have been performed in the U.S., and no nationally representative urinary fluoride levels exist, “hindering the application of these findings to the U.S. population.”

It’s why Dr. Bruce Lanphear, a public health expert and lead author of one of the editorials accompanying the study, calls for more research. “Previous risk assessments focused on dental fluorosis, but the NTP’s meta-analyses show that fluoride may be associated with cognitive function at concentrations found in fluoridated communities,” he writes. “It is time for health organizations and regulatory bodies to reassess the risks and benefits of fluoride, particularly for pregnant women and infants.”

The editorial urging caution in the interpretation of the study’s findings, meanwhile, written by dentist and public health expert Steven Levy, discusses a range of factual errors and methodological problems, including “high risk of bias,” meaning that the results of some of the included studies may be misleading. He stresses that “public policy concerning the addition of fluoride to community water systems and recommendations concerning the use of topical fluoride in its many forms should not be affected by the study findings.”

The new fluoride study dividing the public health world as RFK Jr. calls for a ban on adding it to water