Expected to play a key public health role in the incoming Trump administration, Robert F. Kennedy Jr. has staked out a major policy shift: Discouraging the use of fluoride in drinking water.

For more than half a century, drinking water systems in the United States have added the chemical to reduce tooth decay and lower rates of expensive and potentially painful cavities and other issues. Minnesota requires it for nearly all utilities. The Centers for Disease Control and Prevention (CDC) has hailed the practice as one of the most important public health developments of the last century.

In a recent interview with NBC News, Kennedy pointed to recent research that has linked high levels of fluoride with lower IQ in children.

"I'm not going to compel anybody to take it out," Kennedy said, adding, "I'm going to give them good information about the science, and I think that fluoride will disappear."

Here's what others say about the benefits and risks of fluoridating drinking water:

Why do we add fluoride to water?

Fluoridation has been shown to reduce tooth decay, which can be painful, expensive and lead to life-threatening infections if left untreated, according to CDC. A CDC statement on fluoridation released in May cited that dental decay is reduced 25% in adults and children by repeated, low-level exposure to fluoride through the water supply.

"I would certainly recommend putting fluoride in the water," said Alejandro Aguirre, an endodontist and the president of the Minnesota Dental Association. "You prevent decay, you are going to have healthier children and healthier adults that can be productive for society."

The practice is also endorsed by the American Academy of Pediatricians and the American Dental Association.

The Minnesota Department of Health has a rule requiring fluoridation in almost all public water systems. According to 2023 data, 4.4 million Minnesotans have fluoridated water while only about 56,000 people, mostly on small community water systems that serve hundreds of people or fewer, do not.

"One of the main reasons in favor of community water fluoridation is that it prevents dental [decay] equitably for everyone in the population," MDH wrote in 2020, as it revised its fluoride rule.

Fluoridation rules do not apply to private wells, but fluoride can occur naturally in water supplies.

Does research show it's safe?

Excessive fluoride has long been linked to fluorosis, a condition that discolors teeth with brown, splotchy marks. In the most extreme cases, fluorosis can also affect the skeleton, increasing bone density and creating joint pain. EPA limits fluoride levels in drinking water to 4 parts per million, far above typical fluoridation levels, to avoid fluorosis.

Because many people also use products that contain fluoride, such as toothpaste and black tea, Minnesota in 2020 lowered its target levels for fluoride in drinking water to between 0.5 and 0.9 parts per million.

A more active area of scientific investigation is whether fluoride reduces cognitive abilities in children, a possibility Kennedy has repeatedly mentioned. CDC and many U.S. dental and medical associations are firm in their conclusions that fluoridation is safe and effective.

Many studies outside of the United States, however, have shown that fluoride can harm children's intelligence in very high amounts, said Tewodros Godebo, an assistant professor at Tulane University's department of environmental health sciences. Godebo conducted one of these studies in a region of Ethiopia where fluoride naturally occurs at up to 20 times the level added to water in the United States. He found that these higher levels harmed children's abilities to perform in cognitive tests.

A federal review of the science from the National Toxicology Program (NTP) found harmful effects to child IQ starting at 1.5 parts per million, or a little more than double the recommended level in U.S. water systems.

But below those levels, Godebo said, the science is still not clear on whether low levels of fluoride have an effect on children, with studies pointing in both directions. The NTP review did not find a connection at these low levels in existing studies, and Godebo said more research needs to be done.

"We are not in a position to make a policy decision right now, because the science is all over the place," he said.

Still, the NTP review and other research showing child IQ loss at low levels recently contributed to a court ruling in California. Obama-appointed U.S. District Court Judge Edward Chen ruled that fluoridated water carried an "unreasonable risk" to public health under the Toxic Substances Control Act. Chen ruled that EPA must in turn regulate the chemical, which could range from adding a warning label to the chemical to banning it outright.

An EPA spokesperson wrote in an email that the decision "defers to EPA's expertise as to how to evaluate and regulate fluoride appropriately moving forward." The agency said it and the Department of Justice "will consider all options going forward."

What happens in places that remove fluoride?

Removing fluoride from public drinking water can have its own serious consequences.

The Canadian city of Calgary, Alberta, stopped adding fluoride to its water supply in 2011. Seven years later, researchers found, children in Calgary were experiencing more tooth decay than in nearby Edmonton, which did not drop the additive. Another study found a 21% increase in dental treatments that required general anesthesia for children under 12.

James A. Dickinson, a University of Calgary professor, wrote in an email that it would be easy to assume the widespread use of toothpaste, mouthwashes and other fluoridated products has made drinking water fluoridation less important. But the studies point toward the opposite conclusion, he said.

"Fluoridation helps children regardless of whether their parents take care of their mouth hygiene or not," wrote Dickinson, who works in the departments of Family Medicine and Community Health Sciences.

Cora Constantinescu, a pediatric infectious disease specialist, also reported to Calgary's city council in 2019 that there had been a 700% increase in children referred to her clinic for IV antibiotic treatment for serious dental infections since fluoride was dropped.

Two years later, the city council voted to add fluoride back into the water system; new equipment is expected to come online in 2025.