Fluoridated drinking water has been hailed as one of the top 10 public health achievements of the 20th century by the Centers for Disease Control and Prevention (CDC).

Now it's being called out by Robert F. Kennedy, Jr. -- possibly on track to become head of health initiatives for the incoming presidential administration -- as a practice that should be halted. He recently asserted that Donald Trump would push to remove fluoride from drinking water on his first day in office.

Over the weekend, Trump told NBC News he had not discussed the issue with Kennedy, saying, "but it sounds OK to me. You know it's possible."

On Wednesday morning, Kennedy spoke with NPR, noting on Morning Edition, "We don't need fluoride in our water. It's a very bad way to deliver it into our systems."

Below, a primer on fluoride in drinking water, its history of controversy, and what the science says.

Fluoride is the chemical ion of the mineral fluorine. It is naturally present in trace amounts, according to the CDC, in soil, water, plants, and some food sources including plants and animals. It can also be released from volcanic emissions or as a byproduct of aluminum, fertilizer, and iron ore manufacture.

Once it's inside the body, according to the National Institutes of Health Office of Dietary Supplements, approximately 80% of what's ingested is absorbed in the gastrointestinal tract, with about 50% retained in the body of adults -- all but 1% stored in bones and teeth -- and the other 50% excreted in urine. In young children, up to 80% of absorbed fluoride is retained, as more is absorbed by bones and teeth than in adults.

Fluoride serves to prevent or reverse tooth decay and stimulate new bone formation, according to the NIH.

In 1945, Grand Rapids, Mich. became the first city in the world to fluoridate its drinking water. This came after a doctor's research on fluoride and fluorosis -- the discoloration of tooth enamel from an excess of fluoride -- and his hunch that safe levels might serve to prevent tooth decay.

The Grand Rapids fluoridation became a 15-year project, according to the National Institute of Dental and Craniofacial Research, with researchers monitoring the rate of tooth decay among 30,000 schoolchildren; after 11 years, it was found that the cavity rate among Grand Rapids children born after fluoride was added to the water supply dropped more than 60%. It was considered a scientific breakthrough that could revolutionize dental care.

Since 1962, the U.S. Public Health Service has recommended the addition of fluoride to tap water to reduce the risk and severity of tooth decay, according to the NIH. Currently, the CDC notes, the recommended concentration -- which is not enforceable and is a decision made on a local, not federal, level -- is that of 0.7 mg/L. The CDC says that drinking fluoridated water keeps teeth strong and reduces cavities by about 25% in children and adults. (Another common source of fluoride is toothpaste, which, when you brush with it, sticks fluoride to the tooth's surface, according to the CDC, and increases the amount of fluoride in saliva, which helps rebuild the outer enamel layer.)

Today, fluoridated municipal drinking water -- including tap water and foods and beverages prepared with municipal drinking water -- accounts for about 60% of fluoride intakes in the U.S. In 2022, notes the CDC, more than 209 million people, or 72.3% of the U.S. population served by public water supplies, had access to water with fluoride levels that prevent tooth decay.

The chemicals used to fluoridate drinking water in the United States, according to the NSF, are fluorosilicic acid, sodium fluorosilicate, and sodium fluoride, which are byproducts of the manufacture of phosphate fertilizer.

The CDC has a web page that lists fluoride levels in tap water by county.

Yes, says the CDC, which released a statement on the safety and efficacy of fluoridated water earlier this year.

It noted: "The safety and benefits of fluoride are well documented and have been reviewed comprehensively by several scientific and public health organizations. The U.S. Public Health Service; the United Kingdom's National Institute for Health Research, Centre for Reviews and Dissemination, at the University of York; and the National Health and Medical Research Council, Australia have all conducted scientific reviews by expert panels and concluded that community water fluoridation is a safe and effective way to promote good oral health and prevent decay. The U.S. Community Preventive Services Task Force, on the basis of systematic reviews of scientific literature, issued a strong recommendation in 2001 and again in 2013, for community water fluoridation for the prevention and control of tooth decay."

In his comment posted to X on Nov. 2, Kennedy wrote, "Fluoride is an industrial waste associated with arthritis, bone fractures, bone cancer, IQ loss, neurodevelopmental disorders, and thyroid disease."

While he may be correct regarding its source, the CDC, in its recent statement, disputes the health risks Kennedy raises, noting that the only potential risk is fluorosis from excess fluoride over a long period of time.

"Expert panels consisting of scientists from the United States and other countries, with expertise in various health and scientific disciplines," it noted, "have considered the available evidence in peer-reviewed literature and have not found convincing scientific evidence linking community water fluoridation with any potential adverse health effect or systemic disorder such as an increased risk for cancer, Down syndrome, heart disease, osteoporosis and bone fracture, immune disorders, low intelligence, renal disorders, Alzheimer disease, or allergic reactions."

That doesn't mean the addition of fluoride in water has been controversy-free all these years -- starting in the late 1940s, when activists on the far right of American politics asserted that fluoridation was part of a far-reaching plot to impose a socialist or communist regime.

More recently, in 2016, a Harvard Public Health article questioned the safety of fluoridated drinking water, raising the possibilities of brain toxicity, based on lab-animal studies, and other studies linking it to learning, memory, and cognitive deficits.

That story prompted a cavalcade of letters -- some supportive, including from the dentist, researcher, and former head of Preventive Dentistry at the University of Toronto who worked for years on a comprehensive scientific review of fluoride toxicity. He noted, "I was trained in traditional dentistry and for many years accepted the prevailing opinion of the dental/medical establishment in Canada and the U.S. that water fluoridation is 'safe and effective'...I was mistaken."

But many more responses were harshly critical of the article, such as one from a group of dental professionals, including the dean of the Harvard School of Dental Medicine, asking that it be rescinded, and providing pages of evidence as to where the article went wrong.

According to the NIH, high doses of fluoride -- typically from rare accidents with excessively high levels of fluoridated water or accidental ingestion of fluoride dental products intended for topical use -- can result in nausea, vomiting, abdominal pain, diarrhea, periostitis, and even, rarely, death. But such an acute dose, the NIH notes, "would be virtually impossible to achieve from water or toothpaste containing standard levels of added fluoride."

Another possible result of chronic, excess fluoride intakes of fluoride is skeletal fluorosis, which can lead to symptoms from joint pain to osteoporosis and muscle wasting. But it is "extremely rare" in the U.S., notes the NIH, with no evidence it's caused by the recommended level of fluoride in tap water.

Further, adds the NIH, while one study found an association between higher maternal urinary fluoride concentration in pregnancy with higher rates of neurobehavioral problems in a child at 3 years old, another similar study found no such association.

Regarding the claim that higher fluoride intakes during early development are associated with a lower IQ and other cognitive impairments, the NIH adds, researchers including those behind a National Academies of Sciences, Engineering, and Medicine review consider the evidence to be weak and methodologically flawed.

Finally, with regards to claims of fluoride and bone cancer, the American Cancer Society (ACS) points out that many systematic reviews of the connection have found "inadequate" conclusions and "no clear association." It notes that some of the controversy about the possible link stems from an old (1990) study of lab animals that found higher than expected numbers of osteosarcoma -- a rare bone cancer -- in male lab rats drinking fluoridated water.

Many population-based studies, meanwhile, have looked at the potential link between water fluoride levels and cancer and "have not found a strong link to cancer," the ACS reports.