Fluoridated drinking water was hailed as one of those Top 10 Public Health Achievements of the 20th century by the Centers for Disease Control and Prevention (CDC).

It is now called by Robert F. Kennedy, Jr.— possibly on track to become head of health initiatives for the next presidential administration — as a practice that should be stopped. He recently stated that Donald Trump will push to remove fluoride from drinking water on his first day in office.

Over the weekend, Trump he told NBC News he had not discussed the matter with Kennedy, saying, “but I think it’s OK. You know it’s possible.”

On Wednesday morning, Kennedy spoke to NPR, noting on the Morning Edition“We don’t need fluoride in our water. It’s a very bad way to deliver it to our systems.”

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

What is fluoride?

Fluorine is the chemical ion of the mineral fluorine. It is naturally present in trace amounts, according to CDCin 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 manufacturing.

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

Why is fluoride in drinking water?

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

In 1945, Grand Rapids, Michigan. became the first city in the world to fluoridate their drinking water. This came after a doctor’s research into fluoride and fluorosis – the discoloration of tooth enamel due to an excess of fluoride – and his hunch that safe levels could help prevent tooth decay.

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

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

Today, fluoridated municipal drinking water—including tap water and food and beverages prepared with municipal drinking water—accounts for about 60 percent of fluoride intake in the U.S. In 2022, notes the CDCmore than 209 million people, or 72.3% of the US population served by public water sources, had access to water with levels of fluoride that prevent tooth decay.

The chemicals used to fluoridate drinking water in the United States, according to NSFare fluorosilicic acid, sodium fluorosilicate and sodium fluoride, which are by-products of manufacturing phosphate fertilizer.

The CDC has one web page which lists fluoride levels in tap water by county.

Is fluoride in drinking water safe?

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

It noted: “The safety and benefits of fluoride are well documented and have been comprehensively reviewed by several scientific and public health organizations. US Public Health Service; UK National Institute for Health Research, Center for Evaluation and Dissemination, University of York; and the National Health and Medical Research Council, Australia, carried out 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 US Community Preventive Services Task Forcebased on systematic reviews of the scientific literature, issued a strong recommendation in 2001 and again in 2013 for community water fluoridation for the prevention and control of dental caries.”

Why is fluoride in tap water controversial?

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

While he may be right about 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 of scientists from the United States and other countries with expertise in various health and scientific disciplines,” it noted, “considered the available evidence in the peer-reviewed literature and found no scientific evidence convincing evidence linking community water fluoridation to any potential adverse health effect or systemic disorder, such as an increased risk of cancer, Down syndrome, heart disease, osteoporosis and bone fracture, immune system disorders, low intelligence, kidney disorders , Alzheimer’s disease or allergic reactions.”

That’s not to say that adding fluoride to water has been without controversy all these years — starting in the late 1940s, when far-right activists in American politics stated 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 possibility of brain toxicity, based on laboratory animal studies and other studies linking it to learning, memory and cognitive deficits.

This 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 at a comprehensive scientific review of fluoride toxicity. He RECORDED“I was trained in traditional dentistry and for many years accepted the prevailing view of the Canadian and US dental/medical establishment that water fluoridation was ‘safe and effective’…I was wrong.”

But many other responses harshly criticized the article, like one from a group of dental professionals, including the dean of the Harvard School of Dental Medicine, who called for it to be retracted and provided pages of evidence as to where the article went wrong.

Conformable to NIHhigh doses of fluoride—usually from rare accidents with excessively high levels of fluoridated water or accidental ingestion of topical fluoride dental products—can lead to nausea, vomiting, abdominal pain, diarrhea, periostitis, and even, rarely, death. But such an acute dose, the NIH notes, “would be virtually impossible to obtain from water or toothpaste containing standard levels of added fluoride.”

Another possible result of chronic excess fluoride intake is skeletal fluorosis, which can lead to symptoms ranging from joint pain to osteoporosis and muscle wasting. But it is “extremely rare” in the US, the NIH notes, with no evidence that it is caused by the recommended level of fluoride in tap water.

What’s more, the NIH adds, while one study found an association between higher maternal urinary fluoride concentration during pregnancy with higher rates of neurobehavioral problems in a child at age 3, another similar study found no such association.

As for the claim that higher fluoride intake during early development is associated with lower IQ and other cognitive impairments, the NIH adds, researchers, including those behind a National Academies of Sciences, Engineering and Medicine review, believe that the evidence is weak and methodologically flawed.

Finally, regarding the claims of fluoride and bone cancer, 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 over the possible link stems from an old (1990) study on lab animals that found a higher-than-expected number of osteosarcomas—a rare bone cancer—in male lab rats that drank fluoridated water.

Meanwhile, many population-based studies have looked at the potential link between fluoride levels in water and cancer and “have not found a strong link with cancer,” the ACS reports.

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This story was originally presented on Fortune.com