President-elect Donald Trump has picked Robert F. Kennedy Jr. as his Health and Human Services secretary and pledged to let him “go wild on health,” putting health care high on the agenda for his second term.

While we don’t know what the policies will be, one thing Kennedy has signaled he would look at is fluoridation of water across the United States. He won’t be able to take any action until nominated and then confirmed by the Senate in 2025. But on social media site X, he said that when he is in position, the White House would advise all US water systems to remove fluoride from public water systems, stating: “Fluoride is an industrial waste associated with arthritis, bone fractures, bone cancer, IQ loss, neurodevelopmental disorders, and thyroid disease.”

Fluoride is a naturally occurring mineral which is present in trace amounts in soil, plants, water and food, and at optimal levels helps to prevent cavities by keeping teeth strong. Countries take a range of approaches to ensure fluoride is provided to children - some countries add fluoride to water, like the United States, and others add fluoride to milk or salt to keep it at recommended levels. Other countries opt for school-provided fluoride varnishes and mouthwashes.

Fluoridation in the US

Community fluoridation of water was introduced in the United States in 1945 to help improve oral health in a cost-effective and equitable way. The US Centers for Disease Control and Prevention (CDC) named fluoridation of drinking water one of the 10 greatest health interventions in America in the 20th century alongside vaccination, family planning and safer workplaces.

According to CDC figures, as of 2022, 72% of the US population receives fluoridated drinking water. This varies among states, ranging from 8.5% of the population in Hawaii to 100% of the population in Washington, D.C.

Water fluoridation is not mandatory, and while there is a recommended fluoride concentration in drinking water (0.7 milligrams per liter) from the CDC, that level is not an enforceable standard. The decision to fluoridate the water is taken at a local level and depends on how much fluoride is naturally present.

There are 36,767 water systems in 36 states with publicly available information on the CDC‘s website, a small slice of the estimated number of drinking water systems in the US. As of 2023, just over a third (37%) of those water systems included in the data are fluoridated.

It varies depending on state, too. In Kentucky, 93.8% of these reported water systems are fluoridated, and at the bottom end, only 3.7% of water systems reported in Alaska are fluoridated.

There is also a huge range in the levels of fluoride that are found in these water systems. Not all of the 36 states that report fluoride access report fluoride concentration levels, limiting opportunities for thorough analysis. Among them, 24 states report the fluoride levels in 1,774 public water systems.

Some 53% of these water systems have fluoride concentration within recommended levels - between 0.7mg per liter and 1.2mg per liter. Just under half of them (47%) have fluoride concentration under the CDC’s recommended level.

From the publicly available data on water system fluoride levels, two water systems, both located in South Carolina, reported fluoride concentration levels above WHO guidelines. WHO states that long-term exposure to fluoride levels above 1.5 milligrams per liter (mg/L) in drinking water can lead to health problems.

Does water fluoridation help? Evidence is mixed

It’s long been known that fluoride helps to strengthen teeth and bones - that’s why it has been added to food and tap water for many decades. The CDC holds community fluoridation responsible for a 25% reduction in cavities in children. Cavities can cause pain and sensitivity, which can affect speaking and eating. They can also lead to tooth loss or infection that can spread to the body, causing serious illness or death.

But at certain concentration levels, fluoride can cause harm. The World Health Organization says that while fluoride reduces the incidence of dental caries (cavities) it does have negative effects at higher levels, such as “causing tooth enamel and skeletal fluorosis following prolonged exposure to high concentrations.” Skeletal fluorosis is a serious condition which causes bone weakness and stiffness and pain in joints.

Numerous studies have shown that fluoridated water can help prevent tooth decay and improve oral health. A 2010 Nevada study found that living in a community with fluoridated drinking water was associated with lower incidences of decayed, missing or filled teeth. In 2011, the Canadian city of Calgary removed fluoride from its water due to skepticism about the benefits of fluoride and to cut costs. A later study comparing children in the city with those in Edmonton, where water remained fluoridated, found that over three years, children in Calgary had more tooth decay in their baby teeth. This suggests fluoridated water, without other interventions, was an important source of fluoride for that community.

Yet other studies have found that improvements in oral hygiene awareness and dental practices more generally have lessened the benefits of fluoridated water. A recent report concluded that water fluoridation does help prevent cavities, but not as much as originally thought; authors posit that recent improvements in cavity prevention are due to the popularity of fluoride in toothpaste and mouthwash.

International data also supports this. Across the world, tooth decay has decreased since the 1970s, even in countries where fluoride is not added to water.

Some of the countries where water is not fluoridated take a different approach to oral health, choosing other fluoride-based interventions.

In the UK, for example, Manchester and Blackpool have programs to provide fluoridated milk to local schoolchildren. In the rest of the country, an estimated 10% of the population has access to fluoridated water, either naturally or through local fluoride programs.

In Iceland, water is not fluoridated but there are fluoride rinse programs at schools around the country. Children attending school have twice-monthly fluoride rinses.

Japan began a similar school-based fluoride mouth-rinse program in 1970 which has since expanded to nurseries around the country. Research has found that these fluoride mouth-rinse programs are more effective in decreasing cavities when initiated at younger ages.

In Italy, the Society of Paediatric Dentistry recommends a series of interventions for children such as brushing teeth twice a day with fluoride toothpaste, and reducing sugary food and drinks. High-risk individuals are recommended fluoride tablets, varnishes, gels or sealants depending on their age.

However, exposure to too much fluoride can lead to adverse effects. A study, published in May this year, found that higher exposure to fluoride during pregnancy was associated with increased neurobehavioral problems. And an August, federal review of existing research found that higher estimated fluoride exposure (exceeding WHO guidelines of 1.5mg/L) was associated with lower IQ in children. These levels are rarely seen in the US.

Fluoridation of water is “considered to be particularly beneficial for deprived communities and families who are unable or culturally unwilling to access dental hygiene and dental services,“ water researcher John Fawell, a visiting professor at Cranfield University, told CNN in an email.

Fawell, who has advised the World Health Organization on water quality standards, added: “Fluoride at lower concentrations than [those that] cause dental fluorosis have been associated with a small reduction in IQ in children in some epidemiological studies.

“However in the absence of a clear mechanism it remains an association which is not necessarily causal.”