Fluoride has been added to the water supply in countries across the world for decades, heralded by health authorities as a leading 20th-century public health improvement.
However, the election of US President Donald Trump in November 2024, and the subsequent appointment of Robert F Kennedy Jr as health secretary, has led to a resurgence of claims about its safety.
Opponents of water fluoridation have long claimed it causes neurodevelopmental or other health problems, and Mr Trump has indicated he supports potentially removing it from US water supply.
Mr Kennedy, a leading opponent of fluoridation, has described it as "an industrial waste" responsible for neurodevelopmental disorders, cancer and decreased IQ.
In April, he announced his intention to tell the US's public health agency to stop recommending fluoridation in communities nationwide.
These claims have spread across social media and been repeated by some Australian politicians, such as Senator Ralph Babet.
Oliver Jones, an expert in analytical science and chemistry at RMIT University, told AAP FactCheck fluoride is a naturally occurring mineral found in soil, water, plants and food.
"The benefit of adding fluoride to water (or salt, as many countries in Europe and South America do) is that it helps prevent cavities, particularly in children," Professor Jones said.
Since the middle of the 20th century, it has been added to drinking water in Australia, New Zealand, Canada, Hong Kong, Singapore, the UK and the US.
Australian regulatory authorities permit fluoride to be added to water between a range of 0.6 and 1.1 milligrams per litre (page 1), which is within the World Health Organization's recommended maximum (p7) of 1.5mg of fluoride per litre (mg/L) of water.
According to Australia's National Health and Medical Research Council, fluoride decreases tooth decay by 26 to 44 per cent in children and adolescents, and 27 per cent in adults.
Like many health interventions, it also has risks.
"At high levels of ingestion, fluoride can cause harm – the most obvious is dental fluorosis," University of Melbourne dental public health expert Matt Hopcraft told AAP FactCheck.
This causes white lines or spots on tooth surfaces and in severe cases can lead to brown staining or discolouration, Associate Professor Hopcraft explained.
The biggest source of controversy, however, is fluoride's potential links to neurodevelopmental issues.
Founder of the University of Florida's Fluoride Aluminum Brain and Behavior Lab Ashley Malin said several studies have linked chronic exposure to low levels of fluoride during prenatal development with poorer neurodevelopmental outcomes.
A 2024 paper Dr Malin co-authored tested urine samples from 229 pregnant women and then assessed their children's neurobehavioral outcomes at age three.
It found children who experienced prenatal fluoride exposure were more likely to exhibit behavioural problems, but noted that the urine sample used could be influenced by other food and beverage consumption.
Other experts questioned the study's findings, however, pointing to the small sample size and variable measurements.
In 2019, a Canadian study measured fluoride intake in 512 pregnant women living in six cities via urine samples and self-reported water consumption and later tested their children's IQ levels between three and four years of age.
The researchers took three urine samples throughout the mothers' pregnancy and found an increase in concentration of fluoride of 1mg/L was associated with a 4.49-point lower IQ score in the offspring who were boys, but not for girls.
Based on these sample results and information on beverage intake, the researchers calculated the mother's estimated daily fluoride intake and determined that a 1mg/L increase in fluoride intake was associated with a 3.66-point drop in children's IQ.
However, the study was limited by the mothers' ability to recall beverage consumption and the authors acknowledged their method for calculating maternal fluoride intake was not scientifically validated.
Recent claims about the link between fluoride and neurodevelopment have also been driven by the release of a report from the US National Toxicology Program in August 2024.
The agency published a review of a range of studies investigating fluoride and IQ levels that were conducted in countries including Canada, China, India, Iran, Pakistan and Mexico.
It concluded that drinking water containing more than 1.5 mg/L of fluoride is associated with lower IQs in kids.
However, this level is twice the limit recommended by US health officials and many of the studies were conducted in areas with naturally high fluoride levels.
The report notes that there was insufficient data to determine if the 0.7 mg/L recommended by US authorities has a negative effect on children's IQ.
Dr Malin said these findings are still concerning and pointed to a recent US federal court decision, which determined that research suggests fluoridation of water at 0.7mg/L poses an "unreasonable risk".
The court directed the Environmental Protection Agency to strengthen regulations around fluoride use, but stressed that the decision did not conclude with certainty that fluoridated water endangers public health.
Other studies have found no link between fluoride and IQ levels.
A 2022 Australian study, which included more than 2000 children, found fluoride consumption had no effect on neurobehavioural development.
"Exposure to fluoridated water during the first [five years] of life was not associated with altered measures of child emotional and behavioral development and executive functioning," it states.
A more recent 2024 study assessed the IQ scores of 357 people, aged 16 to 26 years, to see if exposure to fluoride as a child had affected brain development.
It found those who'd consistently been drinking fluoridated water had an IQ score 1.07 points higher on average than those with no exposure.
Dr Hopcraft added that several meta analyses have found the levels of fluoride used in water are not associated with lower IQ scores in children.
However, "there is potentially some association at levels higher than we see in Australia," he added.
Dr Jones agreed: "Multiple high-quality studies in many different countries, including one that followed people over 30 years, testing their IQs at various ages, have consistently found no link between fluoride levels and IQ scores."
A 2023 paper reviewed eight studies that looked at the association between community water fluoridation and children's cognition, concluding that exposure at levels less than 1.5mg/L is not linked to lower IQ scores.
However, it said the association observed at higher levels in endemic fluoride areas requires further investigation.
Dr Jones said context is important when discussing toxicity of a substance.
"A couple of tablets of aspirin or paracetamol will cure your headache, for example, while 20 tablets are likely to cause severe liver damage or even death.
"Just because something can cause a problem under certain conditions doesn't mean it does so at the levels we're typically exposed to.
"There are decades of evidence that water (or salt) fluoridation is a safe and effective way to improve oral health," he said.
