Extract from ABC News
Large studies have analysed data from thousands of children in Australia and the US. (Getty Images: Yarnica)
In the early 2000s, a few small studies done on communities in China and India caught Loc Do's attention.
The studies found these places, which had naturally high levels of fluoride in their water supplies from surrounding rocks, also had children with lower IQ levels.
The fluoride levels in these studies were much higher than they ever have been in Australia.
But Professor Do, a dentistry researcher at the University of Queensland, still thought it was worth investigating.
"I came in with an open mind," he says.
"We know that water fluoridation is effective in preventing decay. But we needed to make sure that the program was also safe for people using it, especially young children."
Now, decades of research — including a large study published today in the US — conclude that consuming fluoride at recommended levels in drinking water has no adverse effects.
Professor Do is very confident there is no link between fluoride exposure and cognitive development.
"I think we can close the book there," he says.
Despite mounting scientific evidence on the safety — and health benefits — of fluoride, there's still concern in some pockets of the community that fluoride is a "neurotoxin", and dangerous to cognitive development.
Even though there is little evidence to support these concerns, they've prompted some local governments in Australia and some US states to remove fluoride from their water supplies.
So is there any truth to the claim? Here's everything you need to know.
Is fluoride really a neurotoxin?
Technically fluoride is a neurotoxin, but not in amounts you're likely to encounter.
Oliver Jones, a chemist at RMIT University, says you'd need to eat huge quantities of fluoride-containing substances to see any toxic effects.
But we're not usually exposed to enough fluoride to cause problems.
"Anything is a toxin if you have enough of it," Professor Jones says.
"Even water is a neurotoxin if you drink enough."
People understand this with other substances, such as alcohol, he says.
"Everyone knows if you drink too much, you're going to get liver damage and you can have cancer."
Fluoride is added to Australian tap water at levels of 0.6-1.1 milligrams per litre. (Unsplash: Jacek Dylag)
Ian Musgrave, a toxicologist at the University of Adelaide, says fluoride is used in high concentrations as a poison, such as in some pest baits.
But studies have been done to establish how much fluoride is required to poison animals, and how much they can safely be exposed to.
There's a wide margin between the level of fluoride that causes harm to animals, and the amount added to water.
"The levels that are in potable water are about 50–200 times lower than the concentration that has no effect on animals," Dr Musgrave says.
While we also get fluoride from other sources, like toothpaste, this still doesn't bring us up to a dangerous exposure level.
"When they set the limits for fluoridation in water, it takes into account all the sources."
This is why toothpaste tubes say "do not swallow": accidentally swallowing some while brushing your teeth is fine, but all the fluoride in the tube of toothpaste could potentially be dangerous in one hit.
"You probably have to eat an entire tube of toothpaste to cause significant harm," Dr Musgrave says.
If you do manage to eat enough fluoride to suffer from its toxicity, Professor Jones says that other parts of the body are affected before the brain is.
Teeth and bones show the first signs of fluoride overexposure, with a condition called "fluorosis" staining and eventually weakening tooth and bone structure.
"The amount that you would need to have negative effects on the brain is extremely high, and you would see other effects first like dental fluorosis or even skeletal fluorosis."
Is there a link between fluoride and IQ?
Large studies have found no link between fluoride and IQ.
A couple of small studies flagged a link between fluoride exposure and lower IQ levels, alongside a heavily criticised review by the US National Toxicology Program earlier this year.
"Most of those studies were not well conducted, without controlling for other factors," Professor Do says.
For instance, some of the places with naturally high fluoride levels also had heavy metal contaminants in their water supply.
However, larger studies have not been able to replicate the link.
No, fluoride in drinking water doesn't lower IQ. Listen to the Health Report to find out why
A study, published today in the journal Science Advances analysed data from academic tests of 26,820 teens who went to more than 1,000 high schools across the US in 1980.
They found no evidence students living in areas with fluoridated water performed worse than those living in unfluoridated areas when they were in high school; there may have even been a small benefit to children in fluoridated areas.
There was also no impact of fluoridation on a subset of students who were followed up 40 years later.
Large studies on Australian children by Professor Do also show no evidence for a link between fluoride and IQ.
In 2022, Professor Do and his colleagues examined cognitive development tests for nearly 2,700 children, over a course of seven or eight years.
They did not find any link between fluoride exposure and cognitive development.
Last year, they specifically tested children with "dental fluorosis": stains on the teeth caused by exposure to levels of fluoride above what is put in tap water.
"Dental fluorosis is a reliable biomarker of fluoride exposure in early childhood," Professor Do says.
Mild dental fluorosis, causing harmless tooth stains, is the first sign of overexposure to fluoride. (Wikimedia Commons: Matthew Ferguson 57, CC BY-SA 4.0)
Despite clearly having slightly more fluoride exposure than recommended, these children also had no difference in their IQ or other development scores.
"We did not find any link between fluoride exposure and IQ. And that has been well accepted by the scientific community," Professor Do says.
Dr Musgrave, who wasn't involved in these studies, says a number of other studies also back the safety of adding fluoride to water.
"Overall, the evidence is that at the concentrations that are present in drinking water, even concentrations that are slightly higher than drinking water … there's no effect on IQ," he says.
Does fluoride accumulate in the body?
Fluoride does build up very slowly in your teeth — that's why it's a useful dental protection — and in your bones, but not elsewhere.
Dr Musgrave says fluoride is very soluble: it dissolves very easily in water, much like chloride from table salt.
"If you're consistently drinking fluoridated water, there'll be a little bit more fluoride in your bone," he says.
"But because bones are slowly turned over, it doesn't really accumulate to the levels that will cause bone fractures, for example, and it doesn't accumulate in your other tissues."
Professor Jones says roughly half of the fluoride ingested by adults gets incorporated into teeth and bone, while growing children tend to absorb slightly more.
"The rest will be excreted, usually by the urine."
What about dangerous fluorine-containing substances?
Fluoride's presence in baits — as well as a number of other concerning substances, like fluorine gas, hydrogen fluoride and PFAS — is often floated as evidence that it's dangerous.
But fluorine atoms themselves (F) are not the risk here: the danger occurs in the molecules they're part of, and — again — in the amount of them.
"It's really the structure of the molecule it's part of, not the individual components," Professor Jones says.
For example, hydrogen (H) can be combined with oxygen (O) to make two completely different molecules.
"H2O is water which we all need, but H2O2 is hydrogen peroxide and that's a very strong bleach."
If you're worried about fluoride or fluorine appearing in poisons, it might be worth considering all the other things it appears in naturally.
Foods like tea, cereals, apples, and almonds all naturally contain fluoride — alongside the ocean, which has about one milligram of fluoride per litre.
Laurence Walsh, a dentistry researcher at the University of Queensland, says he brings this up with patients concerned about fluoride.
"When I run into patients who say 'I'm allergic to fluoride', I say, 'Have you ever swum in the ocean? Have you ever had a cup of tea?'"
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