askST: Can the fluoride in drinking water really lower children’s IQ?

SINGAPORE – A recent controversial US government study has found that children exposed to higher fluoride levels have lower IQ levels, reigniting debate over whether the widespread practice of adding the cavity-reducing mineral to drinking water should continue.

Published in the reputable Journal of the American Medical Association Paediatrics, the study is so contentious that two editorials with opposing viewpoints were released alongside it.

The report comes amid remarks by Mr Robert F. Kennedy Jr, US President Donald Trump’s pick for US health and human services secretary, that fluoride in water causes numerous health problems, including cancer.

But critics have pushed back on such assertions, pointing out that conspiracy theories around fluoride have existed since almost as early as when the mineral was first added to public water supplies in the US in 1945.

What is indisputable is the public health benefits of adding fluoride to water supplies, also known as water fluoridation. It is widely accepted as the most cost-effective way to prevent tooth decay in the population.

Many countries and jurisdictions such as Australia, Hong Kong, Ireland, Malaysia and Britain add fluoride to their water sources for the oral health of their populations.

Singapore has been adding fluoride to its drinking water since 1954. In response to queries on the concerns raised in the study, the Ministry of Health (MOH) said it is aware of the publication.

MOH said it regularly reviews the need for and optimal level of fluoridation of drinking water, based on the latest scientific evidence.

“To date, MOH has not found robust evidence linking water fluoridation at the levels currently used in Singapore to adverse health effects. In fact, research has shown that cessation of water fluoridation programmes generally leads to increased dental caries (or tooth decay) in various populations, as observed in countries like Canada, Finland and Israel,” said the ministry.

The Straits Times unpacks the claims that fluoride can impact children’s IQs to see whether they really hold water, and whether Singaporeans should be concerned for their children’s intelligence and intellectual development.

Q: What is fluoride, and why is it added to drinking water?

Fluoride is a naturally occurring mineral that is found in soil, water and food. It is also known for its role in strengthening teeth and preventing cavities.

The National University of Singapore’s (NUS) Faculty of Dentistry said fluoride works to prevent tooth decay by remineralising tooth structure that has been weakened by acids formed from bacteria in the mouth. The remineralised tooth structure becomes more resistant to tooth decay.

“Fluoride primarily works through a topical effect, and thus water fluoridation provides a constant very low-level concentration in the mouth that helps protect teeth against decay,” said the faculty’s spokesperson.

Q: What does the study say?

Researchers from the US’ National Institute of Environmental Health Sciences reviewed 74 studies on fluoride exposure and children’s IQ conducted in 10 countries, including China, India, Canada and Mexico.

The analysis found a statistically significant association between higher fluoride exposure and lower children’s IQ scores.

For every 1mg per litre increase in urinary fluoride, which is an estimate of exposure from water and other sources, there is a decrease of 1.63 IQ points in children.

However, the study also notably found that there was no significant association with IQ when fluoride was measured at less than 1.5mg per litre in water. That is the World Health Organisation’s (WHO) safety limit.

Q: What do experts have to say about this study?

Professor Eric Chan from the department of pharmacy and pharmaceutical sciences at NUS’ Faculty of Science said that while the results show a link between lower IQ scores and fluoride levels, there is no solid evidence showing that fluoride directly lowers IQ.

Prof Chan also pointed out the difference between statistical significance and clinical significance.

He explained that if a new painkiller lowers pain from 10/10 to 8/10 (where zero means no pain and 10 is unbearable pain), this reduction could be statistically significant. But it is not clinically significant because the patient would still be in severe pain.

“Similarly, IQ scores are grouped in a way that a small drop of one to two points doesn’t make a big difference in performance. While the authors tried to explain the effect on the population as a whole, the impact on an individual is likely very small,” he said.

There are also several concerns about the study’s methodology.

Professor Teo Yik Ying, dean of the Saw Swee Hock School of Public Health at NUS, said the authors attempted to combine evidence across multiple studies in order to come out with a more definitive conclusion.

“However, the authors themselves also admitted the vast majority of the studies – 52 out of 74 – were rated ‘high risk of bias’, which means these studies are unreliable to begin with,” said Prof Teo.

Another criticism levelled at the paper is the use of urine to measure fluoride exposure.

Twenty of the studies analysed did so, using measurements of fluoride in a person’s urine at one point during the day. Such a method is flawed because the amount of fluoride in a person’s urine is also affected by diet.

Dr Shane Snyder, a professor at the Georgia Institute of Technology, said: “The study raises more questions than it produces any answers… If the children in the studies have elevated fluoride (levels) in their urine, it may have been that they were exposed to fluoride somewhere else.”

Q: Is the study applicable to the Singapore context?

No.

MOH said Singapore currently maintains water fluoridation at a concentration of about 0.5mg per litre, within the 0.5mg to 1mg per litre range typically recommended for fluoridation of water supplies, and well below the WHO limit of 1.5mg per litre.

The ministry also reiterated that the study found no robust link between fluoride concentrations of less than 1.5mg per litre and children’s IQ levels.

Q: What are some proven factors that impact a child’s IQ?

Dr Annabelle Chow, clinical psychologist at Annabelle Psychology, said IQ is shaped by a combination of genetics, education and environmental factors, making it a reflection of both nature and nurture.

Education can improve IQ to some degree. The more time children spend in school and learning environments, the better their chances of scoring higher on IQ tests. This is because schooling enhances skills like problem-solving, reasoning and memory, which IQ tests measure.

Factors like a family’s socio-economic status can influence the quality of education a child receives, and their access to nutritious food, both of which are important for brain development. Having access to books, learning tools and enriching experiences can also boost a child’s mental abilities.

Q: Does the study give parents cause for concern?

In short, no.

Associate Professor Richard David Webster from Nanyang Technological University’s School of Chemistry, Chemical Engineering and Biotechnology said: “Everybody in Singapore is exposed to the same tap water, but people have many different IQ levels. Therefore, there are likely to be many other factors that affect IQs, and fluoridation is a very minor factor at worst.”

He added: “People should be cautious about the study until more data is obtained, or other researchers reproduce the findings.”

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