Inadequate support: A review published in Lancet Neurology that referred to fluoride as a neurotoxin had significant methodological flaws. The evidence it provided didn’t support the claim that water fluoridation lowers children’s IQ.
FULL CLAIM: “Fluoride Officially Classified as a Neurotoxin”; after the government “put fluoride in our water and attacked anyone who questioned it”, now it “has declared it ‘reduces the IQ of children’ and is ‘hazardous to human health’”
REVIEW
Fluoride is a mineral that occurs naturally in soil, water, and in small amounts, in many foods. In the 1940s, researchers in the U.S. discovered that low amounts of fluoride strengthen tooth enamel, preventing cavities. This finding set the basis for community water fluoridation in countries such as the U.S., Australia, New Zealand, and Canada as a safe and effective way of preventing tooth decay[1].
On 21 August 2024, the National Toxicology Program (NTP), run by the U.S. Department of Health and Human Services, released a monograph on the “State of Science Concerning Fluoride Exposure and Neurodevelopment and Cognition”. The NTP assessment reported an association between high fluoride levels in drinking water and “lower IQ in children”.
Following the monograph’s release, new and rehashed claims questioning the safety of community water fluoridation appeared on social media platforms. One example is an 8 October 2024 post on X (formerly Twitter) with roughly three million views, claiming that after the government “put fluoride in our water and attacked anyone who questioned it”, public health agencies “declared” that fluoride “reduces the IQ of children” and is “hazardous to human health”.
Around the same time, a post claiming that fluoride had been “Officially Classified as a Neurotoxin in World’s Top Medical Journals” went viral on Facebook.
These posts misrepresent the findings of the NTP and the results of published scientific studies, as we will show below. Available scientific evidence doesn’t indicate that fluoride exposure at the levels added to drinking water is unsafe, as Science Feedback explained in earlier reviews.
The NTP evaluated fluoride exposures higher than the levels added to community water to prevent cavities
The NTP monograph summarized the results of a systematic review of published studies assessing the effects of fluoride exposure on neurodevelopmental and cognitive health.
The NTP found no evidence that fluoride exposure negatively impacted adult cognition. However, it concluded with “moderate confidence” that amounts of fluoride in drinking water exceeding 1.5 milligrams (mg) per liter “are associated with lower IQ in children”.
But these findings don’t support social media claims linking community water fluoridation with reduced IQ in children.
That’s because the NTP assessment of the effect of fluoride on children’s IQ was primarily based on 19 studies conducted at locations in Canada, China, India, Iran, Pakistan, and Mexico, with naturally occurring fluoride levels exceeding 1.5 mg of fluoride per liter of drinking water.
These levels are more than double the amounts added to drinking water in countries such as the U.S. and Canada, currently set at 0.7 mg per liter. In Europe, only Ireland and selected regions in the U.K. currently fluoridate drinking water, at concentrations that don’t exceed 1 mg per liter. The World Health Organization (WHO) recommends a limit of 1 mg per liter for community water fluoridation, well below the levels evaluated by the NTP. The WHO’s guideline value for naturally fluoridated drinking water is 1.5 mg per liter.
The release of the NTP monograph was significantly delayed to address concerns raised during peer review by the National Academies of Sciences, Engineering, and Medicine (NASEM).
One of these concerns was a statement made in a draft version of the monograph that fluoride is “presumed to be a cognitive neurodevelopmental hazard to humans.” This draft was misrepresented on social media as evidence that water fluoridation is neurotoxic, after a lawsuit by opponents of water fluoridation forced the publication of the draft and the NASEM comments in 2023.
However, the NASEM considered that the evidence provided by the NTP didn’t support this conclusion, which was removed in later versions of the monograph. In addition, the NASEM cautioned that the NTP “should make it clear that the monograph cannot be used to draw any conclusions regarding low fluoride exposure concentrations, including those typically associated with drinking-water fluoridation.”
Accordingly, the final version of the monograph explicitly states:
“This Monograph and Addendum do not address whether the sole exposure to fluoride added to drinking water in some countries (i.e., fluoridation, at 0.7 mg/L in the United States and Canada) is associated with a measurable effect on IQ.”
An expert committee at the American Dental Association (ADA) that evaluated the NTP’s assessment also cautioned about this limitation:
“[T]he monograph does not provide any new or conclusive evidence that should necessitate any changes in current community water fluoridation practices for public health policy consideration. None of the studies on IQ included in the organization’s review were conducted in the U.S. and were instead from areas with high levels of naturally-occurring fluoride in water. The report does not provide any evidence to inform the practice of community water fluoridation”.
In the same press release, the ADA stated that the NTP hadn’t adequately addressed what the NASEM identified as “worrisome inconsistencies” in its criteria to determine the risk-of-bias of the studies evaluated. It added that several limitations in the NTP evaluation “undermine[d] the report’s robustness”.
A 2014 review in Lancet Neurology doesn’t demonstrate that fluoride is neurotoxic
The Facebook post claiming that fluoride was “Officially Classified” as a neurotoxin reproduced the headline of an article posted on the website AwarenessAct.com in 2018. According to the fact-checking organization Snopes, identical articles have been published on multiple websites since at least December 2015.
The article’s claim was based on a review of earlier studies published by Grandjean and Landrigan in Lancet Neurology in 2014, which concluded that several chemicals, including fluoride, are “neurotoxicants”[2]. This review was previously cited to support claims that community water fluoridation is harmful.
The article and its headline are misleading in several ways.
Claiming that a substance is “one hundred percent” toxic, as the article did, reflects a misunderstanding of the basic principles of toxicology. Toxicity isn’t an absolute category as the article implied, instead, it’s “the dose [that] makes the poison”. In other words, the toxicity of a substance depends on variables such as how much of the substance a person is exposed to, through what route the exposure occurs, and for how long.
At the time of publication, the review received substantial criticism from experts in the field, who argued that it lacked rigor and that its conclusions were unsupported by the evidence. The journal also received numerous letters from researchers stating that the review’s claims—including those about fluoride—were unsubstantiated. An earlier review by Science Feedback analyzed these concerns in detail.
Briefly, the evidence provided for fluoride neurotoxicity was limited to a single meta-analysis by Choi et al., also co-authored by Grandjean, and published in Environmental Health Perspectives in 2012[3]. The meta-analysis concluded that “children in high-fluoride areas had significantly lower IQ scores than those who lived in low-fluoride areas”.
The meta-analysis included 27 individual studies conducted in poor, rural regions in China, Mongolia, and Iran with high levels of pollution and naturally high levels of fluoride in drinking water. However, the authors of the meta-analysis didn’t control for the potential effect of pollutants, nutritional status, parents’ IQ, socioeconomic status, and other factors that could have impacted IQ scores.
Researchers pointed out that Choi et al. also didn’t clearly state that the groups used as reference had access to water with fluoride levels around those recommended for community water fluoridation. This is relevant because it could provide an alternative explanation to the differences observed: that people exposed to fluoride within the recommended levels have a higher IQ.
In light of these limitations, the meta-analysis doesn’t provide sufficient evidence to support an association between fluoride exposure and neurotoxicity in children.
The National Health and Medical Research Council of Australia also drew the same conclusion on the meta-analysis. Page 215 of its 2016 evidence evaluation report on water fluoridation’s health effects stated, “Overall, this is not convincing or robust evidence of an association between fluoride in drinking water and a reduction in children’s IQ.”
Current evidence doesn’t indicate that fluoride at the levels added to community water is unsafe
Virtually any substance, even water, can be toxic in excessive amounts. Fluoride is no exception.
In some regions, water supplies contain naturally high levels of fluoride that can cause health problems[4]. One common effect of fluoride overexposure is permanent tooth discoloration in children, called dental fluorosis. Over time, exposure to very high amounts of fluoride can cause this mineral to progressively accumulate in the bone. This condition, called skeletal fluorosis, can lead to severe bone and joint damage.
Over the past decade, numerous studies have evaluated the potential impact of fluoride exposure on neurodevelopment and cognition in human populations[5]. In general, older studies have the same limitations as the meta-analysis by Choi et al., namely, high fluoride exposures and a lack of adequate controls[5].
More recent and better-controlled studies specifically evaluating the effects of low fluoride levels on neurodevelopment and cognition have produced inconsistent results[6-10].
One study in Canada reported differences in the IQ scores of three and four-year-olds depending on the fluoride levels detected in their mothers’ urine during pregnancy[6]. However, the study only showed differences in boys but not in girls, and some experts raised concerns about the study’s methodology.
Other recent studies followed participants over time and measured IQ scores at different ages. These studies found either no difference in the IQ scores of children exposed to fluoridated water compared to those unexposed[8] or even reported higher IQ scores among children exposed to fluoridated water[9].
A 2023 meta-analysis of eight studies evaluating the effect of fluoride exposure at the levels recommended for community water fluoridation found no association between fluoride exposure and reduced IQ in children[10].
Very high doses of fluoride, far above the recommended levels for water fluoridation, have shown neurotoxicity in laboratory settings. But overall, current evidence from studies evaluating low fluoride exposures in animals and human populations doesn’t indicate that the low levels of fluoride added to community water cause neurodevelopmental problems[10,11].
In contrast, the benefits of community water fluoridation for dental health are well documented.
The CDC states that drinking fluoridated water “reduces cavities by about 25% in children and adults“. Because of its important role in reducing tooth decay, the CDC included water fluoridation as one of the Ten Great Public Health Achievements of the 20th century.
Medical associations, including the American Dental Association, the American Academy of Pediatrics, the American Academy of Pediatric Dentistry, and the American Association for Dental, Oral, and Craniofacial Research, also endorse community water fluoridation as a safe and effective strategy to prevent tooth decay.
Conclusion
Contrary to social media claims, a 2024 NTP monograph evaluating the effect of fluoride exposure on cognition didn’t show that community water fluoridation lowers IQ. These claims misrepresent the results of the NTP assessment, which only found an association between fluoride and lower IQ at levels that are more than twice the amounts added to community water in some countries.
These high fluoride levels don’t result from community water fluoridation but occur naturally in water supplies in some regions. Exposure to such high levels is known to cause health problems such as dental and skeletal fluorosis. Some studies suggest it might also lead to lower IQ in children. However, many of these studies were also poorly controlled and don’t provide enough evidence that fluoride specifically is responsible for lower IQ in children.
More recently, studies investigating fluoride’s effects on cognition based on exposures representative of community water fluoridation have produced conflicting results. This highlights the need for high-quality studies to better understand the effects of fluoride exposure in humans. Overall, available evidence indicates that community water fluoridation is a safe and effective strategy to reduce tooth decay and improve oral health in the community.
REFERENCES
- 1 – Goodwin et al. (2022) Evaluation of water fluoridation scheme in Cumbria: the CATFISH prospective longitudinal cohort study. Public Health Research.
- 2 – Grandjean and Landrigan (2014) Neurobehavioural effects of developmental toxicity. The Lancet Neurology.
- 3 – Choi et al. (2012) Developmental Fluoride Neurotoxicity: A Systematic Review and Meta-Analysis. Environmental Health Perspectives.
- 4 – Li et al. (2020) Spatial distribution of endemic fluorosis caused by drinking water in a high-fluorine area in Ningxia, China. Environmental Science and Pollution Research.
- 5 – Johnston and Strobel (2021) Principles of fluoride toxicity and the cellular response: a review. Archives of Toxicology.
- 6 – Green et al. (2019) Association Between Maternal Fluoride Exposure During Pregnancy and IQ Scores in Offspring in Canada. JAMA Pediatrics.
- 7 – Malin et al. (2024) Maternal Urinary Fluoride and Child Neurobehavior at Age 36 Months. JAMA Network Open.
- 8 – Do et al. (2022) Early Childhood Exposures to Fluorides and Child Behavioral Development and Executive Function: A Population-Based Longitudinal Study. Journal of Dental Research.
- 9 – Ibarluzea et al. (2022) Prenatal exposure to fluoride and neuropsychological development in early childhood: 1-to 4 years old children. Environmental Health.
- 10 – Kumar et al. (2023) Association between low fluoride exposure and children’s intelligence: a meta-analysis relevant to community water fluoridation. Public Health.
- 11 – Guth et al. (2020) Toxicity of fluoride: critical evaluation of evidence for human developmental neurotoxicity in epidemiological studies, animal experiments and in vitro analyses. Archives of Toxicology.
