Studies suggesting a link between public water fluoridation and reduced intelligence rely on unsubstantiated leaps made possible by studies of “insufficient quality” that were conducted to address entirely different questions.

Africa Studio / Shutterstock.com

Claim: Scientific studies demonstrate that the process of adding fluoride to public water reduces the IQ of the individuals in those areas.



Beginning in the mid-1940s, cities in the United States began adding low concentrations of fluoride to public water supplies in an effort prevent tooth decay. As of 2014, 74.4 percent of the United States populace has access to water from a fluoridated source, and this practice is employed by countries and cities all around the world.

According to numerous meta-studies, the addition of fluoride (the ionic form of the element fluorine that plays a role in tissue mineralization) to public drinking water significantly reduces the incidence of dental caries.

Moderate voices against the practice of water fluoridation point to concerns over the possible negative health effects of fluoride consumption, and a perceived lack of efficacy in terms of dental benefits. More rabid voices, like those of the ever-imaginative Joseph Mercola and Alex Jones, allege widespread conspiracies invoking its (nonexistent) origins in Nazi concentration camps and its use as genocidal chemical weapon employed by the heavy-handed arm of the United States federal government, per Jones’ Infowars.com:

Fluoride, in whatever amount, is nothing less than a chemical weapon. Considering it is applied to entire populations or certain groups within a population, the definition is chemical warfare- a tool most useful to eugenicists who are intent on depopulation [sic] the planet.

These combined efforts have produced myriad confused and self-referential claims that would require multiple posts to debunk. In this particular post, however, we will focus only on the claim that scientific studies have demonstrated that intentional water fluoridation causes neurological damage resulting in a negative effect on a person’s intelligence quotient.

That claim, which is based on an either willful or negligent misreading of actual science, is effectively illustrated in this statement by Joseph Mercola (emphasis ours):

At present [December 2014], a total of 42 human studies have linked moderately high fluoride exposures with reduced intelligence, and over 100 animal studies have shown that fluoride exposure can cause brain damage.

The claim that studies have “linked” fluoride to reduced intelligence is a textbook-ready case of bait-and-switch. In Mercola’s (and many others’) language, the topic has surreptitiously been shifted from the act of water fluoridation as a public health measure to the broader concept of fluoride toxicity in children. Many otherwise benign chemicals can also be harmful in high concentrations. Thiocyanate, a chemical found in kale, may kill you at high doses, for example.

The most famous and commonly cited study (partially because the anti-fluoridation group Fluoride Action Now issued a misleading press release about it that was incorrectly covered as a Reuters report by a number of newspapers) used to link fluoride to reduced IQ was published by a team of Harvard researchers. This paper, a meta-analysis of previously conducted studies that compared populations of children exposed to various levels of naturally occurring fluoride in their water, concluded:

Findings from our meta-analyses of 27 studies published over 22 years suggest an inverse association between high fluoride exposure and children’s intelligence. Children who lived in areas with high fluoride exposure had lower IQ scores than those who lived in low-exposure or control areas.

Scale and context, which are so often ignored by the evangelists of pseudoscience, are crucial when interpreting the results of this analysis, however.

First, the issue of scale. As neuroscientist Steven Novella pointed out in his Neurologica blog, most of these studies’ “high fluoride” groups used concentrations many times higher than allowable limits in the United States, and many of the control groups or “low fluoride” groups had concentrations in the range that is actually targeted as optimal by water fluoridation regulations:

There was a lot of variability across the studies, but generally the high fluoride groups were in the 2-10 mg/L range, while the reference low fluoride groups were in the 0.5-1.0 mg/L range [...]

In other words – fluoridated water in the US has the same level of fluoride as the control or low fluoride groups in the China studies reviewed in the recent article, and the negative association with IQ was only found where fluoride levels were much higher – generally above EPA limits.

Currently, the EPA’s maximum allowable limit for fluoride in drinking water is 4 mg/L with a voluntary recommendation of 2 mg/L. When fluoride is added to a public water supply, the target concentration is 0.7 mg/L.

Supporters of the notion that this Harvard paper’s collection of studies still suggests developmental risks of fluoridation at levels used in public health programs might argue that eight of the studies analyzed in the meta-review focused on concentrations allegedly within this allowable range. This is a factual point that deserves further investigation, as a close look at these eight studies reveals that their contribution to the fluoridation debate is dubious at best.

Of those eight studies, half of them co-investigated fluoride and iodine together (Lin et al 1991, Xu et al 1994, Yang et al 1994, Hong et al 2001) making it hard or in some cases impossible to separate out the combined effects, and two of those four studies reached conclusions that are counter to the hypothesis that fluoridation levels alone are the main driver of a lower IQ:

Xu et al 1994:

When it comes to the parents' level of education, and the children's exposure (or lack thereof) to pre-school education, sIgnificant differences in the intelligence level in the young subjects are shown in both regions affected by thyroid swelling, and their comparative regions. This demonstrates that levels of IQ are not only affected by the amount of trace elements in the environment, but also by social factors such as the education of the subjects, the type of education. and training they are exposed to, etc These social factors most definitely play an important role in this type of research.

Hong et al 2001:

The IQ results of this study show no significant difference between the average IQs of those children from the high fluoride only areas and the high fluoride/high iodine areas, however the result from the high fluoride/low iodine group show statistically significant differences as compared to that of the low fluoride/low iodine group. In short, it appears that the presence or lack of iodine is a more significant factor in both the prevalence of goiter and average IQ [than fluoride levels alone].

Among the eight studies, two of them (Yao et al 1997, Seraj et al 2006) do not appear to actually measure the water intake of the regions in which they are observing differences in intelligence quotient, or at the very least they do not report the total range of values for each region, or any statistical information regarding those values.

Two of those eight studies use a control group with fluoride values that are literally the same or higher than the target range of fluoridation efforts in the United States (Xu et al 1994, Hong et al 2001), seemingly ceding the point that those levels do not affect children’s IQ.

Finally, four of these eight papers (Yang et al 1994, Lu et al 2000, Hong et al 2001, Xiang et al 2003a) are either published (or republished) in the allegedly peer-reviewed journal Fluoride, a publication of the “International Society for Fluoride Research Inc.” — an anti-fluoridation group whose editor-in-chief is a psychiatrist in private practice, with no academic background on the topic of fluoride toxicity.

Collectively, this demonstrates that most of the IQ variance presented in the Harvard study still stems from exposures to extremely high levels of fluoride that would already be considered dangerous in the US, and those studies finding effects on a smaller scale are not sufficient to demonstrate the effects those groups opposed to fluoridation claim they demonstrate.

The next issue is context. Literally none of the studies involved tested populations of individuals exposed to drinking water that was artificially supplemented with fluoride as a public health measure. Instead, all of the studies come from China or Iran, both of which have areas of naturally occurring (endemic) high fluoride pockets of groundwater.

That means that studies utilized in its analysis are wholly irrelevant to the question that advocates claim they are answering. This is significant, as the use of these very specific studies introduces a veritable Homerian epic of confounding details, some of which came up in our analysis of the eight low-level fluoride studies discussed above. Among the most pressing of these are a lack of information on other confounding variables and the quality of the studies they utilized. These issues are noted by the authors of the Harvard study themselves:

“Most reports were fairly brief and complete information on covariates was not available”

“Drinking water may contain other neurotoxicants, such as arsenic”

“The studies [used in the meta-analysis] were generally of insufficient quality”

The authors also explicitly state that the results cannot be used to estimate the possible limits of fluoride exposure with respect to developmental damage, due to lack of data:

Our review cannot be used to derive an exposure limit, because the actual exposures of the individual children are not known.

The data could also be complicated by other factors not accounted for, they say, including the notion that the populations of individuals compared may not actually reflect their fluoride intake:

Misclassification of children in both high- and low-exposure groups may have occurred if the children were drinking water from other sources [...]

Regarding the other neurotoxins that could be in the water, the authors of the Harvard Study make the (unsourced) claim that it is unlikely that other neurotoxicants are present in the groundwater of the areas their studies investigated. As many of the studies did not measure for other compounds, it is impossible to be sure in these specific cases; however, geochemists are not convinced that the same natural processes that lead to high fluoride concentrations don’t also yield high concentrations of other dangerous metals.

Research has, for example, shown that there is a strong correlation between arsenic and fluoride in drinking water, according to a 2011 study in Applied Geochemistry analyzing groundwater in Yuncheng Basin, China:

The strong positive correlation between groundwater F/Cl and As/Cl ratios (r2 = 0.98 and 0.77 in shallow and deep groundwater, respectively) indicates that these elements are mobilized and enriched by common processes.

This should not function as a distraction from the larger point that studying naturally occurring pockets of high fluoride and the assessing the risks of supplementing public drinking water in an effort to have it reach a concentration of 0.7 mg/L are two completely different beasts.

You might ask if any studies have been completed specifically on the effects of IQ on artificially enhanced drinking water. The most recent analysis of these studies comes from an 2015 evidence review performed by the government of Ireland’s Health Research Board, which found only one study sufficient to test that hypothesis:

There was only one study carried out in a non-endemic or CWF [community water fluoridation] area that examined fluoride and IQ. This was a prospective cohort study (whose design is appropriate to infer causality) in New Zealand. The study concluded that there was no evidence of a detrimental effect on IQ as a result of exposure to CWF.

For these reasons, the claim that there are scientific studies that support the notion that water fluoridation can cause developmental problems in children that result in lower IQs is false. No studies that actually investigate that specific question have reached that conclusion. Studies that have been used to make that argument are weak and rely on data collected that does not directly address those questions in the first place.

Featured Image: Africa Studio / Shutterstock.com

Alex Kasprak is a science writer into space, fossils, deep time, and obscure historic events that in some way involve a modicum of science. Based in Los Angeles, he is the real-life basis for that kid Dr. Grant scares with a velociraptor claw in the beginning of Jurassic Park.