By Ken Perrott 31/01/2018

As with most of these fluoride-IQ studies this one is only relevant to areas of endemic fluorosis (This is from a UNESCO paper and has been corrected for New Zealand. Identification of fluorosis in a country does not imply the whole country is high fluoride).

Yes, it’s a bit like groundhog day. Another fluoride-IQ study – and we expect this to be followed by another round of claims by anti-fluoride propagandists that this is the death knell to community water fluoridation. That this study provides the “irrefutable proof” that fluoride is a “neurotoxin.”

But that interpretation is completely wrong. This new study does nothing of the sort – in fact, quite the opposite.

The new study is:

Duan, Q., Jiao, J., Chen, X., & Wang, X. (2017). Association between water fluoride and the level of children’s intelligence: a dose-response meta-analysis

Now, why is this study absolutely useless for those opposing community water fluoridation?

It is not relevant to community water fluoridation

Because it is about a problem in areas of endemic fluorosis – where fluoride dietary intakes are much higher than where community water fluoridation exists.

From its first sentence it concentrates on fluorosis:

“Fluorosis is a progressive degenerative disease that causes skeletal fluorosis and dental fluorosis.”


“Currently, about 500 million people are exposed to environments high in fluoride content, while the incidence of fluorosis has already reached 200 million people worldwide.”

It’s not new research – it’s a meta-analysis of existing studies. Only studies dealing with areas of endemic fluorosis are considered in the meta-analysis. For example, the New Zealand (Broadbent et al., 2014) and Canadian (Barberio et a., 2017) papers which actually studied effects on IQ of community water fluoridation are not included. Nor is the Swedish study (Aggeborn & Öhman, 2016) which considered drinking water fluoride concentrations similar to that used in community water fluoridation.  So far these are the only reliable studies which considered low fluoride concentrations and they all show no effect of fluoride on IQ.

It is concerned with health effects in areas of endemic fluorosis

The meta-analysis includes 26 published studies in the meta-analysis. Most of the papers refer to “high fluoride water,” “fluorosis areas,” “endemic fluorosis” or similar terms in their titles. Low fluoride areas were only considered in the studies as “controls” and studies from areas of community water fluoridation were excluded.

Most of the considered studies simply compared IQ levels in “low fluoride” areas and “high fluoride” areas.  The mean drinking water fluoride concentration in the low fluoride levels of these studies was 0.6 mg/L (0.25 – 1.03 mg/L) and in the high fluoride areas, the mean drinking water concentration was  3.7 mg/L (0.8 – 11 mg/L).

As you can see the control or low fluoride areas, where the studies assumed there were no effects on IQ, have drinking water concentrations similar to that used in community water fluoridation (usually about 0.7 or 0.8 mg/L).

Yes, these studies did show statistically significant differences in IQ levels between the low and high fluoride areas. This is something for health authorities in areas of endemic fluorosis to be concerned about. And this, together with a range of other known health effects of excessive dietary fluoride intake, is the reason why attempts are made to reduce the fluoride levels in drinking water supplies in those areas.

People in high fluoride areas where fluorosis is endemic suffer a range of health problems. Credit: Xiang (2014)

Duan et al (2018) were able to present an overall estimate of the IQ difference between high and low fluoride areas – see figure. This is expressed as a standardised mean difference (SMD) – a necessary measure for a meta-analysis of a range of studies with different variability. The SMD = (difference in mean outcome between groups/standard deviation of outcome among participants) (see Cochrane Handbook).

All of the studies show a lower IQ in high fluoride areas than in low fluoride areas with the overall SMD being 0.52 (-0.62, -0.42 95% confidence interval).

To be clear – this is not 0.52 IQ points but can be interpreted as 0.52 x the standard deviation of IQ  in a population. Unfortunately, the authors do nothing to explain this, leaving readers to make the same mistake many did with a previous IQ meta-study (see Did the Royal Society get it wrong about fluoridation?).

Attempt to derive a dose-response relationship

The authors went on to attempt to derive an overall response curve relating SMD to drinking water fluoride concentration. Unfortunately, their results as presented in their  Fig 4 are confusing and the figure is not properly explained. Also, the modeling methods used to derive the response curve is not well explained.

However, the linear relationship they derived was not statistically significant. (They were able to derive a significant non-linear relationship, but again their methods and reason for doing this were not explained.)

I got the relationship shown in the figure below using the data provided in the paper without further modeling. This relationship is also not statistically significant (p=0.77).

The authors do suggest the possibility that lower intelligence may be associated with medium fluoride concentrations and “that very high fluoride concentration in water was associated with higher intelligence level than
medium fluoride.” However, although the figure above implies that IQ increases at higher fluoride concentrations, I do not think such conclusions are warranted with this data and its variability.

What causes the cognitive deficits?

Authors of these studies often seem to assume a direct chemical fluoride toxicity cause for the cognitive deficit. That also appears to be an assumption behind the desire to produce a dose-response relationship. Of course, anti-fluoride propagandists also prefer this mechanism because it enables them to argue that the effects also occur at low concentrations – they just haven’t been measured yet.

Although a dose-response relationship would be expected for a chemical toxicity mechanism this study did not produce a reasonable dose-response relationship. Some individual studies have claimed such a relationship but these claims are often not supported or the reported relationship is of only minor significance (see my discussion of Xiang et al., 2003 in Perrott, 2018).

The poor or non-existent relationship of cognitive deficits to drinking water fluoride concentration makes me suspect that there is not a direct effect. Rather the real causes of the cognitive deficits observed are dental or skeletal fluorosis or other health effects common in areas of endemic fluorosis. I suggested this in comments on Choi et al.,(2015) who observed a relationship with severe dental fluorosis but not water concentration (see Perrott 2015 – Severe dental fluorosis and cognitive deficits).

There I suggested consideration of the effects of severe dental fluorosis on quality of life and learning difficulties on cognitive deficits.  Another factor could be premature births and low birth weights which are known to influence cognitive development (see Premature births a factor in cognitive deficits observed in areas of endemic fluorosis?)

Duan et al., (2018) in their paper also allude to such possible mechanisms:

“Skeletal fluorosis is another very common and very serious side-effect of high fluoride intake, characterized by changes in the bone density, skeletal deformation, rickets, paralysis, disability, and even death. Patients with skeletal fluorosis have been reported to show neuronal nuclear vacuoles formations, cell loss in the spinal cord, and loss or solidification of Nissl bodies. Moreover, patients experience fatigue, sleepiness, headache, dizziness, and other symptoms related to the nervous system.”


The meta-analysis does confirm that there may be a problem with reduced of intelligence in children in areas of endemic fluorosis. This difference in IQ levels between high and low water fluoride levels is statistically significant.

However, this finding is of absolutely no relevance to community water fluoridation where the drinking water levels are similar to that in the low fluoride areas in the studies used for the meta-analysis.

The summarised data does not appear to be of sufficient quality to determine a reliable dose-response relationship. At least, the derived relationships are not statistically significant. An alternative explanation is that the observed reduced intelligence may not be directly related to drinking water concentration and instead related to dental or skeletal fluorosis, or other health effects common in areas of endemic fluorosis.

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0 Responses to “Yet another fluoride-IQ study”

  • No. The abstract itself states there is an association.

    “In this study, higher prenatal fluoride exposure, in the general range of exposures reported for other general population samples of pregnant women and nonpregnant adults, was associated with lower scores on tests of cognitive function in the offspring at age 4 and 6–12 y. ”

    Leaving aside whether the science is sound and the conclusion is valid or not, your statement is incorrect. The reseach you link does not claim to prove (or even identify) a causal association.

  • Mr. Dempsey, it’s perfectly reasonable to say that this study provides “evidence” that prenatal fluoride exposure reduces IQ, whether the authors claimed to show a casual association or not.

    The same study set out to test for an association between prenatal fluoride exposure and increased levels of ADHD in the children, and they concluded, “Findings are consistent with the growing body of evidence suggesting neurotoxicity of early-life exposure to fluoride.”

  • Hi Ken, thanks for that.

    Are you aware that apparently there has been a large increase in the incidence of dental fluorosis in the USA from 1986 to 2012, where it was found that 65% of children at ages 12 – 15 have some fluorosis, including 30.4% who have moderate to severe fluorosis?

    Also, a study in India of 429 children showed that even in cases of very mild to mild fluorosis, there is a significant observable lessening of IQ, which worsens with the severity of fluorosis.

    If it is true that 3 parts per million of fluoride in drinking water is acknowledged as harmful, and one part per million is acceptable, (quite a small range) how does that take into account the natural variability of susceptibility to fluoride in the population, or the variability of the amount of water people drink for that matter, including the difference between adults and children? If it was found that consuming a really small amount of arsenic cleared up the sinuses nicely, would it be an appropriate “public health initiative” to add arsenic to the water supply so that the whole population should have the benefit of clear sinuses?

    It seems to me that everyone has heard of the dental benefits of fluoride toothpaste and thereby can make their own choice as to how much exposure to this chemical they are willing to accept for them selves and their families – a choice denied to us when this chemical is added to our drinking water.


  • Peter, as you have not pursued your original argument I take it that you agree with me that a statistical association is not evidence of a cause? This issue is fundamental and anti-fluoride campaigners continual misrepresent the nature of statistical associationms in an attempt to confirm their biases.

    You cite the FAN personal Neurath et al who argue that there has been a large increased in dental fluorosis in the USA. You should note that this [paper has been heavily criticised and a thorogh analysis of the data actually shows their conclusions to be wrong. Have a read of “Data Quality Evaluation of the Dental Fluorosis Clinical Assessment Data From the National Health and Nutrition Examination Survey, 1999-2004 and 2011-2016.” ( from the US National Center for Health Statistics. It discusses problems related to quality evaluation and concludes:

    ” The observed increase in dental fluorosis prevalence with age between 2001–2004 and 2011–2014, based on the synthetic cohort analyses, is not biologically plausible. This suggests that there may have been some change in the way the examiners evaluated the level of fluorosis over time. The quality assessment findings in this report should be strongly considered when determining whether these data are appropriate for the user’s analytic objectives, including studies of prevalence
    and trends.”

    Of course, the FAN authors chose to confirm their biases rather than properly evaluate the statistics. A common problem with activists of all sorts.

    See also this Editorial Expression of Concern from the JDR Clinical and Translational Research ( The fact is that the CDC is expressing concern with its own NHANES data so one should take this into account when using the data.

  • Peter, people living in areas of endemic fluorosis suffer all sorts of health problems. IQ deficits are only one of those problems.

    Again, beware of relying on statistical relationships. These may indicate an association but in no way do they indicate a causal relationship. I pointed out other more immediate causes in my article “Severe dental fluorosis and cognitive deficits” ( which commented on a situation where a statistically significant relationship was reported for the extent of dental fluorosis but not for fluoride in drinking water.

    The question of community water fluoridation is an ethical one – not a scientific one as no harm has been shown scientifically. It boils down to democratic decisions by communities. I just wish those campaigning against community water fluoridation could be more honest and discuss the ethical issues (which is their strongest argument) rather than continually misrepresenting the science.

    Your last point about toothpaste is irrelevant. Community water fluodiation has a beneficial effect on oral health even where fluoridated toothpaste is used. It is easy to top understand why when we consider the surface reactions of calcium, phosphate and fluoride at the surface of teeth.