By Ken Perrott 03/02/2016

The “fluoride damages IQ” myth won’t go away – mainly because it is avidly promoted by campaigners against community water fluoride (CWF). This is despite the fact that no link has even been drawn between CWF and IQ (the only elevant study shows no connection).

But that doesn’t stop ideologically driven campaigners who rely on poor quality studies from areas of endemic fluorosis where dietary fluoride intake is higher than in areas using CWF.

There are plenty such studies, but a more recent one illustrates their problems – and the role  confirmation bias seems to play in these studies. It is:

Kundu, H., Basavaraj, P., Singla, A., Gupta, R., Singh, K., & Jain, S. (2015). Effect of fluoride in drinking water on children′s intelligence in high and low fluoride areas of Delhi. Journal of Indian Association of Public Health Dentistry, 13(2), 116.

It’s another study where the IQ values of children from a “high fluoride” area were compared with those for children from a “low fluoride area.” There was a statistically significant difference and the paper goes on to claim:

“High F concentration in the drinking water was found to have marked systemic effects on the IQ of children. Though the precise mechanism by which F crosses the blood brain barrier is still not clean‑cut; enough evidence survives for the influence of F intake via drinking water and low IQ of the child.”

However they do acknowledge:

“Apart from fluoride there are other factors which also affect IQ of children. In the present study, mothers diet during pregnancy also significantly affected the IQ of the children.”

The supporting data is poorly presented and described – for example, no indication is given of the fluoride concentration in the drinking water of the “high fluoride and “low fluoride” areas used. Although they do cite areas in Delhi (where the study was located) with fluoride concentration as high as 32.5 ppm!. And I cannot find any details on “mothers diet during pregnancy” (except perhaps division into two groups – “routine” or “special diet as suggested by the doctor during pregnancy”).

Those confounding factors

These sorts of studies almost always rely on finding a statistically significant difference in the IQ values of children in two different areas or villages. But that statistical significance says nothing about the causal factors involved – it may have nothing to do with differences in fluoride levels.

Kundu et al., (2015) do at least include some data on confounding factors which is often missing from such studies. These show significant difference between the groups from the “high fluoride” and “low fluoride” areas which have no connection with fluoride in drinking water – such as father’s occupation, mother’s education and father’s education) – or only an indirect connection (dental fluorosis).

Here is a summary of the data for the various factors. I have selected the data so to show as two values – equal to “high fluoride” and “low fluoride.”


You get the picture. The areas were chosen according to the concentrations of fluoride in drinking water (whatever they were), but they could equally have been chosen on the basis of parental education, father’s occupation or prevalence of the more severe forms of dental fluorosis.

In fact, rather than concluding drinking water fluoride has a “marked systemic effects on the IQ of children” we could equally have concluded:

  • “The father’s occupation has a marked effect on the IQ of children with the children of unskilled fathers having a lower IQ.”
  • “The mother’s and father’s education has a marked effect on the IQ of children with the children of parents with a higher education having a higher IQ.”
  • “Diet of mothers during pregnancy has a marked effect on the IQ of children.” (The paper did not include data suitable for plotting for this.)

The dental fluorosis factor interests me as I have suggested that, in areas of endemic fluorosis, the physical appearance of defective teeth could lower quality of life and cause learning difficulties which are reflected in lower IQ values (see Severe dental fluorosis the real cause of IQ deficits?Severe dental fluorosis and cognitive deficits – now peer reviewed and Free download – “Severe dental fluorosis and cognitive deficits”).

I think that this is more reasonable as a mechanism than the chemical toxicity mechanism that almost all authors of these sorts of papers assume – but never support with any evidence. Even when dental fluorosis is considered it is usually treated as an indicator of lifetime intake of fluoride (which it is) rather than and independent cause of low IQ.


Most studies like this seem to be motivated by confirmation bias. Despite the possibility of a range of factors being involved, and some of these such as parental education being a more obvious cause, there appears to be an urge to interpret data as evidence of a chemical toxicity mechanism involving fluoride. And there is never any experimental work to confirm this preferred mechanism.

To my mind, if fluoride is implicated in the low IQ values the mechanism involving effects of dental fluorosis on quality of life and learning difficulties appears more credible than an unproven chemical toxicity.

Note: None of this is directly relevant to areas where CWF is used. The prevalence of more serious forms of dental fluorosis is very small in these areas and not related to CWF. Also, no study has yet found an effect of CWF on IQ. Given the higher levels of fluoride used in the studies from areas of endemic fluorosis, and the higher levels of serious forms of dental fluorosis, extrapolation of the results to areas where CWF is used is completely unwarranted.

Featured image: Flickr CC, Indigo Skies Photography.