Once more on the IQ and fluoride myth – why ignore other factors?

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 relevant 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.

Similar articles

4 responses to “Once more on the IQ and fluoride myth – why ignore other factors?

  1. Merely an anecdotal comment, Ken.

    My ancestry originates from the serfs of the UK, moving into the cotton mills of the industrial revolution, with a fair bit of incestuous reproduction along the way. Not exactly the best background for intelligence, I would have to admit, and most people would agree.

    My parents brought me to NZ, and I completed my primary and secondary education in areas with CWF. I am the first person in my extended family to complete tertiary education, coincidentally in medicine, and coincidentally also in an area with CWF.

    My offspring have been brought up in an area with CWF, and both completed tertiary education in areas with CWF. The youngest lost about a third of his brain from cancer at high school, but both went on to complete professional degrees and are working in their chosen professions. Both are objectively more intelligent than their parents.

    The only thing that prevents the attribution of my offspring’s increased intelligence to CWF is the application of science.

    There is no evidence that CWF increases or decreases IQ. Only anecdotes.


  2. Ken you say no-one has found fluoride affects IQ and put up a study which you claim is conclusive that it doesn’t. (Broadbent.)

    I am sorry to hear about Stuartg’s son. I recommend that a study be done to find out how back up of the brain works as other parts take over, in terms of extra stresses which may require back up in addition.

    We may not notice we have kidney damage till much of the capacity has gone. Same with liver. Today’s news has said persons who have an experience 20 years ago and contracted Hep C are just now finding they have it and its associated hepatitis.

    Hepatits, a form of liver trouble, does not only result from disease, it may be from drugs like paracetamol for which an overdose can cause a slow painful death, I understand. The beginning overdose level is not too much above the recommended dose.

    Vitamin B3, which some people are prescribed in pharmaceutical doses to reduce blood lipids, can also damage the liver. However I think in this case it can recover on withdrawal of the medication. The affect happens at a lower dose if the vitamin B3 is made in a time-release formulation. The hepatocytes do not get a chance to recover because the chemical is constantly present in the blood stream.

    Fluoridation of water is said to be more effective against tooth decay since it is more frequently present than if just tooth paste is used.

    The control group for the study Ken cites was said to be in Mosgiel which has about 5mg per litre of nitrate in its water as opposed to close to zero for the fluoridated region. Nitrate in vegetables like silver beet can produce “blue baby” sometimes (methemoglobin it is also called and it needs to be rapidly treated with methylene blue.)

    If the baby is being bottle-fed and the kettle boils down a bit before the formula is mixed there will be greater nitrate than 5 mg/l. The nitrate will be there constantly like time-release. (There are time release nitrate drugs to prevent angina, One would be isosorbide mononitrate. The time release dose is 30 to 60 mg daily. Babies of course would not be getting enough to get the headache side effect, but I do not understand what part the nitrate plays i that yet, as opposed to the rest of the compound.)

    I have marked all the USA states in this http://pubs.usgs.gov/bul/0523/report.pdf which have a mention for nitrate deposits as 1 and the others as 0 and done a Vassarstats Spearman rank correlation with USA state IQs as from http://webcache.googleusercontent.com/search?q=cache:s66B7S0RFgoJ:www.sq.4mg.com/IQ-States.htm+&cd=1&hl=en&ct=clnk&gl=nz

    The result for the 51 states is r= -0.2707 t= -1.97 p one-tailed 0.027252 two-tailed 0.054504.

    I suppose that suggests nitrate bearing deposits in the states rather explain 7.3% of the variation in IQs across USA.

    So given the nitrate in Broadbent’s control area I think another look at his results is suggested. and I am not sure how his study passed peer review.
    I don’t know if a liver or brain effect or what is involved. An IQ test puts a little stress on a subject requiring some back up.

    Someone must have better data than what I have quickly raked up.


  3. Brian –. I would like to see a citation for your claim Mosgiel tap water has “5mg per litre of nitrate” What about a reference to their regular analyses of tap water?


  4. Ken I have tried emailing you a file through Facebook


Leave a Reply: please be polite to other commenters & no ad hominems.

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s