We read with dismay fluoride promoter Ken Perrott’s opinion piece on the US Government-funded study on prenatal exposure to fluoride and its effect on IQ.

Mary Byrne

While the New Zealand Ministry of Health remains silent on a landmark, multi-million dollar,
US Government funded study (Bashash et al)1, and the Minister of Health continues to
claim safety based on out-dated advice, Perrott has sought to discredit the study via the Health Central opinion piece, blog posts and tweets.

Perrott claims that the results were not statistically significant but his analysis is incorrect.
The conclusion by the authors of this study, published in the top environmental
health journal, Environmental Health Perspectives in September 2017, was:

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

Perrott states the study has “a high degree of uncertainty”. But this contrasts with the
statistical analysis and conclusion of the team of distinguished neurotoxicity researchers from Harvard, the University of Toronto, Michigan and McGill. These researchers have written over 50 papers on similar studies of other environmental toxics like lead and mercury.

RESULTS: In multivariate models we found that an increase in maternal urine fluoride of
0.5 mg/L (approximately the IQR) predicted 3.15 (95% CI: −5.42, −0.87) and 2.50 (95% CI: −4.12, −0.59) lower offspring GCI and IQ scores, respectively.

The 95% CI is the 95% Confidence Interval which is a way of judging how likely the results
of the study sample reflect the true value for the population. In this study, the 95% CIs show the results are highly statistically significant. They give a p-value of 0.01 which means if the study were repeated 100 times with different samples of women only once could such a large effect be due to chance.

Perrott comes to his wrong conclusion because he has confused Confidence Intervals with
Prediction Intervals and improperly used Prediction Intervals to judge the confidence in the

A Prediction Interval is used to judge the confidence one has in predicting an effect
on a single person, while a Confidence Interval is the proper measure to judge an effect on a population. In epidemiological studies, it is the average effect on the population that is of
interest, not how accurately you can predict what will happen to a single person.

Despite the authors controlling for numerous confounders, Perrott claimed they did not do a very good job and had inadequately investigated gestational age and birth weight.

Once again Perrott makes a fundamental mistake when he says that the “gestational period < 39 weeks or > 39 weeks was inadequate” and “The cut-off point for birth weight (3.5 kg) was also too high.”

Perrott apparently did not understand the Bashash paper and mistook what was reported in
Table 2 with how these covariates were actually treated in the regression models. The text of the paper plainly states: “All models were adjusted for gestational age at birth (in weeks), birthweight (kilograms)”

Thus, each of these two variables were treated as continuous variables, not dichotomized into just two levels. Perrott’s criticism is baseless and reveals his misunderstanding of the Bashash paper.

Perrott states that the results are not relevant to countries with artificial fluoridation because it was done in Mexico where there is endemic fluorosis. But Perrott is wrong.

The study was in Mexico City where there is no endemic fluorosis. Furthermore, the women’s fluoride exposures during pregnancy were in the same range as found in countries with artificial fluoridation such as New Zealand.

The study reports that for every 0.5 mg/L increase of fluoride in the urine of the mothers
there was a statistically significant decrease in average IQ of the children of about 3 IQ
points. It is therefore correct to say that a fluoride level in urine of 1 mg/L could result in a
loss of 5 – 6 IQ points. This is particularly relevant to the New Zealand situation where
fluoridation is carried out at 0.7 mg/L to 1 mg/L and fluoride urine levels have been found to be in this range. (2)

There is no excuse for Health Minister, David Clark, to continue to bury his head in the sand. This level of science demands that the precautionary principle be invoked and fluoridation suspended immediately.

Mary Byrne is the National Coordinator of Fluoride Free New Zealand an organisation established in 2003 to end water fluoridation in New Zealand.



  1. https://ehp.niehs.nih.gov/ehp655/
  2. http://nutr2015.p.events4you.currinda.com/days/2015-12-02/abstract/307



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  1. Mary’s opinion piece was a response to my blog article “Paul Connett’s misrepresentation of maternal F exposure study debunked”. I have treated it as a “right of reply” and it will be presented as a separste post on my blog – Open Parachute.

    I hope this will encourage a good faith scientific exchange on the important issues involved. Of course I will be responding to Mary’s article in a subsequent post and hope Mary will continue the exchange in the same manner Paul Connett and I did 4 years ago in our “Fluoride Debate.”

  2. Mary’s opinion piece is now poosted on my blog – https://openparachute.wordpress.com/2018/03/13/anti-fluoride-group-coordinator-responds-to-my-article/

    I will respond on Open Parachute to Mary’s article within the next few days but just wish to clarify some mistakes in her claims:

    1: I do not “promote fluoride.” My purpose on this issue has always been to expose the misinformation and distortion of the science surrounding community water fluoridation (CWF). I leave the promotion of health policies to the health experts and authorities.

    2: I have not “sought to discredit the study.” My article Mary responded to was a critique of the misrepresentation of that study by Paul Connett – not an attack on the study itself.

    3: Mary asserts:

    “Perrott claims that the results were not statistically significant but his analysis is incorrect.”

    That is just not true. I have never claimed their reported association was not statistically significant. And my statistical analyses of the Bashish data produced almost exactly the same result that Bashish et al reported.

    I will develop these points further in my forthcoming article responding to this one.

  3. One further note, Mary presents her opinion piece as a response to my opinion piece presented here – http://healthnzme.wpengine.com/opinion-ken-perrott-fluoridation-critics-tour-is-scaremongering-with-irrelevant-research/

    Readers will note that I did not in any way discuss this specific Mexican study in that opinion piece.

    In fact, Mary is responding to my Open Parachute article “Paul Connett’s misrepresentation of maternal F exposure study debunked” – https://openparachute.wordpress.com/2018/03/05/paul-connetts-misrepresentation-of-maternal-f-exposure-study-debunked/

    I encourage readers to read that article for the background information. And, if they are sufficiently interested, to read my upcoming response to Mary’s article and participate in the discussion on it at Open Parachute.

  4. Just a note on point 1 Ken:

    1: I do not “promote fluoride.” My purpose on this issue has always been to expose the misinformation and distortion of the science surrounding community water fluoridation (CWF). I leave the promotion of health policies to the health experts and authorities.

    You are a member and promoter of fluoridation at Making Sense of Fluoride. In the About section it states:

    We are a combination of students, academics and medical professionals advocating that this significant public health initiative be maintained in Australasia given the longstanding scientific consensus on its safety and efficacy. http://msof.nz/about-us/

    Ken, how can you possibly say that you do not “promote fluoridation”?

  5. Yes, Kane – that is a website description – which goes on to say:

    “The purpose/aim of this website is to improve our own and others understanding of the role of fluoride in human health and the social policy towards this. We will do this by exploring and sharing scientific information, dealing with public perceptions on the issue, and discussion with supporters and opponents of social fluoridation policies. We will also explore alternatives to current policies through debate and discussion.”

    It is not the text of the MSoF constitution.

    However, I am serious when I say my interests is to fight misrepresentation. If I find proponents of CWF distorting or misrepresenting the science I will also debunk those claims. This comes naturally from my background in scientific research and from the scientific ethos not to misrepresent our findings.

    To date, the major distortions and misrepresentations are coming from the anti-fluoride groups and activists. You are one of those people (the current chief of the Auckland branch of FFNZ) and it is notable that in your comments here you cannot talk about the science. I do not think, for example, you understand the Bashish et al study or my article debunking Paul Connett’s misrepresentation of that study.

    You are welcome to peruse my writings on the subject – see https://openparachute.wordpress.com/fluoridation/

    Having said that my current scientific understanding is that CWF is both effective and harmless. It is also cost-effective. Given this, I can understand why it is supported and even promoted by sensible people and health experts. Especially as, like many in my generation, I suffered badly from poor oral health in my youth.

  6. That’s right Ken, I spoke to Auckland Council to get the levels of fluoride concentration reduced to 0.7ppm. This was so that it was in line with the US HHS single target of 0.7ppm. Auckland Council followed Fluoride Free NZ’s recommendation and not the NZ MOH’s target of 0.7 – 1ppm (mid point 0.85ppm). I haven’t seen you call for the NZ MOH to reduce their target range to be in alignment with the US Ken. Why not? You don’t appear to have a good track record of going against the status quo of the MOH. Is that because you are biased?

    • I am covering the US HSS recommendations and your little escapade in Auckland in an upcoming article – stay tuned.

      I would never advocate NS aligning with US policies (that would be a disaster, politically, and wrong scientifically) – we should decide our own on the basis of our own conditions. Just think about it.

      However, to get back to the question of apologies for spreading misinformation. Perhaps you should be directing that comment at Mary (see my comment above correcting the misrepresentation of my article) and Paul Connett (see my original article describing Paul’s misrepresentation of the Bashish study.

  7. I see Mary is complaining bitterly about Sciblogs not showing the anti fluoride side of the story. What a shame.
    If anybody tries to put the pro fluoride side on any anti fluoride page they immediately get blocked
    Have to wonder what they are frightened of??

  8. Ken Perrott says at 8.13pm.
    CWF is, effective and harmless. Cost effective.
    CWF supported by sensible people and health experts.
    He then adds the ‘Me’ factor.
    9.29 He says, “I do not promote fluoride”
    Perrott promotes fluoride in all he says. He is a huge proponent and even uses his own teeth to justify CWF.
    Fluoride is a toxic chemical and does not belong in municipal water. Fluoride is the second most toxic substance introduced into the human body following mercury.
    Perrott is just plain wrong. As a ‘scientist’ he should know better.
    Perrott would do better to read up on Heather Gingerich a medical geologist, anti fluoride and approved by the United Nations.
    Scientifically fluoride does not stack up. Perrott is, plain and simple, biased.


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