Category Archives: Science and Society

Fluoridation and child IQ – the problem of counting chickens before they hatch

It’s easy to find studies that confirm ones own bias on health issues. But cherry-picking such studies is unscientific because one could equally cite studies showing the exact opposite. The problem is these studies often rely on poor data, do not sufficiently account for confounding factors and often use poor statistical analysis so contradictory results are inevitable. A more objective requires a meta-analysis of all the available studies.

Anti-fluoride activists do this when they cherry-pick studies to argue that science has “proved” fluoride intake by pregnant women lowers the IQ of their children. These activists rely on just a few studies from two related groups – they cite Bashash et al (2107), Green et al (2019) and Till et al (2020). But what about studies that show the exact opposite – an increase in child IQ and the low fluoride intake relevant to community water fluoridation. Studies they ignore like those of Santa-Marina et al (2019) and Ibarluzea et al (2020).

The publication this month of new extensive data from an important study will be harder to ignore. This establishes a relationship of prenatal maternal intake of fluoride with the cognitive results for children in areas of Spain where community water fluoridation is used. In contrast to the negative results found in the studies anti-fluoride activists rely on, this new study reported: “Maternal fluoride levels were associated with better cognitive scores in childhood.”

Here is the citation for the new study:

Ibarluzea, J., Gallastegi, M., Santa-Marina, L., Jiménez Zabala, A., Arranz, E., Molinuevo, A., Lopez-Espinosa, M.-J., Ballester, F., Villanueva, C. M., Riano, I., Sunyer, J., Tardon, A., & Lertxundi, A. (2021). Prenatal exposure to fluoride and neuropsychological development in early childhood: 1-to 4 years old children. Environmental Research, 112181.

The figure below summarises the results for relationships of child cognitive scores with maternal prenatal urine – a measure of fluoride intake. cognitive scores are from McCarthy Scales tests appropriate for age 4. GCI is a General Cognitive Index.

The data points are for ß coefficients of the linear regressions of cognitive score against material urinary fluoride – the slope in units of amount of cognitive change per unit of urinary F (mg/L or equivalent). The bars are the 95% confidence intervals – if they include zero in the range this indicates the score is not beta value for the score is not statistically significant.

As we can see all the cognitive scores a positive and statistically significant for boys – hence the conclusion that “Maternal fluoride levels were associated with better cognitive scores in childhood.” However, none of the changes in scores for girls was statically different from zero

Don’t get me wrong. I am certainly not arguing that maternal intake of fluid will increase the cognitive levels of offspring male children. I am not attempting to confirm a pro-fluoride bias here. Remember, I said that objective assessment requires a meta-analysis of all the available studies.

The figure below shows the results of a meta-analysis of all the available studies which considered the relationship of the cognitive score with prenatal maternal urinary F.

As we can see, some of the beta values are positive and some are negative. Many are not significantly different from zero – any effect is not statistically signficant. Over all the studies, on average the score is positive (ß = 2.1 for 1 mg/L increase in urinary fluoride, or the equivalent measure) but not statistically significant from zero (95% confidence level is -1.9 to 6.1).

So the best objective conclusion to date is that child cognitive scores are not influenced by prenatal maternal fluoride intake at the levels expected in these studies which are relevant to community water fluoridation.


These studies illustrate the danger of cherry-picking studies to draw conclusions. or even simply taking the data from only a few studies that have so far been published. But that is what Grandjean et al (2021) have done in the benchmark analysis of cherry-picked studies. They concluded that a safe level for maternal urinary fluoride is as low as about 0.2 mg/l.

Image credit: Cherry-Picking

At best they counted their chickens before they had hatched, given that some important studies had not been published in scientific journals. At worst the cherry-picked to confirm a bias.

I suspect the latter because the important Spanish data had been published as conference papers so they should have been aware of them. The authors of the benchmark paper are all associated with the limited number of studies they considered (particularly those of Bashash et al (2107), Green et al (2019) and Till et al (2020)) and were members of just two groups obviously coordinating their work. Authors from this group have been very active in promoting their own findings, studiously ignoring the findings of voters and also using their work to promote the arguments of anti-fluoride activists against community water fluoridation.

Just imagine if some pro-fluoride activists chose to claim that fluoridation will actually increase child IQ and cherry-picked the Spanish studies to support their claim. I can’t actually see that happening, but if it did it would be as bad as the approach used by the authors of  Grandjean et al (2021).

This tactic of cherry-picking studies may be used to provide a scientific basis for activist arguments – but that is not true science.

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Anti-fluoridation group tells porkies about NZ fluoridation review

FFNZ spreads misinformation about the NZ fluoridation review – yet again

New Zealand opponents of community water fluoridation (CWF) are at it again. Their only response to the recently upgraded fluoridation review is to call it “propaganda” and to completely misrepresent it. But it’s interesting to look at their misrepresentation because it does highlight a basic flaw in the studies the anti-fluoride campaign has been promoting.

Fluoride Free NZ (FFNZ) claim in their recent press release (Chief Science Advisor Appears To Deliberately Mislead On Fluoride Science):

“among the many mistakes and reliance on out-of-date science, the most glaring issue is that she refers to two of the best studies ever carried out on fluoride and IQ (Mexico and Canada) as “having high prenatal exposure”. This is probably the most egregious misrepresentation in the review and hard to believe it was not done to purposely misrepresent.”

But this is completely false. In discussing the Canadian study the review actually says it:

“found that the mother’s exposure to fluoride during pregnancy was associated with lower IQ scores [54] in boys (but not girls), even at optimally fluoridated water levels (i.e. between 0.7-1.2 mg/L). If this finding were replicated in robust studies, it would potentially be concerning as Aotearoa New Zealand recommends fluoridation of water between 0.7 and 1.0 mg/L. There was significant and valid criticism of aspects of the study by many subject-matter experts when it was released (see for example, ‘expert reaction to study looking at maternal exposure to fluoride and IQ in children’). The study used sub-group analysis to find an association that is not explained in the paper (i.e. why were only boys affected [55] and why verbal IQ was not impacted), the effect appeared to be driven by the minority of participants that had much higher fluoride exposures (i.e. higher than those in Aotearoa New Zealand).” [My emphasis]

So the review does refer to the Canadian study being conducted at “optimally fluoridated water levels (i.e. between 0.7-1.2 mg/L)” – not at the elevated levels leading to “high prenatal exposure” that FFNZ falsely (and “egregiously”) asserts. But the key assertion by the NZ fluoridation review is that “the effect appeared to be driven by the minority of participants that had much higher fluoride exposures.”

Outliers lead to false conclusions

Canadian study promoted by opponents of community water fluoridation relies on just a few outliers  Image credit: The problem with outliers

It’s quite simple really. Even within a group exposed to levels of fluoride expected with CWF there can be some individuals who receive higher exposes (f0r instance through consumption of fluoridated toothpaste or industrial pollution).

Looking at the data in the Canadian study in the image below taken from Green et al (2019) we can see that while most data points are clustered together at urinary F concentrations less than 1 mg/L there are a few data points at high urinary F concentrations and these do appear to drive the relationship they report – particularly for boys.

For the more statistically inclined reader, the table below summarises the relationships obtained by linear regression analysis. While the authors reported a statistically significant relationship for all the urinary fluoride concentrations up to 2.5 mg/L when the four high-end outliers (> 2.0 mg/L) are removed there is no significant relationship.

So I think the suggestion of the updated NZ fluoridation review is quite correct. The effect reported by Green et al (2019) is driven by just a few outliers and there is no statistically significant relationship when those four outliers are removed. That gives a false impression of the effect of CWF and in fact, their data shows absolutely no difference between IQ in fluoridated areas and unfluoridated areas.

Note 1: There is a discrepancy in the first table between the relationship reported by Green et al (2019) and that based on digitally extracted data points. Unfortunately, only 82% of the claimed data points could be extracted which is strange as usually close to 100% of data points can be extracted. Other commenters have reported the same problem. So it appears the authors have not included all their data in the figures and they have so far refused to make their data available for independent statistical analysis.

Although the Green et al (2019) paper did not cite R-squared values in her Master thesis did cite an R-squared value of 0.049 for boys. The low R-squared values (meaning the inclusion of the coefficient explains at most only a few per cent of the variation) and relatively high regression standard errors suggest that the reported coefficients are meaningless (they can be ignored in any model) – even if statistically significant.

Note 2: In case anyone suggests I have neglected the FFNZ reference to the Mexican study. That study took place in an area of endemic fluorosis and the authors have no record of the water fluoride levels mothers were exposed to. Bashesh et al (2017) reported:

“By virtue of living in Mexico, individuals participating in the study have been exposed to fluoridated salt (at 250 ppm) and to varying degrees of naturally occurring fluoride in drinking water. Previous reports, based on samples taken from different urban and rural areas, indicate that natural water fluoride levels in Mexico City may range from 0.15 to 1:38 mg/L. Mean fluoride content for Mexico  City’s water supply is not available because fluoride is not reported as part of water quality control programs in Mexico.

Despite this, the Bashash study is often unjustly included with studies from areas of CWF by coauthors of Bashash and Green (see for example Farmus et al 2001). Anti-fluoride activists almost always make this mistake. Sure, they may attempt to justify their treatment of Bashash et al (2017) as relevant to CWF based on urinary fluoride values. But these a subject to so much variation and usually involve different collection and correction methods making comparison unjustified.

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Opponents of fluoridation all at sea with new legislation

Opponents of community water fluoridation (CWF) should be supporting the proposed fluoridation legislation instead of organising opposition to it.

Think about it.

If Fluoride Free NZ (FFNZ) was honest in its claim that they are “New Zealand’s leading advocate for science in the fluoridation debate” (see their press release  Open letter to Hon Andrew little, Minister of Health) then they should be supporting science rather than ideology and insist on the best scientific scrutiny of information relating to fluoridation.

If they honestly accept the claims of those cherry-picked anti-fluoride scientists they are quoting in their social media memes then they should welcome the opportunity to expose the research of those scientists to a proper critical review.

With scientific backers like this, opponents of community water fluoridation should be welcoming the new fluoridation legislation (example of social media memes promoted by the Fluoride Action Network)

And they should welcome the proposal that the proper place for such a scientific review is the office of the Director-general of health which can call on the best scientists for information and review of the evidence. That proposal is an integral part of the draft legislation which requires that the Director-General must consider the scientific evidence related to community water fluoridation before making a direction that CWF be introduced or stopped in a region (see clause 116E : Director-General may direct local authority to add or not to add fluoride to drinking water in the Supplementary Order Paper).

So, it appears strange that instead of welcoming the new legislation FFNZ is mobilising their supporters to oppose it. And their US colleagues at the Fluoridation Action Network are pouring their resources into the FFNZ campaign.

But why? It is ridiculous for pro-science people to campaign to retain the old system where the evaluation of evidence and decisions on CWF were made by scientific and political naive councillors in local bodies. Councillors who could be easily captured by activists and fooled by their misrepresentation of the science., Councillors who are more concerned with their next election or chances of claiming the Mayor’s job than any science. And councillors who are already predisposed to the claims of the activists, who may indeed be activists themselves, who were more concerned with ideological orientation than any science.

If the fluoridation opponents organised by FFNZ are really “leading advocates for science” and want recent research they are promoting to be considered in fluoridation decisions then they would be supporting the new legislation rather than opposing it.

Ideological distortion of science

I really wonder at a group of ideologically motivated people making submissions promoting their understanding (or misunderstanding) of the science to the Parliamentary Health Committee when that committee is simply not tasked with considering the science. Its job is to consider proposals for the reorganisation of the mechanism for making fluoridation decisions – nothing to do with science itself.

Instead of wasting their submissions on this bill, they should be saving them for promotion of their beliefs about what the science claims to the Director-General of Health and his/her staff. Once this bill is passed the Director-General of Health’s office should be a great place for these claims to be properly considered and reviewed.

That would be a vast improvement on the old situation when they took their arguments to scientifically and politically naive local body councils. Or brought in US anti-fluoridation spokesmen to speak to audiences of homoeopaths, head massagers and other alternative health advocates and their followers.

Or perhaps I am the naive one. Perhaps fluoridation opponents prefer to make their arguments to those local body councillors instead of scientifically capable people. Perhaps FFNZ is dishonest to claim they are ““New Zealand’s leading advocate for science in the fluoridation debate.” 

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Update of NZ fluoridation review timely and useful

Image credit: Kurt:S/Flickr (CC BY-SA 2.0).

An updated New Zealand scientific review of community water fluoridation (CWF) finds that CWF in New Zealand is safe and effective. The new review, Fluoridation: an update on evidence, was published by the New Zealand Prime Minister’s Chief Science Advisor Juliet Gerrard FRSNZ HonFRSC. It examines scientific findings published since the last New Zealand fluoridation review in 2014 – Eason et al (2014). Health effects of water fluoridation: A review of the scientific evidence. It states:

“Royal Society Te Apārangi published a comprehensive review in 2014 looking at the health effects of water fluoridation [Eason et al. 2014]. The review found that there were no adverse effects of fluoride of any significance arising from fluoridation at the levels used in Aotearoa New Zealand. We have considered new research on fluoridation and comprehensive reviews published subsequently, and find that the conclusions of the Royal Society Te Apārangi remain appropriate.”

So, in effect, nothing new there as far as conclusions are concerned. New Zealanders should continue to welcome CWF without any fear of negative effects. But some readers may be interested in the details – specifically the review’s conclusions about the new research which anti-fluoride activists have been promoting of late. Opponents of CWF have in the last year or so almost exclusively concentrated their campaigns on claimed harmful neurodevelopmental and cognitive impacts. The review considers this in a section “Could fluoride have possible neurodevelopmental and cognitive health effects?” and concludes:

“Recent studies continue to show that at very high levels and with chronic exposure, fluoride could potentially have negative neurodevelopmental and cognitive impacts. However, this is not a concern at levels used in fluoridation of water supplies in Aotearoa New Zealand.”

In particular, the new review considered the studies opponents of CWF claim show harmful effects at the water fluoride concentrations used in CWF. It found problems with these studies and states:

“The conclusion reached by Royal Society Te Apārangi remains appropriate. While there is some evidence that high fluoride concentrations may have an adverse effect on developing brains, there is no convincing evidence of neurological effects at fluoride concentrations achieved by fluoridation of water supplies in Aotearoa New Zealand.”

The new review could be better

Of course, the review is written for policymakers and politicians who want the conclusions but don’t want to see the detailed evidence. They will have confidence that behind the scenes proper scientific scrutiny was made of the studies discussed. But, as a scientist, I would have liked to see more details about that scrutiny. For example, a lot more could be said about the deficiencies of the Canadian study (Green et al., 2019) than:

“There was significant and valid criticism of aspects of the study by many subject-matter experts when it was released (see for example, ‘expert reaction to study looking at maternal exposure to fluoride and IQ in children’). The study used sub-group analysis to find an association that is not explained in the paper (i.e. why were only boys affected and why verbal IQ was not impacted), the effect appeared to be driven by the minority of participants that had much higher fluoride exposures (i.e. higher than those in Aotearoa New Zealand).”

I find this statement in the review a bit misleading:

“The results found in the study undertaken in Canada [56] are also in contrast to those found in the study undertaken in Aotearoa New Zealand [57], which has the advantage of more accurately reflecting local contextual factors.”

It refers to the Green et al., (2019) and Broadbent et al (2015) studies claiming their results are contradictory. But in terms of their comparison of IQ in fluoridated and unfluoridated areas this table shows their findings were exactly the same – absolutely no difference. (Till et al 2020 used essentially the same Canadian data) I think this comparison should have been made clear in the review because it is important but is most often overlooked because opponents of fluoridation, and the study’s authors,  never consider it. They remain silent about the facts in this table. This is hypocritical considering the attempts anti-fluoridation critics made to discredit the same finding reported by Broadbent et al (2015) when their paper was published.

Fluoridation opponents have instead concentrated on the relationships between maternal urinary fluoride and child IQ outcomes reported by Green et al (2019). However, all those relationships were extremely weak (explaining only a few per cent of the variation) or statistically non-significant.

The bias of the authors of the Green et al (2019) and Till et al (2020) studies is evident not only in their complete absence of discussion of their own results showing no difference between fluoridated and unfluoridated areas. It is also shown by the fact neither Green et al (2019) or Till et al (2020) referred to the findings of Broadbent et al (2015) or include that paper in their discussion or reference list. (Contrast this with the fact Green et al 2019 included four papers published in the journal Fluoride, known for its anti-fluoridation bias and poor quality).

These omissions are serious as Broadbent et al (2015) is the only other study of IQ in areas including CWF. This defect in the papers appears not to have been picked up by the publishing journals’ referees – which brings me to question the quality of the peer review used for these papers.

We really need to have a more extensive critique of these studies with proper consideration of the methodologies used (e.g. measurement of urinary fluoride and cognitive factors), the statistical relevance of the reported relationships and the quality of their literature reviews and discussions.

A timely, authoritative and useful review

This new review is timely. The NZ Parliament will soon pass legislation removing the current local body control of regional fluoridation decisions over to the Director-General of Health. Anti-fluoride activists will actively campaign against this and will be promoting misinformation about the recent research. The new review provides lawmakers and the media with up to date scientific information, presented in a popular format, which will counter the anti-fluoridation campaigners.

The backing of the office of the New Zealand Prime Minister’s Chief Science Advisor provides scientific authority. As do the 12 reputable scientists who provided peer reviews of the document.

The more science-savvy members of the public who want to see evidence rather than simply rely on recommendations will also find the review and its bibliography useful.

At this stage, it is not clear how anti-fluoride organisations will attempt to discredit the new review and its findings. In 2014 they worked hard to discredit the NZ Fluoridation Review prepared by the Royal Society of New Zealand and the Office of the Prime Minister’s Chief Science Advisor (Health effects of water fluoridation: A review of the scientific evidence.) They prepared their own critique (promoted as a critique by “international scientists “but actually written by, and “peer-reviewed” by, the usual small group of anti-fluoride scientists and activists) –  with their report Scientific and Critical Analysis of the 2014 New Zealand Fluoridation Report. I critiqued and analysed the report in NZ Community Water Fluoridation is Safe and Effective: A critique of Fluoride Free NZ’s criticisms of the NZ Fluoridation Review which can be downloaded as a pdf.

Many opponents of CWF may be distracted by other things like the Covid19 vaccination issue at the moment. However, the anti-fluoridation campaign has big financial backers locally and internationally so it is likely they will come up with something a lot more effective than the recent attempt by Fluoride Free NZ to pretend that the new review effectively supports their claims.

I look forward to analysing any substantial critique they can make of the updated NMZ Fluoridation review.

A meme promoted by Fluoride Free NZ seeks to pretend the new fluoridation review supports their claims. It doesn’t.

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Fluoridation contribution to heavy metals in drinking water is too low to measure

Anti-fluoride activists claims of fluoridation chemicals being laced with toxic metals just do not stand up to scrutiny

Anti-fluoridation activists claim fluoridation adds toxic heavy metals to drinking water because the fluoridation chemicals are “waste products” from industry (the phosphate fertiliser industry). Despite the fact that published research shows this not true.

The activists rarely give evidence for their claims but when they do they cite chemical data from certificates of compliance supplied to local bodies by the fluoridation chemical providers. But these activists simply have no concept of what these figures mean and always ignore the huge dilution involved in water treatment.

Example of continually tweeted scaremongering misinformation from the Fluoride Action Network’s press officer.

Anti-fluoride people particularly go on about arsenic yet a 2015 Canadian study showed that the difference in arsenic levels from unfluoridated and fluoridated water treatment plants was infinitesimal. That study is:

Peterson, E., Shapiro, H., Li, Y., Minnery, J. G., & Copes, R. (2015). Arsenic from community water fluoridation: quantifying the effect. Journal of Water and Health.

The data shows that even after treatment the concentration of arsenic due to natural sources is about 0.44 ppb. Fluoridation added a mere 0.07 ppb to this! (ppb = parts per billion = micrograms/litre = μg/L).

See Another defeat for anti-fluoridation claims about arsenic for a discussion of this paper.

The authors point out that all the drinking water systems in their study were compliant with the Canadian drinking water guideline for arsenic of 10 μg/L (10 ppb [parts per billion] – the same as in New Zealand) and the estimated amount attributable to fluoridation from this study is less than 1% of this guideline. Their results were also consistent with other published estimates of the likely contribution of fluoridation chemicals to arsenic in drinking water.

The fluoridation chemicals used in New Zealand have lower levels of impurities than those used in North America so the resulting contamination of drinking water is even lower than in Canada.

Here I look at the heavy metals contaminants in our drinking water and the effect of fluoridation on those levels

What are the concentrations of contaminants in our tap water?

They are very low – in fact, they are regulated to be very low. The regulations set maximum allowable levels (MAV) and providers must keep their contaminant below these MAV levels.

The figure below uses data taken from reports for the Hamilton City Council Water Supply Annual Compliance Report 2018/2019. I have converted the data from units of g/m3 to parts per billion – ppb. I have used data from  a certificate of compliance for the hydrofluorosilicic acid (HFA) supplied to the Hamilton City Council in 2015 (referred to in my article Chemophobic scaremongering: Much ado about absolutely nothing)

I have made this figure tall in an attempt to show the calculated contribution of contaminants concentrations from the fluoride treatment (red) alongside the actual measured levels in the tap water (green). The Y-axis is different for Barium and Aluminium because the concentrations are much higher. Click on the image to see a larger version of the figure

Here are some relevant points about this data:

1: Lead is not included because the amount is below the detection limit. The measured levels of the other contaminants of interest (green) are very low – well below the MAV values.

Waikato River water contains relatively high levels of arsenic – higher than MAV recommendations. But water treatment reduces the levels to well below the MAV level (and in some samples the levels are below the detection limit).

2: The calculated levels for contaminants due to added fluoridation chemicals (HFA – hydrofluorosilicic acid) are extremely low. In fact, in all cases, they are well below the levels that could be detected by the analytical methods used – in most cases far less than 1% of the detectable levels.

This means that the final concentration in drinking water of heavy metals added with the fluoridation chemicals is far too low to even be detected by the sensitive chemical analytical methods used. It is simply misleading for anti-fluoride activists to rave on about the contaminant levels in the concentrated chemicals without taking into account the huge dilution involved.

3: The figure below illustrates that the fluoridation treatment makes only a minuscule (undetectable) contribution of contaminant elements to the tap water.  Well under 1% of the contaminant contribution coming from the source water itself.


Fluoridation chemicals add such a minuscule amount of heavy metals to drinking water that their contribution cannot be detected. The amounts are below the detection limit of the analytical methods used.

The claims of anti-fluoride activists are emotional rather than factual. It is meaningless to publish images of compliance certificates. When they cite chemical data for the fluoridation chemicals used they simply have no concept of what the figures mean and completely ignore the high dilution factor involved in treating drinking water.

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Hip fractures in the elderly and fluoride – contradictory evidence

Room for cherry-picking to confirm a bias. Separate Swedish studies report that fluoride can either prevent or promote the risk of hip fracture in the elderly. Image credit: Are hip fracture patients treated quickly enough?

Anti-fluoride activists are promoting a recent study linking fluoride intake and bone fractures. No surprise there. But they are cherry-picking a single study to support their agenda and scientifically literate people should see the wider picture and not ignore other studies which, on the whole,  convey a different story. This issue illustrates problems with epidemiological studies producing variable results and shows why people should avoid cherry-picking and look at the full range of studies in a field.

Here I consider just two studies on fluoride intake and bone fracture which produced different conclusions. Both studies involved people from Sweden where the natural fluoride levels in drinking water vary across the country.

Drinking water fluoride may protect against hip fractures

First a study from 2013:

Näsman, P., Ekstrand, J., Granath, F., Ekbom, A., & Fored, C. M. (2013). Estimated drinking water fluoride exposure and risk of hip fracture: A cohort study. Journal of Dental Research, 92(11), 1029–1034.

The main findings are illustrated in the figure showing the calculated Hazard Ratios for people of different ages living in areas of Sweden with “very low” (less than 0.3 mg/L), “low” (0.3 – 0.69 mg/L), “medium” (0.7 -1.49 mg/L) or “High” (greater than 1.5 mg/L) fluoride levels in the drinking water. The Hazard Ratio in the figure below is a measure of the number of hip fractures at these levels compared with the number of hip fractures at “Very low” fluoride concentration. The bars represent the 95% confidence intervals. The Hazard Ratios for the “very low” group are 1.0 and Hazard Ratios statistically significantly different to 1 (no effect) are coloured red.

Considering all people there is no statistically significant increase in the number of hip fractures for any level of water fluoride concentration compared with the “very low” levels. The number of hip fractures experienced by people in the two lower age groups (less than 70 years and 70 – 80 years) was significantly lower at higher water fluoride concentrations than at the “very low” concentrations. The authors say:

this “suggests a protective effect of fluoride among the younger (age younger than 80 years): however, the majority of fractures occurred above the age of 80 years (median age at time of fracture, 82.0).”

So a study suggested that the fluoride in Swedish drinking water does not encourage bone fractures and may actually protect against them in the lower age groups.

Fluoride may encourage hip fractures

Now a study from 2021 – the one anti-fluoride activists are promoting (for obvious reasons):

Helte, E., Vargas, C. D., Kippler, M., Wolk, A., Michaëlsson, K., & Åkesson, A. (2021). Fluoride in Drinking Water , Diet , and Urine in Relation to Bone Mineral Density and Fracture Incidence in Postmenopausal Women. Environmental Health Perspectives, 129(April).

Unlike Näsman et al (2013) which used drinking water fluoride concentrations as a measure of fluoride exposure, Helte et al (2021) used urinary fluoride and estimated dietary fluoride intake as measures of fluoride exposure. The Hazard Ratios were calculated from the number of hip fractures in the Tertile 2 groups (0.88 – 1.30 mg/g urinary fluoride or 1.74 – 2.41 mg/day dietary fluoride intake) and Tertile 3 groups (1.30 – 116.51 mg/g urinary fluoride or 2.41 – 11.16 mg/day dietary fluoride intake) compared with hip fractures in the tertile 1 groups (0.14 – 0.88 mg/g urinary fluoride or 0.26 – 1.74 mg/day dietary fluoride intake).

Note: The urinary fluoride units of mg/g represent mg of urinary F/g urinary creatinine. Creatinine levels were used to correct the spot values for dilution.

The Hazard Ratios that statistically significantly different to 1 (no effect) are coloured red in the figure below.

A bit complicated I know, but what the figure shows is no statistically significant increase in hip fracture numbers for the tertile 2 groups compared with the lower F intake tertile one group. But a significant increase in fracture numbers for the tertile 3 groups except for the women exposed to constant water fluoride concentrations since 1982 in the dietary F group.

Hertle et al (2021) also considered other types of fracture. There were no statistically significant increases in fractures in the upper tertiles for either the “all fractures” or “major osteoporotic fractures” classes.

So, a bit of a mixed bag but this paper is currently being promoted by anti-fluoride activists as evidence of a harmful result from community water fluoridation (CWF).

Critically assessing the evidence for bone fractures

It is easy to see why supporters of CWF may cite Näsman et al (2013) as evidence for lack of harm and opponents may cite Helte et al (2021) as evidence of harm from CWF. But neither approach is really scientific. The methodological differences and choice of factors considered can easily explain variable results. One should critically and rationally assess both of these papers, together with the many other papers reporting similar studies, before coming to any conclusion.

On balance, the published studies probably support the findings of Näsman et al (2013) and not Helte et al (2021). In fact, a systematic review and meta-analysis published in 2015 concluded that chronic exposure to fluoride in drinking water was not associated with a significant increase in hip fracture risk. The citation for this review is:

Yin, X.-H., Huang, G.-L., Lin, D.-R., Wan, C.-C., Wang, Y.-D., Song, J.-K., & Xu, P. (2015). Exposure to Fluoride in Drinking Water and Hip Fracture Risk: A Meta-Analysis of Observational Studies. PLOS ONE, 10(5), e0126488. 

It’s worth reproducing one of the figures from that review because it illustrates how epidemiological studies may, individually, support a claim of harm but when considered as a whole these studies do not support the claim. The figure below shows the range of Hazard Ratios obtained by a number of studies.

The lesson here is to be very careful of claims made on the basis of single cherry-picked studies. Especially when those making the claim have a bias they wish to confirm. Every claim should be critically and rationally considered using all the available studies.

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An open letter to Paul Connet and the anti-fluoride movement

Paul Connett and Vyvyan Howard have, through the local Fluoride Free New Zealand activist group, published an open letter addressed to NZ scientists and educators (see An Open Letter To NZ Scientists And Educators). It is strange to encourage scientific exchanges through press releases but if they are seriously interested in an exchange of informed scientific opinion on the research they mention I am all for it.

In fact, I renew my offer to Paul Connett for a new exchange on the new relevant research along the lines of the highly successful scientific exchange we had in 2013/2014 summarised in Conett & Perrott (2014) The Fluoride Debate.

Connett and Howard say they felt “let down” by the reception they received in their 2018 visit. But they should realise this sort of ridicule is inevitable when a supposedly scientific message is promoted by activist fringe groups with known funding by big business (in this case the “natural”/alternative health industry). The science should be treated more respectably and discussed in a proper scientific forum or via a proper scientific exchange rather than political style activist meetings.

It is this sort of respectable, informed and open scientific exchange I am offering to Paul Connett and Vyvyan Hoard.

Connett and Howard argue that there has recently been  “a dramatic change in the quality of these [fluoride] studies.” I agree that new research occurs all the time and there is plenty of scope upgrading of the scientific exchange we had in 2013/2014 to cover that new research. Consideration of the new research requires the objective, critical and intelligent consideration scientists are well used to and this is not helped by activist propaganda meetings. So I encourage Connett and Howard to accept my offer. after all, if they are confident in their own analysis of this research what do they have to lose?

Inaccuracies in “open letter”

One can see an “Open letter” as displaying a willingness to enter into a proper scientific exchange. However, Connett and Vivyan’s open letter includes inaccuracies and misinformation on the new research which simply demonstrates that a one-sided presentation cannot present the research findings properly.

For example, they misrepresent the 2014 New Zealand fluoridation review of Eason et al (2014). Health effects of water fluoridation: A review of the scientific evidence. Even to the extent of mistaking the authors (not Gluckman & Skegg as they claim) and misrepresenting the small mistake made in the summary which was later corrected. That attitude does not bode well for the proper consideration of the research.

Connett and Howard concentrate on new research relating child IQ to fluoride intake but they ignore completely the fact that all the research comparing IQ in fluoridated and unfluoridated areas show absolutely no effect. I have summarised the results for the three papers involve in this table.

Instead, they concentrated on a few extremely weak relationships reported in a few papers. But even here they get this wrong – for example, they say there is “a loss of about 4 IQ points in offspring for a range of 1 mg/liter of fluoride in mother’s urine.” The paper they refer to (Green et al 2019) actually found no statistically significant relationship between child IQ and maternal urinary fluoride for all children considered. The relationship Connett and Howard mention was actually for male children (no relationship for female children or for all children) and was very weak. These sort of weak relationships are commonly found in epidemiological research and are usually meaningless. In this case, Connett and Howard have simply cherry-picked one value and misrepresented it as applying to all children.

Both the Green et al (2019) and Till et al (2020) papers Connett and Howard refer to suffer from selecting a few weak statistically significant relationships and ignoring the larger number of non-significant relationships they found for the data they investigated. Connett and Howard also completely ignored the new studies that don’t fit their claims. For example that of Santa-Marina et al (2019). Fluorinated water consumption in pregnancy and neuropsychological development of children at 14 months and 4 years of age. which showed an opposite positive relationship of child IQ with maternal urinary fluoride. Similar they ignored the large Swedish study of Aggeborn & Öhman (2020). The Effects of Fluoride in the Drinking Water showing no effect of fluoride on IQ but positive effects on oral health and employment possibilities in later life.

In conclusion, I reiterate that genuine open scientific exchanges do not take place via press release and activist meetings. However, the fact that Connett and Howard have issued an “Open Letter” could be interpreted as inviting others to participate in a proper exchange. I endorse that concept and offer Connett and Howard space for a free and open exchange on the new research at this blog site.

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Data dredging, p-hacking and motivated discussion in anti-fluoride paper

Image credit: Quick Data Lessons: Data Dredging

Oh dear – another scientific paper claiming evidence of toxic effects from fluoridation. But a critical look at the paper shows evidence of p-hacking, data dredging and motivated reasoning to derive their conclusions. And it was published in a journal shown to be friendly to such poor science.

The paper is:

Cunningham, J. E. A., Mccague, H., Malin, A. J., Flora, D., & Till, C. (2021). Fluoride exposure and duration and quality of sleep in a Canadian population-based sample. Environmental Health, 1–10.

Data dredging

This study used data from a Canadian database – the Canadian Health Measures Survey. Databases with large numbers of variables tempt researchers to dredge for data or relationships which confirm their biases. Despite the loss of statistical significance in this approach data dredging or data mining is quite common in epidemiological studies.

Cunningham et al (2021) looked for relationships using two separate measure of fluoride exposure and four different measures of possible sleep disturbance. They found a “statistically significant (p<0.05) relationship between lower sleep duration and water fluoride. But no relationships for higher sleep duration, trouble sleeping or daytime sleepiness with either water fluoride or urinary fluoride. Their results for logical regression analysis are summarised in this figure. (Error bars crossing an Odds Ratio value of 1.0 indicate that the relationship is not statistically significant and p<0.05).

Of the 8 relationships investigated only 1 was statistically significant.


I discussed the problem of p-hacking in Statistical manipulation to get publishable results.

With a large dataset, one can inevitably find relationships that satisfy the p<0.05 criteria – because this p-value value is meaningless when multiple relationships are considered. One can even find such “statistically significant relationships” when random datasets are investigated (see Science is often wrong – be criticalI don’t “believe” in science – and neither should you, The promotion of weak statistical relationships in science  and Can we trust science). Once multiple relationships are investigated the chance of finding accidental relationships is much greater than 1 in 20 signified by the p<0.05 value.

So, one of the 8 relationships above satisfied the p<0.05 criteria when considered alone. But as part of multiple investigations, the chance of finding such a relationship by chance is much greater than 1 in 20.

Motivated reasoning

This paper smacks of motivated reasoning. The authors obviously have a commitment to the concept that fluoride causes problems with the pineal gland and drag up anything they can find in the literature to support this – without critically assessing the quality of the cited work or even mentioning the fact that the cited studies were made at much higher fluoride concentration on non-human animals. In effect, they are attempting to convert very weak results, obtained by data dredging and p-hacking, to a fact. They are attempting to make a purse out of a sow’s ear.

This research group is not new to this game. I commented on this in my critique of another sleep disorder paper from the group (see ).

Many of the same researchers are listed as authors on both papers – yet Cummingham et al (2021 ) cite the previous paper as if it was an independent study. They say “As far as we are aware, this is only the second human
study investigating the effects of fluoride exposure on sleep outcomes” which is simply disingenuous considering the involvement of the same researchers in both papers.

Both these papers were also published in the same journal – Environmental Health – a pay-to publish-journal that is known to be friendly to anti-fluoride researchers and uses very sympathetic peer reviewers (see ). The Chief editor, Philippe Grandjean, is well known for his opposition to fluoridation. I commented on his refusal to consider a paper of mine that critiqued an anti-fluoride paper published in his journal (see Fluoridation not associated with ADHD – a myth put to rest).


Yet another very weak study, published in an anti-fluoride friendly pay-to-publish journal with poor peer review. Despite the weaknesses due to data dredging, p-hacking and motivated reasoning, anti-fluoride activists will cite the single “statistically significant” result as gospel and ignore the 7 relationships that are not significant. As for inadequate consideration of confounders or other risk-modifying factors, this study ignores completely the fact that city size and geographic factors have a strong effect on both sleep patterns and water fluoride concentrations (see Perrott 2018). Such inadequate consideration of confounders is another common problem in epidemiological studies.

Oh, well, we are not a rational species. More a rationalising one. And in such areas motivated rationalisation and confirmation bias is rife.

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Embarrassing knock-back of second draft review of possible cognitive health effects of fluoride

We have come to expect exaggeration of scientific findings in media reports and institutional press releases. But it can also be a problem is original scientific publications where findings are reported in an unqualified or exaggerated way. Image Credit: Curbing exaggerated reporting

This is rather embarrassing for a US group attempting to get the science right about possible toxic effects of fluoride. It’s also embarrassing for the anti-fluoride activists who have “jumped the gun” and been citing the group’s draft review as if it was reliable when it is not.

The US National Academies of Sciences, Engineering, and Medicine (NAS) have released their peer-review of the revised US National Toxicity Program (NTP) draft on possible neurodevelopmental effects of fluoride (see Review of the Revised NTP Monograph on the Systematic Review of Fluoride Exposure and Neurodevelopmental and Cognitive Health Effects).

This is the second attempt by the NTP reviewers to get acceptance of their draft and it has now been knocked back by the NAS peer reviewers for a second time.

Diplomatic but damning peer-review

Of course, the NAS peer reviewers use diplomatic language but the peer review is quite damning. It criticises the NTP for ignoring some of the important recommendations in the first peer review. One which is quite critical was the lack of response to the request that NTP explains how the monograph can be used (or not) to inform water fluoridation concentrations. The second NAS peer review firmly states that the NTP:

“should make it clear that the monograph cannot be used to draw any conclusions regarding low fluoride exposure concentrations, including those typically associated with drinking-water fluoridation.”


“Given the substantial concern regarding health implications of various fluoride exposures, comments or inferences that are not based on rigorous analyses It seems to me that there is soime internal politicsshould be avoided.”

It seems to me there is some internal politics involved and some of the NTP authors may be promoting their own, possibly anti-fluoride, agenda. Certainly, the revised NTP draft monograph continues to obfuscate this issue. It continues to state that “fluoride is presumed to be a cognitive neurodevelopmental hazard to humans” – a clause which anti-fluoride campaigner consistently quote out of context. Yes, it does state that this is based on findings demonstrating “that higher fluoride exposure (e.g., >1.5 mg/L in drinking water) is associated with lower IQ and other cognitive effects in children.” But this is separated from the other fact that the findings on cognitive neurodevelopment for “exposures in ranges typically found in drinking water in the United States (0.7 mg/L for optimally fluoridated community water systems)” are “are inconsistent, and therefore unclear.”

Monograph exaggerates by enabling unfair cherry-picking

So, you see the problem. The draft NTP monograph correctly refers to IQ and other cognitive effects in children exposed to excessive levels of fluoride. The draft also correctly refers to that lack of evidence for such effects at lower fluoride exposure levels typical of community water fluoridation. But in different places in the document.

The enables activist cherry-picking to support an anti-fluoride agenda and that is a fault of the document itself. It should clearly state that the monograph should not be used to draw any conclusion at these low exposure levels. This is strongly expressed in the peer-reviewers’ comments.

I find the blanket “presumed to be a hazard for humans” quite misleading. For example, no one says that calcium is “presumed to be a cardiovascular hazard to humans.” Or that selenium is “presumed to be a cardiovascular or neurological hazard to humans.” Or what about magnesium – would you accept that it is a “presumed neurological hazard to humans?” Would you accept that iron is a “presumed cardiovascular, cancer, kidney or erectile dysfunction hazard to humans?” Yet all those problems have been reported for humans at high intake levels of these elements.

No, we sensibly accept that various elements and microelements have beneficial, or essential benefits, to humans at reasonable intake levels., Then we sensibly warn that these same elements can be harmful at excessive intake. To proclaim that any of these elements are “presumed” to be hazardous – without clearly saying at excessive intake levels, is simply distorting or exaggerating the data.

What does “presumed” mean?

A lot of readers find the use of “presumed” strange. But it’s meaning is related to the levels of evidence found by reviewers.

No, don’t believe those anti-fluoride activists who falsely claim that “presumed” is the highest level of evidence and that the finding should be treated as factual. They are simply wrong.

Some idea of the word’s use is presented in this diagram from the NTP revised draft monograph.

So “presumed” means that the evidence for the effect is moderate. That the effect is not factual or known. But as further evidence comes in the ranking of fluoride as a hazard may increase, or decline.

As the monograph bases this “presumed” rating solely on evidence from areas of endemic fluorosis where fluoride intake levels are high it is correct to avoid stating the effects as factual. For example, consider these images from areas of endemic fluorosis in China (taken from a slide presentation by Xiang 2014):

Clearly, people in these areas suffer a range of health effects related to the high fluoride intake. The cognitive effects like IQ loss from these areas could result from these other health effects, not directly from fluoride (although excessive fluoride intake leads to the health effects).

So we can “presume” that fluoride (in areas of endemic fluorosis where fluoride intake is excessive) is a “cognitive neurodevelopmental hazard for humans” but we can not factually state that the neurodevelopment effects are directly caused by fluoride. That would require further scientific work to elucidate the specific mechanisms involved in creating that effect.

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The promotion of weak statistical relationships in science

Image credit: Correlation, Causation, and Their Impact on AB Testing

Correlation is never evidence for causation – but, unfortunately, many scientific articles imply that it is. While paying lip service to the correlation-causation mantra, some (possibly many) authors end up arguing that their data is evidence for an effect based solely on the correlations they observe. This is one of the reasons for the replication crisis in science where contradictory results are being reported. Results which cannot be replicated by other workers (see I don’t “believe” in science – and neither should you).

Career prospects, institutional pressure and the need for public recognition will encourage scientists to publish poor quality work that they then use to claim that have found an effect. The problem is that the public, the news media and even many scientists simply do not properly scrutinise the published papers. In most cases they don’t have the specific skills required for this.

There is nothing wrong with doing statistical analyses and producing correlations. However such correlations should be used to suggest future more meaningful and better-designed research like randomised controlled trials (see Smith & Ebrahim 2002Data dredging, bias, or confounding. They can all get you into the BMJ and the Friday papers. ). They should never be used as “proof” for an effect, let alone argue that the correlation is evidence to support regulations and advise policymakers.

Hunting for correlations

However, researchers will continue to publish correlations and make great claims for them because they face powerful incentives to promote even unreliable research results. Scientific culture and institutional pressures provide expectations demanding academic researchers produce publishable results. This pressure is so great they will often clutch at straws to produce correlations even when the initial statistical analyst produces none. They will end up “torturing the data.”

These days epidemiological researchers use large databases and powerful statistical software in their search for correlations. Unfortunately, this leads to data mining which, by suitable selection of variables, makes the discovery of statistically significant correlations easy. The data mining approach also means that the often cite p-values are meaningless. P-values measure the probability the relationship occurs by chance and often cited as evidence of the “robustness” of the correlations. But probability is so much greater when researchers resort to checking a range of variables and that isn’t reflected properly in the p-values.

Where data mining occurs, even to a limited extent, researchers are simply attempting to make a purse out of sow’s ear when they support their correlations merely by citing a p-value < 0.05  because these values are meaningless in such cases. The fact that so many of these authors often ignore more meaningful results from their statistical analyses (like R-squared values which indicate the extent that the correlation “explain” the variation in their data) underlines their deceptive approach.

Poor statistical relationships

Consider these correlations below – two data sets are taken from a published paper – the other four use random data provided by Jim Jones in his book Regression Analysis: An Intuitive Guide.

You can probably guess which correlations were from real data (J and M) because there are so many more data points All of these have correlations low p values – but of course, those selected from random data sets resulted from data mining and the p-values are therefore meaningless because they are just a few of the many checked. Remember, a p-value < 0.05 means that the probability of a chance effect is one in twenty and more than twenty variable pairs were checked in this random dataset.

The other two correlations are taken from Bashash et al (2017). They do not give details of how many other variables were checked in the dataset used but it is inevitable that some degree of data mining occurred. So, again, the low p-values are probably meaningless.

J provides the correlation of General Cognitive Index (GCI) scores in children at age 4 years with maternal prenatal urinary fluoride and M provides the correlation of children’s IQ at age 6–12 y with maternal prenatal urinary fluoride. The paper has been heavily promoted by anti-fluoride scientists and activists. None of the promoters have made a critical, objective, analysis of the correlations reported. Paul Connett, director of the Fluoride Action Network, was merely supporting his anti-fluoride activist bias when he uncritically described the correlations as “robust.” They just aren’t.

There is a very high degree of scattering in both these correlations, and the R-squared values indicate they cannot explain any more than about 3 or 4% of the variance in the data. Hardly something to hang one’s hat on, or to be used to argue that policymakers should introduce new regulations controlling community water fluoridation or ban it altogether.

In an effort to make their correlations look better these authors imposed confidence intervals on the graphs (see below). This Xkcd cartoon on curve fitting gives a cynical take on that. The grey areas in the graphs may impress some people but it does not hide the wide scatter of the data points. The confidence intervals refer to estimates of the regression coefficient but when it comes to using the correlations to predict likely effects one must use the prediction intervals which are very large (see Paul Connett’s misrepresentation of maternal F exposure study debunked). In fact, the estimated slopes in these graphs are meaningless when it comes to predictions.

Correlations reported by Bashash et al (2017). The regressions explain very little of the variance in the data and connect be used to make meaningful predictions.

In critiquing the Bashash et al (2017) paper I must concede that at least they made their data available – the data points in the two figures. While they did not provide full or proper results from their statistical analysis (for example they didn’t cite the R-squared values) the data does at least make it possible for other researchers to check their conclusions.

Unfortunately, many authors simply cite p-values and possible confidence intervals for the estimate of the regression coefficient without providing any data or images. This is frustrating for the intelligent scientific reader attempting to critically evaluate their claims.


We should never forget that correlations, no matter how impressive, do not mean causation. It is very poor science to suggest they do.

Nevertheless, many research resort to correlations they have managed to glean from databases, usually resorting to some extent of data mining, to claim they have found an effect and to get published. The drive to publish means that even very poor correlations get promoted and are used by ideologically or career-minded scientists, and by activists, to attempt to convince policymakers of their cause.

Image credit: Xkcd – Correlation

Remember, correlations are never evidence of causation.

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