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Data extraction and synthesis
Extensive data extraction. Risk of bias assessment using the OHAT tool. A narrative synthesis of
the results was carried out.
Results
The review included 89 studies in humans,199 in animals and 10 reviews of in vitro studies. Where
there was consistent evidence of a positive association, in relation to a water fluoride
concentration of <20 ppm (mg F/L), and where studies were judged to be acceptable or high
quality, health effects were taken forwards for further examination of causality using Bradford
Hill's 9 criteria. Of the 39 health outcomes reviewed, 4 were further assessed for causality. The
authors reported 'strong' evidence of causality for dental fluorosis and reductions in children's IQ
scores, 'moderate' strength evidence for thyroid dysfunction, 'weak' for kidney dysfunction, and
'limited' evidence for sex hormone disruption.
Conclusions
The authors conclude that moderate dental fluorosis and reductions in children's IQ scores are the
most appropriate health outcomes to use when setting an upper safe level of fluoride in drinking
water. For reductions in children's IQ, the authors acknowledge a biological mechanism of action
has not been elucidated, and the dose response curve is not clear at lower concentrations, limiting
the ability to set an upper safe threshold.
Subject terms: Fluoridation, Dentistry
A Commentary on
Taher M K, Momoli F, Go J et al.
Systematic review of epidemiological and toxicological evidence on health effects of fluoride in
drinking water. Crit Rev Toxicol 2024 54 2-34.
GRADE Rating: 6000
Commentary
The safety of water fluoridation has been frequently questioned and concerns have been raised
about many different health effects. Recently, studies investigating neurodevelopment and IQ have
generated increasing attention. In the USA, there is an ongoing court case 'Food & Water Watch, Inc.
et al. v. Environmental Protection Agency (EPA) et al. in which the plaintiffs, a coalition of anti -
fluoridation groups, claim that fluoride poses an 'unreasonable risk' to health on the grounds that
it is neurotoxic, and should be regulated as such under the Toxic Substances Control Act of 1976
(TSCA)I.
A key piece of evidence in the trial is a US National Toxicology Programme (NTP) systematic
review of fluoride exposure and neurodevelopmental and cognitive health effects. The NTP
systematic review was started in 2016 and the first draft (2019) concluded that "fluoride is
presumed to be a cognitive neurodevelopmental hazard to humans" The US National Academies of
Science, Engineering and Medicine (NASEM) have twice peer reviewed the draft NTP review and
requested revisions on the basis that the conclusions were not adequately supported by the
evidence?.
An important issue is that many of the studies de nstrating adverse effects of fluoride on IQ are
related to very high naturally occurring wat uoride concentrations, far higher than the
recommended (and legal, in the UK an trope) maximum of 1.5 mg F/L, and the 1 mg F/L target
for water fluoridation programme . dditionally, many studies have been conducted in developing
countries in populations who may also be exposed to environmental pollutants such as lead,
mercury or arsenic through contaminated drinking water or coal smoke pollution-. At the time
of writing, the final NTP review has not been published, however, the most recently available third
draft (Sept 2022) is more cautious, stating with "moderate confidence" that "higher fluoride
exposure... [>1.5 mg F/L] is consistently associated with lower IQ in children, and acknowledges
that "more studies are needed to fully understand the potential for lower fluoride exposure [<1.5
mg F/L] to affect children's IQ"4.
This brings us back to the present systematic review, by Taher et al:-�. In terms of the methods, the
scope of the review is extensive. The main manuscript is 29 pages, and there are over 1000 pages of
supplementary materials. It is therefore surprising that there is no indication that the review
protocol was registered in advance, important to maintain rigour and reduce bias. Relatedly, there
is no justification given for the'top-up search' between 2021 and Feb 2023, for only IQ -effects
studies. It is not clear that data extraction was performed in duplicate, or that the data extraction
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not imply endorsement of, or agreement with, the contents by NLM or the National Institutes of
Health.
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Evid Based Dent. 2024 Jun 1;25(2):95-97. doi: 10.1038/s41432-024-01022-6
Fluoride and children's IQ: evidence of causation lacking
Deborah Moore 1°E1, Anne -Marie Glennv 2
Author information Article notes Copyright and License information
PMCID: PMC11213703 PMID: 38824186
Abstract
Data sources
Human, animal, and in vitro studies. Extensive literature search of multiple bibliographic
databases, trial registries, major grey literature sources and bibliographies of identified studies.
Study selection
The authors aimed to identify studies which could be used to determine the maximum safe level
for fluoride in drinking water. To identify new studies published since a 2016 Australian review,
the search period was 2016 to July 2021. Studies which evaluated the association between either
naturally or artificially fluoridated water (any concentration) and any health outcomes were
included. No restrictions on study design or publication status. Articles published in a 'non -Latin
language' were excluded. Screening of abstracts and full texts was in duplicate. For IQ and dental
fluorosis, a top -up search was conducted between 2021 and Feb 2023.
forms were piloted. Indeed, a large volume of information has been extracted but not all of it is
used in the analysis or manuscript, and there are some reporting inconsistencies.
Our main area of concern reflects the risk of bias assessment and how it has been applied. The
authors used the OHAT Tool for Human and Animal Studies. A requirement of the tool is that
important confounding factors should be agreed in advance with subject matter experts, for each
health outcome. There is no indication that this was done, therefore we have limited confidence in
the authors' assessment as we are not sure what confounders they would expect to be considered
as a minimum. In addition, the study summary table in the main manuscript presents study type,
country, and direction of association for each health outcome, but does not include any indication
of the strength of the association or the water fluoride concentration, which makes it difficult to
evaluate the relevance of the findings to water fluoridation programmes.
The human evidence included in the Taher review overlaps with the draft NTP review-,, and other
systematic reviews on the neurological and health effects of fluoride in water, including a recent
Irish government review (Lambe et al.6), and a Canadian government review (CADTH)7. There
appear to be differing perspectives on the quality of this evidence from reviewers with a
toxicology perspective (Taher et al 5 and the draft NTP review), versus those with an evidence -
based medicine perspective-67. The latter have stated that the current evidence base related to IQ
is insufficient to draw conclusions, and that further high quality research is needed (Lambe et al.
and CADTH)�,.
To illustrate, a Canadian prospective cohort study by Till. et al.!, assessed as `high quality / low
risk of bias' by both Taher et al. and the draft NTP review1, was assessed as `low quality / high risk
of bias' by the Lambe et al. and CADTH reviews�2. Bias concerns stated were: selection bias (low
participation rate, high loss to follow-up); validity of the fluoride and IQ measurements; and
insufficient adjustment for confounding, including by maternal IQ and marital status6,7. Similar
differences in risk of bias assessments are evident for other papers included in the multiple
reviews. The paper by Till et al. (2020) uses data from the Maternal -Infant Research on
Environmental Chemicals (MIREC) birth cohort database in Canada, as do several other recent
fluoride -IQ papers (including Farmus et al.2, also assessed as high quality by Taher et al., and low
quality by Lambe et al .-'�). The MIREC cohort was not designed to evaluate fluoride and the studies
using these data have been extensively critiqued2 1110, with recent authors stating that these studies
should be "considered unacceptable for legal and policy purposes"!-1.
In conclusion, there is ongoing and high -profile debate regarding the impact of fluoride in drinking
water on IQ with very different perspectives on bias according to discipline. High -quality
prospective longitudinal studies based on individual -level exposures, in populations exposed to
fluoride concentrations of relevance to fluoridation programmes, and taking account of all
important confounding factors are necessary to provide higher -quality information.
Practice Points.
• Practitioners may need to respond to patient concerns regarding the
neuro developmental toxicity of fluoride in drinking water.
• Recent proposals to extend water fluoridation in the North East of England mean this
issue is very topical.
• Practitioners should be aware the evidence base is much disputed with ongoing
concerns regarding the validity, applicability, and risk of bias in many of the studies.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published
maps and institutional affiliations.
References
1. Court Listener. Food & Water Watch, Inc. et al. v. Environmental Protection Agency et al.
(3:17-cv-02162). 2024 [cited 2024 Apr 10]. Available from:
https://www.courtlistener.com/docket/6201332/food-water-watch-inc-v-environmental-
protection-agencv/?filed after=&filed before -&entry gte=&entry lte=&order by=desc
2. National Academies of Science Engineering and Medicine. Review of the Revised NTP
Monograph on the Systematic Review of Fluoride Exposure and Neurodevelopmental and
Cognitive Health Effects: A Letter Report (2021). The National Academies Press; 2021 [cited
2023 Jan 11]. Available from: https: nap. nationalacademies.org catalog/26030/review-