Appraisal Neurodevelopmental Disorders

An example of poor methodology and bias in studies claiming fluoride decreases IQ

Publication reviewed:

Arsenic and fluoride exposure in drinking water: children’s IQ and growth in Shanyin County, Shanxi Province, China

Wang SX, Wang ZH, Cheng XT et al — Environ Health Perspect. 2007; 115:643–7.

Many claims that fluoride decreases IQ are made across the internet. But the research base behind these claims are dominated by studies with weak methodology and high probability of biases such as the ecological fallacy. Dr. Ismail Jolaoso appraised an example of one of these studies.

Fluoride decreases IQ claims based on studies in regions such as Shanxi province
Temples in Shanxi Province, China, the location of the research claimed to indicate fluoride decreases IQ but appraised to be from a weak study.


The major findings in the study indicate that exposure to high levels of fluoride and arsenic is associated with decreases in IQ. The relationship between high arsenic in drinking water and neurotoxicity is well supported by other research and accepted by the scientific community but that is not the case with fluoride. A carefully conducted animal study failed to show neurotoxic effect of fluoride. The authors could not determine if each child in the high fluoride group actually drank high fluoride water. The authors also did not measure arsenic in the majority of children in the high fluoride group.


  • A – Strong methodology and unbiased, appeared in peer-reviewed in respected science journal
  • B – Strong methodology and unbiased, not in peer-reviewed journal
  • C – Weak methodology and/or biased
  • F – Not a scientific finding


  • High – All the peer-reviewed research to date support these findings, and a significant amount of research has been done in this area.
  • Medium – Most, but not all, peer-reviewed research to date support these findings, and a significant amount of research has been done in this area.
  • Low – Not a lot of research has been done in this area, or some, but not most, other peer-reviewed research supports these findings.
  • Not Supported – No other studies support this study’s conclusions.
  • Contradicted – Most studies contradict this study’s conclusions.


The authors provided detailed information on the study methodology and results to generate hypothesis for future testing in Shanxi Province of China.


This is an ecological study. Therefore, it is subject to ecologic fallacy. The biologic evidence supporting the study hypothesis is weak. It is difficult to draw a causal inference that fluoride decreases IQ – there is no evidence of temporality and dose-response relationship. The study measures exposures to arsenic and fluoride but, in the high fluoride group, not all children were assessed for arsenic exposure. It is not clear if the study participants are representative of children living in Shanxi province. Because the comparison group also was exposed to fluoride at 0.5 mg/L on average, a level that is consistent with the optimal level for fluoridation in China, this study is not relevant for fluoridation policy in the US.


The validity of this study to determine whether fluoride causes IQ deficits is weak. The cross-sectional study design makes it difficult to examine the effect of fluoride and arsenic. The design is more appropriate for generating hypothesis for future testing. The exposure to fluoride was based on a sample of water from some wells and may not be representative of all wells. Furthermore, there was no individual level measurement of exposure. In addition, fluoride level in the control group was 0.5 mg/L, a level consistent with optimal level of fluoride exposure for China. The authors did not control for confounding variables such as parental education and arsenic level. Even though urinary arsenic was low (Table 2) in 101 of 253 children in the high-fluoride group, there is the possibility that the urinary arsenic could be higher in the 142 children who were not evaluated. The study is not relevant to fluoridation or fluoride use in the United States. The standard of living and fluoride exposure cannot be compared with that in the US. The authors’ conclusion that fluoride in water is associated with neurotoxic effects cannot be supported.