Overview
As of June 2021, a total of 77 studies have investigated the relationship between fluoride and human intelligence, and over 60 studies have investigated the relationship between fluoride and learning/memory in animals. Of these investigations, 69 studies have found that elevated fluoride exposure is associated with reduced IQ in humans, while over 60 animal studies have found that fluoride exposure impairs the learning and/or memory capacity of animals. The human studies, which are based on IQ examinations of 26,142 children (67 studies) and 245 adults (2 studies), provide compelling evidence that fluoride exposure during the early years of life can damage a child’s developing brain.
After reviewing 27 of the human IQ studies, a team of Harvard scientists concluded that fluoride’s effect on the young brain should now be a “high research priority.” (Choi, et al 2012). Other reviewers have reached similar conclusions, including the prestigious National Research Council (NRC), and scientists in the Neurotoxicology Division of the Environmental Protection Agency (Mundy, et al). In the list of the IQ studies, we summarize the results from the 69 studies that have found associations between fluoride and reduced IQ and provide links to the abstracts or full-text copies, whe available, of the studies.
For a discussion of the 8 studies that did not find an association between fluoride and IQ, click here.
Quick Facts About the 69 Studies:
- Location of Studies: China (41), India (14), Iran (4), Mexico (3), Canada (3), Egypt (1), Kenya (1), Pakistan (1), and Sudan (1).
- Sources of Fluoride Exposure: 60 of the 69 IQ studies involved communities where the predominant source of fluoride exposure was drinking water; nine studies investigated fluoride exposure from coal burning.
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Fluoride Levels in Water:
IQ reductions have been significantly associated with fluoride levels of just 0.7 mg/L (Green 2019); 0.15 to 1.38 mg/L (Bashash 2017); 0.7 to 1.2 mg/L (Sudhir 2009); 0.88 mg/L among children with iodine deficiency. (Lin 1991); 0.50–1.90 mg/L (Yu 2021). Other studies have found IQ reductions at 1.4 ppm (Zhang 2015); 1.8 ppm (Xu 1994); 1.9 ppm (Xiang 2003a,b); 0.3-3.0 ppm (Ding 2011); 2.0 ppm (Yao 1996, 1997); 2.1 ppm (Das 2016); 2.1-3.2 ppm (An 1992); 2.2 ppm (Choi 2015); 2.3 ppm (Trivedi 2012); 2.38 ppm (Poureslami 2011); 2.4-3.5 ppm (Nagarajappa 2013); 2.45 ppm (Eswar 2011); 2.5 ppm (Seraj 2006); 2.5-3.5 ppm (Shivaprakash 2011); 2.85 ppm (Hong 2001); 2.97 ppm (Wang 2001, Yang 1994); 3.1 ppm (Seraj 2012); 3.15 ppm (Lu 2000); 3.94 ppm (Karimzade 2014); and 4.12 ppm (Zhao 1996).
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Fluoride Levels in Urine:
About a quarter of the IQ studies have provided data on the level of fluoride in the children’s urine, with the majority of these studies reporting that the average urine fluoride level was below 3 mg/L. To put this level in perspective, a study from England found that 5.6% of the adult population in fluoridated areas have urinary fluoride levels exceeding 3 mg/L, and 1.1% have levels exceeding 4 mg/L. (Mansfield 1999). In Canada, Till et al (2018) performed a national survey of urinary fluoride levels of pregnant women in fluoridated and non-fluoridated communities. The fluoride levels of women living with water fluoridation had the same levels as found in the Bashash (2017) study. The U.S. has the same “optimal concentration” of 7 mg/L in their fluoridation schemes as Canada. Although there is an appalling absence of urinary fluoride data among children in the United States, the excess ingestion of fluoride toothpaste among some young children is almost certain to produce urinary fluoride levels that exceed 2 ppm in a portion of the child population.
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Fluoride Levels in Hair, Nail, Urine, and Water
The most recent IQ study from Yu et al. (June 2021) recorded these measurements for the 952 children in their study. The authors noted, “Our study has several strengths. Using four fluoride exposure indicators including water fluoride, urinary fluoride, hair fluoride and nail fluoride, which reflect the external and internal, and short-term and long-term exposures, makes the evaluation of fluoride exposures more comprehensive and reliable. Besides, due to the relatively rare studies on low-to-moderate level fluoride exposure in hair and nail, our results also enrich the epidemiological evidence across different fluoride indicators and levels.”
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In-Utero Exposure: also called the Mother-Offspring studies.
In the Mother-Offspring studies by Green 2019, Valdez Jiménez 2017, and Li 2004, the authors evaluated the association of in utero exposure to fluoride on the offspring. Fluoride exposure was determined through urine analysis during the pregnancy and the levels were paired with cognitive testing of the offspring at various ages up to 12 years. Thomas et al. (2018) also performed this study finding an effect, however it is only available as an abstract, and it hasn’t yet been added to the list of IQ studies.
Methodological Limitations
As both the NRC and Harvard reviews have correctly pointed out, many of the fluoride/IQ studies have used relatively simple designs and have failed to adequately control for all of the factors that can impact a child’s intelligence (e.g., parental education, socioeconomic status, lead and arsenic exposure). For several reasons, however, it is unlikely that these limitations can explain the association between fluoride and IQ.
First, some of the fluoride/IQ studies have controlled for the key relevant factors, and significant associations between fluoride and reduced IQ were still observed. This fact was confirmed in the Harvard review, which reported that the association between fluoride and IQ remains significant when considering only those studies that controlled for certain key factors (e.g., arsenic, iodine, etc). Indeed, the four studies that controlled for the largest number of factors (Green 2019; Bashash 2017; Rocha Amador 2007; Xiang 2003a,b) reported some of the largest associations between fluoride and IQ to date.
Second, the association between fluoride and reduced IQ in children is predicted by, and entirely consistent with, a large body of other evidence. Other human studies, for example, have found associations between fluoride, cognition, and neurobehavior in ways consistent with fluoride being a neurotoxin. In addition, animal studies have repeatedly found that fluoride impairs the learning and memory capacity of rats under carefully controlled laboratory conditions. An even larger body of animal research has found that fluoride can directly damage the brain, a finding that has been confirmed in studies of aborted human fetuses from high-fluoride areas.
Finally, it is worth considering that before any of the studies finding reduced IQ in humans were known in the western world, a team of U.S. scientists at a Harvard-affiliated research center predicted (based on behavioral effects they observed in fluoride-treated animals) that fluoride might be capable of reducing IQ in humans. (Mullenix 1995)
Summary
When considering their consistency with numerous animal studies, it is very unlikely that the 53 human studies finding associations between fluoride and reduced IQ can all be a random fluke. The question today, therefore, is less whether fluoride reduces IQ, but at what dose, at what time, and how this dose and time varies based on an individual’s nutritional status, health status, and exposure to other contaminants (e.g., aluminum, arsenic, lead, etc). Of particular concern is fluoride’s effect on children born to women with suboptimal iodine intake during the time of pregnancy, and/or fluoride’s effects on infants and toddlers with suboptimal iodine intake themselves. According to the U.S. Centers for Disease Control, approximately 12% of the U.S. population has deficient exposure to iodine.
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