An Open Letter from your Dental Hygienist on Fluoride and IQ

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A study and subsequent articles last year cited lower IQ measurements for children whose mothers ingested fluoride during pregnancy, creating a paranoid frenzy among parents. The article stated that the ingestion of fluoridated water by an expectant mother decreased the IQ of the child by 2.5 points on a 200-point IQ test scale.1

The study in question is deeply flawed. Please allow me as your dental hygienist to break it down for you.

Let me start by saying that, before 2008 and a revision by the World Medical Association, there was no law that stated a study with human subjects should be publicly disclosed at all.2 This came under fire several years ago when a popular lorcainide clinical trial from 1993 surfaced, indicating that the anti-arrhythmic drug killed several people during the study; it was never published.3 That drug went on to more clinical trials and more deaths, because of the publication bias that prohibited it from being reported. The laws now require all human-subject studies to be disclosed, and studies pop up all the time. Many of those studies, though, are clinically insignificant.

Low Economic Status of Subject Group

The study in question relating to IQ tests was flawed not only in design but in execution. The reasons are even outlined in the discussion section of the Environmental Health Perspectives study itself.1 To begin, one of the major subject groups studied was representative only of a small population in Mexico City known for low-to-moderate income level and socioeconomic status. Studies have long shown that low socioeconomic status influences IQ score based on a variety of factors, including the language used in standardized testing and an inability to understand the questions.4

The IQ test, in this case, was even translated from English to Spanish — the consistency of which has never been evaluated.1 There are many studies with various subjects, including even rats, that indicate environmental enrichment at a young age improves learning and memory.5 The implication of that information relates again back to socioeconomic status and an inability to afford the purchase of programs and items that assist with environmental enrichment and stimulation in early development.

What Excessive Exposure?

Secondly, the population that had statistically significant data indicating changes in IQ scores was far beyond the level of fluoride considered within normal limits in this country. The results section of the study even said that there is no statistically significant change in IQ scores associated with levels below 0.8mg/L.

The recommended level of fluoride in drinking water in the United States is 0.7mg/L. This level is indicated by the U.S. Department of Health and Human Services.6 The study also states that the level of fluoride exposure of the children during early development was not associated with changes in IQ—only during gestation.

Differing Methods of Fluoridation

Finally, the fluoride in this study was measured and adjusted based on urinary levels, not water fluoridation levels. In fact, community water fluoridation levels were never even collected nor analyzed because there is no reporting system for that information in the study’s location.1 There is no community water fluoridation program in Mexico.

Why would a study intended to evaluate fluoride ingestion even choose a location without a regulated community water fluoridation program? In fact, there was a huge variation in the levels of community water fluoridation, ranging from 0.15-1.38mg/L for those involved in the study, supplemented also with the use of 250 part-per-million fortified salt in addition to that. Fluoridated salt is not even available for use in the United States.

Comparing fluoride levels in a country with unregulated community water fluoridation with the addition of fluoride salts to a country with extremely well-regulated community water fluoridation programs with no salts is comparing apples to oranges.

It may also be useful to mention that participants had roughly 25% more blood mercury levels than those excluded from the study — not to mention that many in the study group also had elevated levels of lead exposure. Lead has long been linked to cognitive impairment, along with mercury.7-8

To top it all off, the variations in IQ status reported in this study, even at those elevated levels of fluoride, were a mere 2.5 points. The Journal of Psychoeducational Assessment indicates that re-test stability among the same child taking the same test more than once over the span of 12-88 days can vary beyond 2.5 points.9

In conclusion, don’t believe everything you read and please take the variety of confounding factors of any study into consideration before you believe the headline. Actually read it. If you don’t understand it, bring it to your health professional and discuss it with them. We are here for you. We know what p-values are, and we are advocates in your health just as much as you are.

The beauty of science is that it is ever-changing. With reliable, factual new studies changing our views of what we have always accepted to be true, we are completely open to evolving along with them—this study simply isn’t one of them.

Now Listen to the Today’s RDH Dental Hygiene Podcast Below:


  1. Bashash M., Thomas D., Hu H., et al. Prenatal Fluoride Exposure and Cognitive Outcomes in Children at 4 and 6-12 Years of age in Mexico. Environmental Health Perspectives. 2017 Sep; 125(9): 097017.
  2. NIH U.S. National Library of Medicine. History, Policies, and Laws. 2008 Declaration of Helsinki Revision Promotes Trial Registration and Results Dissemination. 2019 Aug.
  3. Cowley J., Skene A., Stainer K., et al. The effect of lorcainide on arrhythmias and survival in patients with acute myocardial infarction: an example of publication bias. International Journal of Cardiology. 1993; 40:161-166.
  4. Hanscombe B., Trzaskowski M., Hayworth C., et al. Socioeconomic Status (SES) and Children’s Intelligence (IQ): In a UK-Representative Sample SES Moderates the Environmental, Not Genetic, Effect on IQ. PLoS One. 2012; 7(2): e30320.
  5. Cortese P., Olin A., O’Riordan K., et al. Environmental Enrichment Improves Hippocampal Function in Aged Rats by Enhancing Learning and Memory, LTP and mGluR5-Homer1c Activity. Neurobiol Aging. 2018 Mar; 63: 1–11.
  6. U.S. Department of Health and Human Services. HHS Recommendation for Fluoride Concentration in Drinking Water for Prevention of Dental Caries. 2015.
  7. Hou S., Yuan L., Jin P., et al. A clinical study of the effects of lead poisoning on the intelligence and neurobehavioral abilities of children. Theor Biol Med Model. 2013; 10: 13.
  8. Azevedo B., Furieri L., Pecanha F., et al. Toxic Effects of Mercury on the Cardiovascular and Central Nervous Systems. J Biomed Biotechnol. 2012; 949048.
  9. Review of the Wechsler Abbreviated Scale of Intelligence, Second Edition (WASI-II).  Journal of Psychoeducational Assessment. 2012 May; 31(3):337-341.
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Jenna Arcaro, RDH
Jenna Arcaro, RDH, has practiced in both pediatric and general private practices since graduating from Northampton Community College in 2013. Dubbed the office Perio Queen, she embraces teaching, from peer tutoring in college to fostering high school intern programs, all the way to developing continuing education courses. A competitive runner and wildly unsuccessful fitness influencer, she enjoys hiking with her chubby Labrador Retriever and settling in with a Freida McFadden novel. The author of three mystery novels herself, much of her time has been spent writing, achieving publication in many directions, from dentistry to psychology.