A new study is being touted as showing no relationship between fluoridated water and IQ. But here’s what you should know.
The study focuses on a population in Wisconsin that was born in 1940 — which is five years before any communities in the U.S. began fluoridating their water. This is an important limitation because the key window of vulnerability for fluoride’s cognitive effects is the early years of life (in utero and infancy) when the brain is undergoing rapid development and the blood brain barrier does not yet adequately protect the baby from toxic chemicals.
Curiously, the authors *claim* they measured the effect of fluoridation “from birth.” This is where it gets interesting.
The authors admit they have no data on water fluoride levels from the 1940s. None. That’s a pretty serious limitation by itself. But it gets worse.
To get around this data gap, the authors relied on a 2020 publication that provides test results for about 2 wells per county in Wisconsin. To use this data, the authors needed to make several big assumptions. This is where it gets bad, real bad.
The authors assumed that if ONE well in a county had “optimal” fluoride levels (0.7 mg/L) then EVERY CHILD in that county was exposed to fluoridated water. This assumption, which is critical to the study, has basically no connection to reality. Why? Because the level of fluoride in a single well tells you virtually nothing about the level of fluoride in other wells, let alone in *all* other wells, let alone in *public water supplies* (which are often derived from surface waters, not well water).
Why does this matter? Because the study’s use of a clearly erroneous measurement of early life exposure to fluoridated water made it virtually impossible for this study to detect an association between early-life fluoride exposure and IQ. It’s like trying to determine if prenatal exposure to Tylenol is associated with autism by determining the Tylenol use of the child’s neighbor rather than the child’s mother.
In an excellent analysis, James Lyons-Weiler calls the study “policy advocacy masquerading as science”. Precisely.
Dig Deeper
FAN’s Chris Neurath shows three main reasons why this study likely could not have been able to detect an effect of F on IQ:
1.) The IQ tests were given to high school students in 1956 when they were 16 years old, so they would have been born in 1940. That is 5 years before the first artificial fluoridation trial in the US had even begun, so not a single person in this dataset had prenatal exposure from fluoridated water, or childhood exposure to fluoridated water from birth to past age 5 years. Almost all prior studies finding neurotoxicity found it from prenatal or early childhood F exposure. By age 5+ fluoride may have little effect on IQ. This study is incapable of assessing whether F is a developmental neurotoxicant.
2.) It measured IQ at an older age (16 y or even older) when it becomes difficult to detect a neurotoxic effect that occurred in utero or early infancy for several reasons even if there had been early life F exposure.
3.) It had only group-level fluoridation exposure information, and not even at a small local level but only at the level of a county in some cases and a school district in other cases. A 1969 US Public Health Service report shows that only about 3% of the population of Wisconsin had drinking water with naturally elevated water F above 0.7 mg/L. Combine that with the fact that there was no artificial fluoridation anywhere in the US for the first 5 years of life for this cohort and we can say that this study is incapable of detecting developmental neurotoxicity from either artificial or naturally occurring F in drinking water.
Limitations 2) and 3) produce greater random measurement error, with the age 16y IQ tests producing greater random (non-differential) error in the IQ score outcome measure and the group-level F exposure producing random error in the F status exposure measure. Combine both of these random errors and there is likely too much “noise” to detect a “signal” from an effect of F on IQ.