Fluoride Data Deep Dive
In August 2024, the National Toxicology Program (NTP), part of the US Department of Health and Human Services, published a report on the “State of the Science Concerning Fluoride Exposure and Neurodevelopment and Cognition: A Systematic Review”:
“Seventy-two studies assessed the association between fluoride exposure and IQ in children. Nineteen of those studies were considered to be high quality; of these, 18 reported an inverse association between estimated fluoride exposure and IQ in children.
This review finds, with moderate confidence, that higher estimated fluoride exposures (e.g., as in approximations of exposure such as drinking water fluoride concentrations that exceed the World Health Organization Guidelines for Drinking-water Quality of 1.5 mg/L of fluoride) are consistently associated with lower IQ in children.”
It doesn’t get more mainstream than the US Department of Health and Human Services - they are also extremely rigorous in their methodology.
The report caveats their conclusion with “More studies are needed to fully understand the potential for lower fluoride exposure to affect children’s IQ.” In the US, the target level of our water fluoride programs is 0.7 mg/L, so under the 1.5 threshold they use to define “high exposure” in their results. While there are ~1.9 million Americans living in areas with 1.5 mg/L, the vast majority are in areas closer to the 0.7 target.
However, upon digging deeper into the included studies and others, I think the risk of lower IQ is still relevant to the broader US context for a number of reasons, each of which I’ll go on to explore in more detail:
The report includes three compelling studies from Canada and one from Mexico where the “high fluoride water” group has levels comparable to our fluoride levels. There are additional studies from these regions that find adverse cognitive effects but use measures other than IQ, so weren’t included in the report’s primary conclusion.
Even among the report’s studies in areas with much higher fluoride than ours, the effect seems to be “dose-dependent” meaning more fluoride is associated with greater IQ deficit, rather than it just being some magic threshold. This suggests to me that less fluoride is probably safer here in the US too, balancing the dental concerns.
In addition to fluoridated water, another major source of fluoride is toothpaste. Research shows that parents consistently put too much toothpaste on their children’s brush, and young children swallow the majority of it. If you calculate the amount ingested in daily brushings, this could bring children above the report’s threshold.
The NPT report itself says: “Additional exposures to fluoride from other sources would increase total fluoride exposure. The moderate confidence conclusions may also be relevant to people living in optimally fluoridated areas of the United States depending on the extent of their additional exposures to fluoride from sources other than drinking water.”
#1: Summary of studies in Canada and Mexico, with similar fluoride levels to the US
These studies all use the “cohort design” methodology where they follow the same group of people over different periods of time. This makes the findings even more compelling than the “cross-sectional” studies, which just compare groups at a snapshot in time.
Green 2019: “Association Between Maternal Fluoride Exposure During Pregnancy and IQ Scores in Offspring in Canada” (source)
It is well established that fluoride readily crosses into the placenta (at around 60% transfer)
This study looked at prenatal fluoride exposure by assessing mothers’ self-reported water/beverage intake (green and black teas are very high in fluoride) and their urinary fluoride measured across all 3 trimesters. IQ scores of the children were later measured at 3-4 years old.
The participants came from fluoridated and non-fluoridated Canadian cities - where the average levels were 0.6 mg/L and 0.1mg/L respectively - so very comparable to American fluoridated (0.7 mg/L) and non-fluoridated levels. They controlled for a bunch of important maternal and child characteristics, like education level.
Looking at water/beverage consumption, a 1mg increase in fluoride consumed during pregnancy was associated with a decrease in IQ score of 3.66 points
While the results were statistically significant, there was a wide confidence interval for this finding, meaning there was lots of variation
Looking at maternal urinary fluoride, a 1 mg/L increase was associated with a 4.49-point IQ decrease for boys only
Not all the studies on this topic find this gender difference, but there are a few more that do, for instance:
In this study in Mexico, there was a relationship between prenatal fluoride exposure and toddler’s performance on other cognitive tests for boys only
This study on rats also found that male rats’ brains were more sensitive to prenatal fluoride exposure, whereas females’ were more sensitive postpartum
While the mechanism isn’t well understood, evidence from studies in other areas suggests the developing male brain is more sensitive to environmental changes in utero than female’s, for instance, mother’s obesity levels (study with humans) and stress levels (study on rats) on male offspring’s hippocampus and memory.
Till 2020: “Fluoride exposure from infant formula and child IQ in a Canadian birth cohort” (source)
This study used the same cohorts as the previous one (with Canadian fluoridated water systems that are equivalent to ours in the US), but looked at postnatal exposure from formula
After controlling for prenatal fluoride exposure, they found a 0.5 mg increase in fluoride intake from formula was associated with a 7.6 point lower score on the “Performance” portion of the IQ test
Performance IQ measures non-verbal reasoning and visual-motor coordination skills (as opposed to Verbal IQ)
This finding that non-verbal cognition is more impacted than verbal appears in a few other studies at well
The skills needed for these different forms of intelligence involve different brain structures, so it’s likely that some parts of the brain are more sensitive to fluoride than others
For example, a study with rats found that the hippocampus (essential for memory) is especially vulnerable to high levels of fluoride
As you read in my “Recommendations” section, I think it’s a no-brainer to avoid fluoridated water when preparing infant formula, especially since you aren’t balancing the risk against any dental benefit at this stage
Farmus 2021 “Critical windows of fluoride neurotoxicity in Canadian children” (study)
This paper, again with the same cohort of participants, brings it all together by directly comparing the association between fluoride and performance IQ (PIQ) across three different exposure windows (prenatal, infancy, and childhood) and across sexes.
Overall, the strongest association between fluoride and PIQ was prenatal, followed by a significant association during infancy, and a weaker association during childhood
This makes sense when you think about brain development: the “blood-brain barrier” - which does a good job protecting the adult brain from various toxic agents - is less developed and effective in a fetus, newborn and young child, in that order (source)
When looking by sex, the most critical period for boys appears to be prenatal whereas for girls it’s infancy
Just as in the previous paper, the authors cite numerous animal and human studies that suggest boys and girls respond differently to neurotoxicants, and their development of specific brain regions differs
Bahash 2017: “Prenatal Fluoride Exposure and Cognitive Outcomes in Children at 4 and 6–12 Years of Age in Mexico” (study)
Followed pregnant women and their children in Mexico city, where the water fluoride levels range from 0.15 to 1.38 mg/L (so all under the 1.5 level in the NTP report)
Controlled for maternal characteristics like age and IQ, and for children’s characteristics including their own urinary fluoride levels, to isolate the impact of prenatal exposure
An increase of 0.5 mg/L of maternal urinary fluoride predicted a 2.5-point lower IQ score in children
It’s important to note that this association didn’t hold under 0.8 mg/L. However, that number is close enough to our target US level of 0.7 mg/L that it still gives me pause, especially when you consider:
0.7 is the target but there is variation, for instance San Francisco tap water ranges from 0.2 to 0.9 (source)
We consume fluoride from other sources in addition to tap water: for instance, green and black tea has a high fluoride content. A Danish study found that a third of popular tea bags and a similar proportion of loose black tea had fluoride levels above 1.5 mg/L, even when brewed with fluoride-free water (source)
Note: the IQ differences across these studies may sound small, but remember how the IQ scale works:
| IQ Range | Category |
|---|---|
| 130 and above | Very Superior (Gifted) |
| 120-129 | Superior |
| 110-119 | High Average |
| 90-109 | Average |
| 80-89 | Low Average |
| 70-79 | Borderline |
| 69 and below | Extremely Low (Intellectual Disability) |
The NTP report also touches on studies that use cognitive measures outside of IQ, such as ADHD symptoms, toddler’s performance on other cognitive assessments, etc.
In 8 of the 9 studies deemed high-quality, there was evidence of significant associations between fluoride exposure and cognitive performance
A number of these studies involved “high” fluoride levels that are similar to those in the US
For instance, Riddell 2019 “Association of water fluoride and urinary fluoride concentrations with attention deficit hyperactivity disorder in Canadian youth” which found that higher fluoride levels were associated with more hyperactivity, inattention and ADHD diagnosis (study)
In this study, the mean water fluoride level in the “fluoridated” region was only 0.49 mg/L, so even lower than the US target of 0.7 mg/L
#2: Impact of fluoride on IQ appears to be dose-dependent
Many of the cross-sectional studies in the NTP report reveal a “dose dependent” effect - as you go up in fluoride concentration, the IQ impact also goes up, rather than it being a binary effect.
This is observed both at the group level (i.e. comparing populations across 4 different water fluoride levels) and the individual level (i.e. comparing the full range of urinary fluoride levels)
While most of these studies take place in areas where the fluoride levels are far beyond anything we’d see in the US (these are naturally high fluoride regions in India, China and Iran), it seems reasonable that the trend would continue downward too
In fact, one of the cross-sectional studies in India compared IQ among children from four different fluoride-level regions, including a 0.7-1.2 mg/L region and a <0.6 mg/L regions, which is more relevant to the US.
They found a small but significant IQ difference comparing those two lower fluoride regions, suggesting the effect may hold even at lower concentrations (source)
The authors of the NTP report also conducted a separate meta analysis on this topic of “dose-dependence”, which is “in press” so should be available soon. But the findings are mentioned in the current report and include a significant dose-response relationship at both the group and individual level.
When it comes to establishing that this relationship remains significant at lower levels of fluoride, sample size may be a limitation in some studies - in statistics, as differences get smaller, you need a larger sample size to power the effect.
#3: Fluoride toothpaste may result in overexposure for young children
In addition to consuming fluoridated water, the official stance from dentists is that as soon as the first tooth erupts, children should also be brushing teeth with fluoride toothpaste twice daily, initially with a “tiny smear” and later a “pea size” around age 3
However, research has shown that parents significantly overdose the amount of toothpaste (by a factor of 6-7x) (source) and young children swallow most of the toothpaste (especially when it comes in yummy flavors!) (source)
These studies are concerned with fluorosis risk - the staining/mottling of teeth. This is only an aesthetic issue and far less important to me than the potential cognitive impacts.
Given this reality, one of the above studies estimates how much fluoride a young child may really be ingesting:
Mean amount actually dosed by parents = 263 mg (compared to the 45 mg “reference dose” done by dentists)
Assuming twice daily brushing and 100% swallowing, 526 mg of toothpaste consumed
The standard fluoride concentration in toothpaste is 1000 ppm or 0.1%, so the child would be ingesting 0.526 mg/day
This is much higher than 0.1 - 0.25 mg estimate for children 0-5 years old given by the NIH
This inspired me to do my own calculation to answer the question: if we take into account toothpaste swallowed, along with water consumption, might US children actually be consuming fluoride closer to the 1.5 mg/L level of the NTP report?
I’ve made a few tweaks for my toothpaste model:
A “smear” is supposed to be ~100 mg; let’s be conservative and say parents are overdosing by 4x (rather than the 6-7x vs. the dentist’s “reference dose” in the study)
To be more realistic, I’ll assume only 1 daily brushing (right?!?!)
To be more conservative, I’ll assume only 75% is swallowed, which was the mean for 1.5-2.5 year olds in a study that measured this issue directly
Putting those numbers together, we get 400mg x 75% = 300 mg of toothpaste swallowed
Given the fluoride concentration of 0.1%, that is 0.3 mg of fluoride per day from toothpaste
Let’s add in the consumption via fluoridated water
Children are supposed to drink about the same number of 8 oz cups of water as their age in years, so a 2 year old would drink 16 oz, which converts to 0.47 L
0.47 L x 0.7 mg/L F (average SF fluorination levels) = 0.33 mg of fluoride per day from water
Total is thus 0.3 + 0.33 = 0.63 mg of daily fluoride
The threshold at which the report’s findings officially apply is 1.5 mg/L. If we convert this to the amount a 2 year old would consume daily, that’s 0.47 L x 1.5 mg/L = 0.7 mg of fluoride per day
So using fairly conservative measures, the true fluoride exposure for a 2 year old - incorporating both water and toothpaste - may be much closer to the report’s threshold where adverse cognitive effects occur (0.63 vs. 0.7 daily mg of fluoride)
I didn’t attempt to incorporate fluoride from foods, but some foods that are popular with children (raisins, oatmeal, potatoes, etc) have around 0.08mg fluoride per serving, so could bring the total up (source)
But what is the minimum level of daily fluoride needed to obtain the dental benefits?
Official sources say 0.05 mg per kg of bodyweight
So returning to our 2 year old, if she is an average weight of 12 kg (26.5 lbs), that would be 0.6 mg of fluoride - which is exactly our estimated total with both fluoride toothpaste and fluoride water
This puts us in a bit of a conundrum: as this report from the Scientific Committee on Health and Environmental Risks of the European Commission puts it, “The emerging picture from all risk assessments conducted on fluoride is that there exists a narrow margin between the recommended intakes for the prevention of dental caries and the upper limits of exposure.”
A few things ease my concerns on the dental front, though:
The 0.05 mg per kg figure is very much a subject of debate. Studies actually looking at fluoride intake and dental outcomes find wide variation, with a good percentage of children consuming less than that amount, yet having no cavities (source)
Official recommended amounts tend to be cast as a safety net to ensure that the majority is covered. So if you believe your family has a healthier diet compared to the average American (consider sugary foods and drinks), you can probably get away with less
Finally, on a more personal note, if I had to choose a little more risk of cavities or a little more risk on my child’s brain, I would take the cavity risk
But there is a strong genetic factor when it comes to cavity risk, so keep in mind you and your partner’s dental history when making your own judgment call (source)
