[Este artículo está disponible en español. La traducción al español apareció por primera vez en la revista Pensar.]
The Centers for Disease Control and Prevention named community water fluoridation one of 10 great public health achievements of the 20th century. In a recent statement, the American Dental Association (ADA) stressed that it is “committed to fluoridation of public water supplies as the single most effective public health measure to help prevent tooth decay” and pointed out that “this commitment is shared by many national and international organizations, including the World Health Organization, US Public Health Service, the Centers for Disease Control and Prevention, the American Medical Association and the American Academy of Pediatrics.” Nevertheless, many people continue to strongly oppose water fluoridation. Several countries have banned fluoridation, preferring other methods of providing fluoride. Opponents are quick to capitalize on any reports of harm. When a recent study in Canada reported that prenatal exposure to water fluoridation was associated with decreased IQs in children, the media were all over it, stressing the danger. Critical thinkers will want to avoid emotion-filled debate and instead will carefully evaluate the details of the evidence. What is the reality behind the fears and the hype?
Objections to fluoridation
There are many objections to fluoridation. Some of the most common ones:
- Fluoride causes fluorosis, mainly at higher doses
- Additional sources of fluoride (toothpaste, etc.) can lead to higher doses than intended
- Water fluoridation forces medication on everyone, not just those who will benefit
- Other ways of providing fluoride are equally effective
- A Cochrane systematic review found small increases in caries-free children (15% for baby teeth and 14% for permanent teeth) and found that at 0.7 ppm 12% of people will have fluorosis of aesthetic concern. But it found that the quality and quantity of research was poor and inadequate:
- Most of the studies (71%) were done before fluoride toothpaste was introduced
- There was a high risk of bias in the studies
- Many studies were of high natural levels of fluoride rather than of fluoridated water
- There was substantial variation in results
- There was inadequate evidence for a benefit in adults
- There was inadequate evidence for the effects of stopping fluoridation
- There is evidence that intrauterine exposure decreases children’s IQ
Benefits of fluoridation
The ADA responded to the Cochrane review. They said more than 70 years of rigorous scientific research analyzing fluoride intake from all sources has consistently shown that:
- an optimal level of fluoride (0.7 ppm) in community water is safe and effective
- plus, it prevents tooth decay by at least 25% in both children and adults.
They offered a booklet of “Fluoride Facts” explaining the evidence.
Studies prior to Canada study
Before the Canadian study captured the attention of the public, studies in several countries had looked for a possible association between fetal exposure to fluoride and measures of children’s intelligence. Studies of laboratory animals had shown that fluoride could be neurotoxic, with effects on learning and memory. And fluoride is known to cross the placenta, so the idea was not implausible; but it needed to be tested in humans. Let’s review some of the results of human studies done before the Canada study.
The 2003 China study
A research report by Q Xiang et al. in 2003 found that higher drinking water fluoride levels from natural sources were significantly associated with higher rates of mental retardation (IQ <70) and borderline intelligence (IQ 70–79).
The 2012 Harvard systematic review
Choi et al. published a systematic review and meta-analysis of studies on fluoride and neurobehavioral development. It found that children in high-fluoride areas had significantly lower IQ scores than those who lived in low-fluoride areas. Its methodology was criticized and the statistical significance was not thought to reflect clinical significance.
The review had major flaws:
- The studies reviewed were not of fluoridation; they were from China and Iran. China does not fluoridate, but the natural water in some areas has a high fluoride content and there is exposure to fluoride in the smoke from coal-burning.
- The high fluoride exposures were much higher than with fluoridation; fluoridation has the same level of fluoride as the low-exposure control groups in China.
- Confounding factors were not considered
- The review actually showed that fluoridation was safe.
Steven Novella, in his Neurologica blog, pointed out “There are many weaknesses to the epidemiological studies reviewed in the recent article – high heterogeneity, poor controlling for other variables, no indication of blinding of IQ assessments, and many others. But even taken at face value they do not indicate any association between lower IQ and the fluoride levels added to drinking water in the US. In fact, those levels of fluoride were used as the controls in these studies showing higher IQ. (There was a lot of variance of the effect size, but the net effect size on IQ in the meta-analysis was -0.45 IQ points)
The 2015 Choi pilot study in China
In a 2015 pilot study by Choi et al. 51 Chinese first-grade children in a community with stable lifetime fluoride exposure found that deficits were associated with increased fluoride exposure. Dental fluorosis score was the exposure indicator that had the strongest association with the outcome deficits, and the WISC-IV digit span subtest appeared to be the most sensitive outcome, where moderate and severe fluorosis was associated with a digit span total score difference of -4.28.
The 2015 New Zealand study
A study by Broadbent et al. in New Zealand was a prospective study of a general population sample of over a thousand subjects that found no difference in IQ from fluoride exposure. It concluded:
“ These findings do not support the assertion that fluoride in the context of CWF (Community Water Fluoridation) programs is neurotoxic. Associations between very high fluoride exposure and low IQ reported in previous studies may have been affected by confounding, particularly by urban or rural status.”
Steven Novella pointed out that there was a trend toward higher IQ in the fluoride exposed group, but this was not statistically significant, so the study was negative. The lack of correlation held up even when they corrected for possible confounders, like socioeconomic status. He said, “This is as good as it gets for an observational study – a nicely controlled environment, large population, good long term follow up.”
A Mexican study in 2017
A 2017 study by researchers from the US, Canada, and Mexico examined prenatal fluoride exposure and cognitive outcomes in Mexican children. The subjects were 229 mother-child pairs. Fluoride was measured in archived urine samples taken from mothers during pregnancy. Children were tested with the General Cognitive Index (GCI) of the McCarthy Scales of Children’s Abilities at age 4 and with full scale intelligence quotient (IQ) from the Wechsler Abbreviated Scale of Intelligence (WASI) at age 6–12. They found that higher levels of maternal urinary fluoride during pregnancy (a proxy for prenatal fluoride exposure) were associated with lower scores on tests of cognitive function in the offspring at ages 4 and 6–12 years. IQ scores were estimated to be 2.5 points lower for every 0.5 increase in prenatal fluoride exposure, but the effect may be limited to higher fluoride exposures.
Another Mexican study in 2017
Another study in Mexico tried to more accurately quantify fluoride exposure. Data from 65 mother-infant pairssuggested that cognitive alterations in children born from mothers exposed to fluoride could start in early prenatal stages of life.
The Canada study
Now let’s examine the new study in Canada that raised so much alarm. It was a 2019 study by Green et al. in JAMA Pediatrics that examined 512 mother-child pairs in Canada. A 1-mg/L increase in maternal urinary fluoride was associated with a 4.49-point lower IQ score in boys, but there was no statistically significant association with IQ scores in girls.
The scientific community pushed back.
A leading Canadian health agency evaluated the article. The Canadian Agency for Drugs and Technologies in Health (CADTH) published a report evaluating the Green study. It concluded:
“The evidence is weak due to multiple limitations (e.g., non-homogeneous distribution of data, potential errors and biases in the estimation of maternal fluoride exposure and in IQ measurement, uncontrolled potential important confounding factors); therefore, the findings of this study should be interpreted with caution.”
- “It’s hard to believe that a drop of 4.49 IQ points in boys was missed by prior research, and why is that effect so large while there is no effect in girls (just a non-statistical trend toward increased IQ).”
- The study is at odds with a large body of prior research showing no association.
- Even if high levels of exposure affect IQ, there is no association at less than 2 ppm, and fluoridated water typically has 1 ppm.
- When outliers were eliminated, the effect became non-significant. Two of the boys had extremely low IQs, in the 50s.
- There was a disconnect: performance IQ decreased but verbal IQ increased
- The unadjusted IQ differences were not statistically significant
- They didn’t look at post-natal lead exposure, which can cause severe decreases in IQ.
- Urine concentrations are variable. They only represent one point in time and have not been validated as a measure of total maternal intake.
- The results are consistent with noise in the data
The study actually supported the safety of water fluoridation. Even one of its authors said that although she thought the study’s results were valid, she continues to support fluoridation programs.
The way the media reported the Canadian IQ study was unfortunate. They raised undue alarm over findings that many scientists have criticized as questionable. Fluoridation is still the best way to reduce the risk of tooth decay for all children, including those from the poorest families and those who never see a dentist. Surely no parent would want to want their children to suffer unnecessarily from dental decay (except possibly a few who are callously profit-motivated and make their living selling dentures?) and surely everyone would agree that preventing cavities is a worthwhile goal. In my opinion, it’s fine for people to reject public water fluoridation, but only if they find another practical way to ensure that all children are provided with fluoride in another form.
The bottom line: High doses of fluoride can be toxic, but the recommended concentration (0.7 mg/L) used in community water fluoridation is not harmful or toxic. No, public water fluoridation is not going to decrease your children’s IQs. You can stop worrying.
This article was originally published in Skeptical Inquirer as a Reality Is the Best Medicine column.
