• The danger du jour: Fluoride

    Every once in a while chain emails or Facebook posts make their way back into prominence. It happened last year with HPV killing girls. This week, it seems to be Fluoride. A couple friends have sent me this post from last year which features this meta-analysis published two years ago on Fluoride and IQ. I give you the dates so that you know this isn’t “breaking news”. Here’s the abstract:

    Background: Although fluoride may cause neurotoxicity in animal models and acute fluoride poisoning causes neurotoxicity in adults, very little is known of its effects on children’s neurodevelopment.

    Objective: We performed a systematic review and meta-analysis of published studies to investigate the effects of increased fluoride exposure and delayed neurobehavioral development.

    Methods: We searched the MEDLINE, EMBASE, Water Resources Abstracts, and TOXNET databases through 2011 for eligible studies. We also searched the China National Knowledge Infrastructure (CNKI) database, because many studies on fluoride neurotoxicity have been published in Chinese journals only. In total, we identified 27 eligible epidemiological studies with high and reference exposures, end points of IQ scores, or related cognitive function measures with means and variances for the two exposure groups. Using random-effects models, we estimated the standardized mean difference between exposed and reference groups across all studies. We conducted sensitivity analyses restricted to studies using the same outcome assessment and having drinking-water fluoride as the only exposure. We performed the Cochran test for heterogeneity between studies, Begg’s funnel plot, and Egger test to assess publication bias, and conducted meta-regressions to explore sources of variation in mean differences among the studies.

    Results: The standardized weighted mean difference in IQ score between exposed and reference populations was –0.45 (95% confidence interval: –0.56, –0.35) using a random-effects model. Thus, children in high-fluoride areas had significantly lower IQ scores than those who lived in low-fluoride areas. Subgroup and sensitivity analyses also indicated inverse associations, although the substantial heterogeneity did not appear to decrease.

    Conclusions: The results support the possibility of an adverse effect of high fluoride exposure on children’s neurodevelopment. Future research should include detailed individual-level information on prenatal exposure, neurobehavioral performance, and covariates for adjustment.

    So if we constrain ourselves for a second just to this publication, we can see that the researchers wanted to look at studies examining a relationship between fluoride exposure and delayed neurobehavioral development. They found 27 epidemiological studies. They found that when they combined the results, those who were exposed to high levels of fluoride has an IQ which was lower by 0.45 points. It was statistically significant.

    Now, let’s discuss. Of the 27 studies, 25 appear to be from China. The other two are from Iran. I bring this up not to disparage those countries’ abilities to do research, but to point out that these countries have very different background levels of fluoride than we see in the US. In some of these studies, fluoride levels reached 11.5 mg/L, compared to NYC, which shoots for 0.7 to 1.2 mg/L. Moreover, some of the kids got their fluoride from inhaling it from coal burning, or because it was a pollutant. That’s… not the same as fluoridation here.

    These were not RCTs, but epidemiologic studies. Lots of other factors could be contributing. Schooling, arsenic, iodine, even lead (which often wasn’t measured) could be in play here. We can’t say which of these things is “causing” the result.

    And what does a difference in 0.45 in IQ scores mean? It’s really small. In fact, as the researchers point out in the paper, it may be within the measurement error of the test itself.

    This isn’t the first time people have gotten up in arms about Fluoride. The Fluoride Action Network, an organization dedicated to educating people about the dangers of fluoride, offers many arguments against the policy of adding fluoride to water supplies.  Among their claims, they note that fluoride has minimal benefit when it is swallowed and that fluoride is not recommended for babies.  They propose that fluoride has many risks, including those to the brain, thyroid gland, and bones.  Others make much more alarmist statements, calling fluoride a “corrosive poison.”   Websites and publications raising concerns about fluoride offer “scientific references” and “expert quotes” that really could cause concern that the fluoride in your water not only doesn’t help you, but that it could be hurting you.  Let’s review the data.

    First of all, overwhelming evidence has existed for over 60 years that proves the efficacy of water fluoridation.  Some of the best studies come from the early days of fluoridation because it was easier then to find groups of people who were not exposed to any fluoride sources.  In a 15 year landmark study from Grand Rapids, Michigan, children who received fluoridated water from birth had 50-63% less tooth decay than children from a nearby city in Michigan who drank non-fluoridated water.  A comparison study in Newburgh, New York found that 6- to 9-year-olds had 58% less tooth decay than kids in a nearby city with fluoride-deficient water 10 years later, and they had 70% less decay 15 years later.  A huge survey in the United States of almost 40,000 school children found that tooth decay rates were declining overall (probably because of higher quality toothpastes, mouthwashes, and supplements that all have fluoride), but even when scientists controlled for all of those other things the kids might be exposed to, kids who had fluoride in their drinking water still had 25% less tooth decay.

    The evidence for how well fluoride works is massive.  A compilation of the results of 113 studies in 23 countries showed reductions in tooth decay for both baby teeth and adult teeth.  People with fluoride in their water had 40-49% less tooth decay in baby teeth and 50-59% less tooth decay in adult teeth. Another compilation of studies conducted between 1976 through 1987, showed reductions in tooth decay between 15-60%, with the highest benefit for those with baby teeth, but significant benefits for adults who already have their permanent teeth.  Yet another systematic review of the literature analyzed 214 studies to determine whether fluoride in drinking water was effective.  This systematic review found that when you combine all 214 of those studies, children who got fluoride in their drinking water had fewer teeth with cavities and were more likely to have no cavities at all compared to the children who did not have fluoride in their drinking water.  The 350 peer-reviewed references compiled by the American Dental Association in their publication, Fluoride Facts, also support how well fluoride works.

    Of course, preventing cavities might not be worth endangering your health in other ways. Some people are concerned about water fluoridation leading to cancer, thyroid problems, neurological problems, heart disease, and other toxicities.  Is fluoride really safe?  Once again, the scientific evidence overwhelmingly supports the safety of adding fluoride to a community water supply.  Reviews of the safety of fluoride by the Institute of Medicine, Food and Nutrition Board, the National Research Council of the United States, the U.S. Department of Health and Human Services, Public Health Service, and the World Health Organization have all led to the conclusion that fluoride supplementation is safe, effective, and recommended for community water supplies.  A 2000 systematic review, published in the BMJ, analyzed 214 studies on fluoridation and found no evidence of potential adverse effects except for dental fluorosis (which I’ll get to in a moment). Periodic reviews every six years by the U.S. Environmental Protection Agency (EPA) continue to find no harmful effects related to fluoride in drinking water. Good scientific studies demonstrate that drinking fluoridated drinking water does not increase the risk of hip fractures.  More than 50 extremely large studies do not show any association between fluoridation and the risk of cancer. One small study from the 1950s in 15 patients with an overly active thyroid (hyperthyroidism) tried to use large amounts of fluoride as a treatment for hyperthyroidism and found it seemed to help some patients.  On that basis, concerns have been raised about whether fluoride in drinking water adversely affects the thyroid gland.  Again, much better science says that the answer is no.  Studies from people with drinking water with naturally high levels of fluoride found that it had no effect on their thyroid gland size or function, and this matches results from animal studies.  Furthermore, two studies have found no association between the level of fluoride in water and thyroid cancer.

    The National Research Council of the National Academy of Sciences supports the conclusion that drinking optimally fluoridated water is not a genetic hazard.  There is no known association between the drinking fluoridated water and Downs Syndrome.  One psychiatrist in the 1950s published two studies claiming that the two were connected, but four subsequent studies have found no connection and experienced researchers have noted significant problems with how that psychiatrist analyzed his data. There is also no generally accepted scientific evidence establishing a link between fluoridated water and other neurological disorders, including attention deficit disorder.

    One study, in which rats were administered fluoride at 125 times the level in community fluoridated water, concluded that the rats showed some behavioral changes. However, this study did not use any sort of control group to compare the rats who had the fluoride to rats that did not, and scientists who reviewed the results of this study have concluded that it is significantly flawed and cannot be used to draw conclusions about problems with fluoridated water.  Moreover, a seven year long study in human children from birth to age six found no effects from the fluoridated water on the children’s health or behavior, using both mothers’ ratings and teachers’ ratings of the children’s behavior.

    There is one real problem that can result from too much fluoride – dental fluorosis.  This is a discoloration of the teeth that can occur when a child ingests more fluoride than is recommended.  With mild dental fluorosis, the teeth get white flecks or spots, but with severe fluorosis the teeth can get a permanent brown stain.  About ten percent of the mild fluorosis seen in children does probably come from the fluoridation of water (although the dentists argue that these small white flecks are a small price to pay for avoiding cavities, tooth decay, missed school, and so on).  The biggest problem with fluorosis is that kids do sometimes have a habit of swallowing their toothpaste.  Fluorosis is the reason that the American Dental Association recommends that children under six do not use more than a pea-sized amount of toothpaste.  We assume that little kids are going to swallow their toothpaste, and if they swallow a lot more than the pea-sized amount, they are more likely to get fluorosis. Toothpaste delivers a much more concentrated amount of fluoride than the drinking water does. So, the biggest cause of fluorosis is likely swallowed toothpaste, not the water.

    Many experts have concluded that fluoride is both beneficial and safe in the drinking water of our communities, and they agree that the risk of fluorosis is far outweighed by the benefits of preventing tooth decay.  The American Dental Association (ADA), the Centers for Disease Control and Prevention (CDC), the American Medical Association (AMA), and the U.S. Surgeon General have all issued statements supporting how well adding fluoride to water works to prevent tooth decay.  In fact, the CDC declared fluoridation of public drinking water to be one of the 10 biggest public health achievements of the 20th century.

    Too much of anything can be bad, including Fluoride. I don’t dispute that. But we shouldn’t overlook the fact that dental caries are still the most common chronic disease in kids age 6-19. Let’s not go backwards. Ignore this chain email.


    Some of this was adapted from our first book, Don’t Swallow Your Gum, copyright © 2009 by the authors and reprinted by permission of St. Martin’s Griffin. If you like this kind of stuff, go buy a copy. References for much of the above include the following:

    1. American Dental Association.  Fluoride & Fluoridation: Fluoridation Facts Introduction. (Accessed 16 May 2008, at .
    2. CDC. Achievements in public health, 1900-1999: fluoridation of drinking water to prevent dental caries. JAMA 2000;283:1283-6.
    3. Horowitz HS. The effectiveness of community water fluoridation in the United States. J Public Health Dent 1996;56:253-8.
    4. Knox EG. Fluoridation of water and cancer: a review of the epidemiological evidence: report of the working party. London: H.M.S.O.; 1985.
    5. McDonagh MS, Whiting PF, Wilson PM, et al. Systematic review of water fluoridation. BMJ 2000;321:855-9.
    6. Murray JJ. Efficacy of preventive agents for dental caries. Systemic fluorides: water fluoridation. Caries Res 1993;27 Suppl 1:2-8.
    7. National Research Council (U.S) Subcommittee on Health Effects of Ingested Fluoride, Wagner BM, National Research Council (U.S) Board on Environmental Studies and Toxicology, National Research Council (U.S).Commission on Life Sciences. Health effects of ingested fluoride. Washington, D.C.: National Academy Press; 1993.
    8. Newbrun E. Effectiveness of water fluoridation. J Public Health Dent 1989;49:279-89.
    9. Ripa LW. A half-century of community water fluoridation in the United States: review and commentary. J Public Health Dent 1993;53:17-44.
    10. Ten great public health achievements–United States, 1900-1999. MMWR Morb Mortal Wkly Rep 1999;48:241-3.

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