The Evidence Behind the Risk of Fluoride Use
By: Margherita Fontana, DDS, PhD; Carlow Gonzalez-Cabezas, DDS, MSD, PhD; Livia Tenuta, DDS, MSc, PhD
Reprinted, with permission, from the Journal of the Michigan Dental Association, December 2024 issue.
In the last several months, the debate around use of fluoride for caries control has reignited, fueled by a government report and a court ruling directing the EPA to issue new rules about fluoride levels in tap water. Here is an examination of the evidence and why continued support for the use of fluoride is indicated.
For example, the majority of the higher quality studies included in the report (19 in total) were conducted in areas of endemic fluorosis in China, India, Iran, and Mexico, where the population is exposed to naturally high levels of F in the water. Their results point to an association between higher exposure to F [more than 1.5 ppm (milligrams per liter)] and lower IQ in children. The NTP report rightfully treated this association with concern. However, it is important to understand that association does not mean causation; for example, there may be other unknown contaminants in the water that could be responsible for the effects on IQ, which at this time remain unidentified in these studies. The World Health Organization (WHO) has set a safe limit for F in drinking water of 1.5 ppm to prevent the development of moderate/severe fluorosis. Yet, the vast majority of the evidence on the association between IQ and F comes from communities where living conditions do not allow for the implementation of the WHO recommendation.
In an effort to understand how F might affect IQ, the NTP investigated mechanistic studies, most of which explored the potential impact of F on thyroid hormones, and subsequent neurodevelopment. The NTP concluded that the data does not support this mechanism. No other mechanisms have been clearly identified. Therefore, at this stage, the data available on F and IQ remains based on association rather than causation, without a demonstrated plausible biological mechanism.
In addition, the NTP found no evidence that F exposure adversely affects adult cognition, nor that the low F level of 0.7 ppm, currently recommended for U.S. community water fluoridation programs, negatively impacts children’s IQ, even when considering studies which were conducted in artificially fluoridated areas in Canada.9,10
Given their potential impact on the use of water fluoridation as a public health measure, these studies included in the NTP report have undergone extensive scrutiny by the scientific community. In fact, a detailed assessment of the limitations of the methods and outcomes used to measure F exposure (e.g., F in urine) and IQ (e.g., tests performed by different assessors in fluoridated and non-fluoridated areas, potentially affecting outcomes due to inter-assessor variability) has been recently published.11 These methodological design limitations, present in the majority of the studies published on this topic, highlight that the current level of evidence is largely based on associations, which fall short of the rigor preferred by scientists to guide public health policies.
Furthermore, other important longitudinal studies have failed to demonstrate an association between F use via fluoridated water and IQ. For example, a longitudinal study conducted in New Zealand where subjects were followed over 38 years showed that the IQ scores of teens and adults living in fluoridated communities during infancy and childhood were no different than the scores of people who lived in communities without fluoridated water.12 A different longitudinal study in Australia showed that exposure to fluoridated water during the first five years of life was not associated with long-term altered measures of child emotional and behavioral development or executive functioning.13 These specific studies focusing on IQ and intelligence add to the extensive body of evidence, which shows that, apart from dental fluorosis, no other health concerns are linked with chronic F use at recommended levels.
The U.S. Public Health Service currently recommends 0.7 ppm F, and as mentioned above, the WHO recommends drinking water to have a maximum of 1.5 ppm F. A 2020 critical review of the evidence did not support the presumption that F should be assessed as a human developmental neurotoxicant at the current exposure levels in Europe.14 The International Association for Dental Research (IADR) reviewed its position statement on community water fluoridation in 2022, considering the neurotoxicity risks, and decided to continue endorsing water fluoridation.15 Thus, while additional research to better understand the association at very high levels of chronic exposure and potential biologic mechanisms is important, there is currently no evidence to suggest concern with the strategies we currently use.
As a response to the NTP report, numerous commentaries in support of water fluoridation as currently provided in the United States have been written by respected groups such as the American Dental Association,16 the American Academy of Pediatrics,17 the American Association for Dental, Oral and Craniofacial Research,18 the American Fluoridation Society,19 and the American Water Works Association,20 among others. Also, the National Institute of Dental and Craniofacial Research has recently sponsored an excellent free webinar reviewing the science behind water fluoridation (available at https://www.nidcr.nih.gov/news-events/events/2024/puttingprevention-action-community-water-fluoridation-webinar#recording).
As clinicians, it is of upmost important we always consider the evidence supporting risks and benefits of strategies we use to care for our patients and communities. The NTP report, as well as all existing evidence to date, strongly supports the safety and efficacy of F for caries control as currently used and recommended in the United States.
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Margherita Fontana, DDS, PhD, is the Clifford Nelson Endowed Professor of Dentistry and chair-elect of the Department of Cariology, Restorative Sciences and Endodontics at University of Michigan School of Dentistry in Ann Arbor, MI. She serves as director of the Global Initiatives Program in Oral and Craniofacial Health at the School of Dentistry. She has more than 180 papers published in peer reviewed journals, such as the Journal of Dental Research, Caries Research, Journal of Dental Education, Pediatric Dentistry, Advances in Dental Research, Journal of Public Health Dentistry, Journal of the American Dental Association, and the British Dental Journal. She is the 2023-25 president of the Organization for Caries Research (ORCA).
Carlos González-Cabezas, DDS, MSD, PhD, is the Richard Christiansen Collegiate Professor of Oral and Craniofacial Global Initiatives at the Department of Cariology, Restorative Sciences and Endodontics at University of Michigan School of Dentistry. He is the school’s associate dean for Academic affairs and has directed or co-directed several pre-doctoral and graduate courses in cariology. He has particular research interest in professionally applied anticaries products and translational caries models.
Livia Tenuta, DDS, MSc, PhD, is an associate professor at the Department of Cariology, Restorative Sciences and Endodontics at the University of Michigan School of Dentistry. Tenuta has been involved in leadership roles in the International Association for Dental Research, serving as president of the IADR Cariology Group, the Organization for Caries Research (ORCA), and the American Academy of Cariology. She is currently the editor-in-chief of the scientific journal Caries Research. She has published more than 100 peer-reviewed articles on dental biofilm cariogenicity and the role of fluoride in caries prevention.