Evidence-based water fluoridation policy

Abstract

A national-scale fluoridation study addresses policy-relevant exposure levels and provides evidence that adverse neurodevelopmental effects do not result from municipal fluoridation.

HISTORY OF FLUORIDATION

Water fluoridation has once again become a hot-button issue. Polarized political debate surrounds the topic with each side claiming that they are protecting public health. Back in the 1950s, consensus was reached regarding the health benefits of water fluoridation for preventing dental caries (1). However, the current challenge focuses on the claim that fluoride produces adverse neurodevelopmental health effects and that fluoridation no longer provides dental health benefits. The dental public health community and the Centers for Disease Control and Prevention have forcefully promoted fluoridating drinking water supplies to benefit the entire population, most notably for those of limited economic means who may have limited access to dental care. However, a new set of critics has emerged, an odd alliance of environmental health advocates who believe that fluoride has adverse neurodevelopmental effects and “MAHA” proponents who distrust the public health establishment and almost any governmental action forced on them.

The early history of fluoridation opposition provides a notable parallel to the current battle. Fluoridation was once viewed as a communist plot to poison the population and take over the government, later as a cause of cancer and AIDS, and more broadly as an infringement of freedom of choice (2). Often the antifluoridationists sought to downplay the importance of dental caries as a health problem as well. Since decisions about fluoridation are made one community at a time, there were (and currently are) heated local political battles over this seemingly simple public health measure, most recently in Miami, where Florida’s surgeon general is opposed to fluoridation and the Miami-Dade County Commission voted to end the practice but the mayor of Miami refused to do so.

Putting aside politics and ideology, two critical scientific questions should drive decisions about water fluoridation: (i) Is water fluoridation a necessary and effective means of preventing dental caries? (ii) Are there adverse neurodevelopmental effects of fluoride at the levels recommended for fluoridation—currently 0.7 mg/liter?

Water fluoridation is widespread in the United States and Australia (Fig. 1) and quite rare in Europe, for example. International patterns of water fluoridation provide a fascinating and inexplicable global pattern, widespread in Singapore, Gabon, Malasia, and Ireland and virtually absent in Mexico, Thailand, and France (https://worldpopulationreview.com/country-rankings/water-fluoridation-by-country#:~:text=The%20three%20countries%20with%20the,constitutes%20only%20275%20thousand%20people). In the modern era of fluoride-containing toothpaste and more effective and widely available dental care, fluoridation may not have the same benefit that it did in the 1950s and 1960s. If no clear public health benefit exists, then why add fluoride to drinking water?

Fig. 1. A new study addresses exposure levels and human impacts of municipal fluoridation.

Credit: herstockart / iStock.

WHY THE CONCERN?

In recent years, the question that has received a great deal of attention from researchers and increasingly from the public is whether fluoride at the levels used to prevent dental caries has adverse neurodevelopmental effects resulting from exposure during pregnancy and infancy. A National Toxicology Program report included a meta-analysis of a sizable number of studies in locations with high levels of naturally occurring fluoride, largely in rural areas of China and India. The report documented an association between fluoride concentration and reduced cognitive ability as reflected in IQ scores (3). However, the levels of exposure studied, the settings in which these studies were conducted, and the methodological deficiencies of many of the studies call into question both the validity and applicability of those studies to fluoridation policy in the United States.

The currently recommended fluoride level of 0.7 mg/liter is intended to provide the desired benefit without risk of dental fluorosis, a permanent discoloration of the teeth. Excess fluoride is recognized as harmful, but the question is whether the levels below those that cause dental fluorosis have other adverse effects. This question motivates the study by Warren et al. (4) in this issue of Science Advances and the lens through which it should be evaluated.

In addition to the meta-analysis reporting an association between fluoride and reduced IQ in children (3), several other studies have addressed the association between urinary fluoride levels during pregnancy and IQ in the offspring. A series of high-quality cohort studies of very similar design in Mexico (5), Canada (6), Spain (7), and Denmark (8) examined urinary fluoride and children’s IQ, generating an array of notably inconsistent findings.

The study in Mexico found clear associations between maternal urinary fluoride and small reductions in IQ (6), but, interestingly, in Mexico, fluoride is introduced in salt rather than through drinking water. The Canadian study reported weak indications of an association between urinary fluoride and IQ overall limited to boys but not girls (7). The Spanish study found an anomalous increase in IQ associated with consumption of fluoridated water (8), and the Danish study found no association between fluoride level and IQ (9). Cumulatively, these studies hardly make a sufficient case for challenging a beneficial public health policy.

WHAT DOES THE CURRENT STUDY ADD?

Warren and coworker’s study has several features that make it especially applicable to informing fluoridation policy. Assigning exposure based simply on whether an individual resides in an area served by fluoridated water may seem crude relative to the studies that evaluate urinary fluoride concentrations, but policy determines water fluoridation, not urinary fluoride. Biomarkers may be helpful in quantifying the relationship between fluoride intake and health by incorporating variation in water consumption and other sources of fluoride, but the policy question concerns the health impact of water fluoridation.

The national scale of the Warren study and its attempt at representativeness (impaired by extensive nonresponse) also make it more directly applicable to public health policy. The focus on lifetime cognitive ability is broadly relevant to public health despite having less precision than rigorous neurobehavioral testing in children. Much of the argument regarding potential adverse effects concerns cognitive ability, specifically IQ, and contends that even very small reductions of one or two points would have lifelong impact on the population.

The Warren study addresses lifelong impact on cognitive ability and finds no evidence of a negative effect of fluoridation and possibly even a small benefit. This is consistent with a Swedish study that similarly related early life exposure to fluoridated water to adult cognitive capability and likewise found no indications of adverse cognitive outcomes (but interestingly did report favorable dental health outcomes) (9). A study in Calgary, Canada took advantage of the change from fluoridated to nonfluoridated water during an ongoing pregnancy cohort study and found no indication that fluoride exposure during pregnancy had adverse effects on children’s cognitive ability (10). Biomarker studies in carefully selected cohorts have distinct strengths but fundamental limitations when it comes to policy guidance. Simple comparisons of health in areas served by fluoridated drinking water and areas served by nonfluoridated drinking water are needed to inform decisions, and more such studies would be helpful.

WHAT’S NEXT?

Despite historical evidence of fluoride’s benefit in preventing dental caries, reassessment of that finding at present would be of great value. Cessation of water fluoridation in some communities in the United States and Canada will provide an opportunity for researchers to assess the consequences of that decision with spatial and temporal comparisons of dental health. Documentation of the expected benefits and particularly assessing the impact on socioeconomically deprived populations would be pertinent to a comprehensive assessment of risks and benefits. If dental health benefits are not found, policy predicated on that assumption should be revisited.

Such research on risks or benefits needs to examine the range of fluoride concentration that is relevant to the policy decision, 0.7 mg/liter versus none. Studies of more highly exposed populations may contribute to our general understanding of whether fluoride can cause neurodevelopmental harm, but studies of elevated naturally occurring fluoride levels are only possible in limited locations around the world, requiring extrapolation both to lower levels of fluoride and notably different settings.

An issue that may warrant consideration is the choice between fluoridating the water supply and providing fluoride through other means as several other countries do. While fluoridation of drinking water is not consistent around the world, introduction of some form of supplementary fluoride is widespread throughout high-income and most middle-income countries. Fluoride is provided in salt in many countries including Mexico, Switzerland, and Germany (11). Milk is fluoridated in Russia, China, and much of the United Kingdom (12). Consideration of risks and benefits from these alternative means of delivery may warrant evaluation by some assessment of the effectiveness of the different approaches in different locations. Fluoride in the food supply is less subject to local control than drinking water, which would undoubtedly prove politically challenging to implement, but given the complexity and vulnerability to misinformation, expecting each municipality to have the necessary expertise for an informed evaluation may not be ideal.

Until clear evidence exists that water fluoridation lacks public health benefit or compelling evidence of harm at the level of fluoride exposure in fluoridated water, neither of which has occurred, it seems foolhardy to interfere with a long-established and well-recognized public health success. With due credit to the folk wisdom of Bert Lance, director of the Office of Management and Budget under President Jimmy Carter, “If it ain’t broke, don’t fix it.” Warren and colleagues move the needle a bit further into the “ain’t broke” range.