In 1944, the cities of Newburgh and Kingston in New York State agreed to participate in an experiment whereby one city, Newburgh, added sodium fluoride to the public drinking water while the other city did not (Ast 1950). Why did they do this? Extensive epidemiological studies from all over the world had indicated that regions with naturally higher levels of fluoride in the environment (from rocks or soil) had children with remarkably fewer cases of dental decay (dental caries). So it was decided that, to determine whether this effect was in fact due to fluoride, a real-world experiment needed to be done using towns that were otherwise very comparable.
The initial findings, after only three years, showed a marked reduction in tooth decay in children exposed to fluoride during the eruption of permanent teeth (ages 6–9) (Ast 1950). These findings spurred many communities in the United States, Canada, and other countries to add fluoride to their public water supplies (McLaren 2021). Since then, fluoridated water has been considered an important tool for preventing tooth decay and promoting remineralization in teeth (Monjaras-Avila 2025).
How does fluoride help prevent decay? Tooth enamel is mostly made of calcium crystallized with phosphate and hydroxide. Fluorine replaces the hydroxide, forming stronger crystals (Armfield 2007, Wikipedia) that are more resistant to acid attack. It also appears to be one of the most effective agents for remineralizing areas of teeth at risk for caries (Monjarás-Ávila 2025).
In recent years, however, some communities have stopped adding fluoride to their water, including the entire state of Utah and some communities in Canada, including the city of Calgary. Why are they doing this?
How dangerous is fluoride in drinking water?
That fluoridation prevents tooth decay is not controversial among legitimate scientists and practitioners. More than 30 countries fluoridate their water and have done so for decades (Armfield). Many studies, including recent ones, support a consistently positive effect of fluoridated water (Wang 2004, Armfield 2007). When cities stop fluoridating water, there is a corresponding increase in tooth decay in children (Yazdanbakhsh 2025, McLaren 2022). Because of this, a city in Canada—Windsor—that had stopped fluoridating its water eventually reinstated it due to concerns about rising tooth decay (Canadian Broadcasting Company 2018).
That said, fluoride levels several times higher than those used in municipal water supplies have been associated with neurological harm, typically demonstrated in “preclinical studies” (e.g., laboratory animal studies rather than real-world human studies). In addition, higher levels of fluoride can lead to “mottled teeth.” But again, these levels are well above what is added to public water. In fact, a carefully conducted study from Australia found no effect of early childhood exposure to low levels of fluoride on cognition (Do 2024).
So, as with many (most?) things, more is not better. Another example of this effect is that low levels of fluoride appear to encourage colonies of beneficial gut microbes, while higher levels seem to be detrimental to beneficial microbes and may encourage potentially pathogenic ones (Yasin 2025).
What are the stakes?
Dental decay is not trivial for anyone, but it is particularly damaging for children, whose teeth are still developing. Dental decay causes pain, and when left untreated—such as in under-resourced communities or in those that avoid modern medical care—it can lead to sepsis (McLaren 2022) and even death. I remember a few years ago when a teenage boy in the District of Columbia died of sepsis caused by a decayed tooth that his family could not afford to have treated.
Who is behind the anti-fluoride movement?
Unsurprisingly, many allegations against fluoride come from entities that have something to gain by scaring people about the public water supply, such as bottled water companies, sellers of water filtration systems, and “purveyors of alternative medicines and therapies” (Armfield 2007), including supplement vendors. More recently, there has been a dramatic increase in fear-mongering on social and other media, presumably for commercial or political advantage.
It is also interesting that, historically, objections to water fluoridation have tended to mirror the dominant anxieties of the time. In the 1950s, for example, fluoridation was claimed to be a Communist plot (Armfield 2007), or alternatively a German or Russian drug designed to cause schizophrenia and make people easier to control. Today, conspiracy theories are more likely to blame domestic governments rather than foreign adversaries.