What’s behind the wave of distrust about fluoride in water?

For decades, fluoridation in water has been celebrated as a milestone in global public health. Ever since the city of Grand Rapids, Michigan (USA), became a pioneer in fluoridation in 1945, the practice has gained worldwide recognition as an effective, safe and affordable strategy to combat one of humanity's oldest diseases: tooth decay. Almost a century later, however, water fluoridation has been under attack by controversial studies and misinformation.

In 2025, the US states of Utah and Florida passed laws banning the addition of fluoride to public water supplies. The decisions were prompted by concerns about potential risks to children’s neurological development, in a debate that has damaged public perception of an established and, until now, almost unquestioned practice.

The starting point was not a change in position by major scientific entities. On the contrary: the World Health Organization (WHO), the United States Centers for Disease Control and Prevention (CDC) and the American Dental Association (ADA) continue to recommend water fluoridation. What has reignited the controversy are recent studies, such as the one published in March in the journal Environmental Health Perspectives , which have alarmed public administrators.

Conducted by researchers at the Karolinska Institute in Sweden, the study followed about 500 children from a rural population in Bangladesh from prenatal care until they turned 10. During this period, the researchers analyzed urine samples to measure exposure to fluoride from various sources: drinking water, food and dental products. The research concluded that there is an association between relatively low levels of fluoride exposure during pregnancy and childhood (below the limit of 1.5 mg/L recommended by the WHO) and a decline in the intelligence and cognitive abilities of these children at age 10.

The study suggests that even small declines in cognitive performance, when applied to large populations, can have a significant impact on society. Although the researchers emphasize that the study does not prove a definitive cause-and-effect relationship and that more research is needed in other socioeconomic and environmental contexts, findings like this have generated alarm and reopened the debate on the validity of fluoridation as a public health policy.

“In recent years, some studies have suggested that exposure to high levels of fluoride may have adverse effects, especially in children in the neurological development phase,” explains dentist Bruna Fronza, a professor of the undergraduate course in Dentistry at the Albert Einstein Israelite School of Health Sciences (FICSAE). “However, it is important to emphasize that these studies have methodological limitations, such as the lack of control for other environmental variables or additional sources of fluoride exposure, which compromises the reliability of the data and conclusions presented.”

Success strategy

The logic of fluoride in water is simple but ingenious. The substance strengthens tooth enamel by helping to remineralize teeth, reversing small daily mineral losses caused by acids produced by bacteria in contact with sugary foods and drinks in the mouth. Thus, it acts as a protective shield, hindering the development of cavities. Furthermore, because it is universally distributed in water, it reaches the entire population, especially those with less access to dental products and dental services.

Dentist Paulo Frazão, a professor in the Department of Politics, Management and Health at the School of Public Health at the University of São Paulo (USP), explains that all water sources contain fluoride, but the strategy of adjusting the concentration has proven to be a low-cost, highly effective preventive option that is safe for human health. “It is the only known measure to reduce the inequality of tooth decay among children and adolescents of different levels of family income, as long as treated and fluoridated water reaches both rich and poor neighborhoods,” he says.

In Brazil, water fluoridation was established by Law No. 6,050 of 1974, after a long history of studies and international recognition of the effectiveness of this strategy in preventing tooth decay. With the federal law that required the addition of fluoride to public water supply systems, the country saw a significant drop in tooth decay rates over the following decades.

“Statistics regarding the number of decayed, missing and filled teeth have evolved positively in recent decades and reveal how important fluoridation of public water supplies is in ensuring the protection of the oral health of Brazilians,” says Claudio Miyake, president of the Federal Council of Dentistry (CFO).

In 1980, the Ministry of Health estimated the national DMFT at 12 years of age — an index that assesses the average number of decayed, missing and filled teeth in permanent teeth — at 7.3. The number, high according to WHO criteria, placed Brazil in the category of poor countries with a high incidence of tooth decay. The decline was gradual: 6.7 in 1986, 3.1 in 1996, 2.8 in 2003, 2.1 in 2010 and reached 1.7 in 2023, placing Brazil among the countries with a low incidence of tooth decay.

Real risks and assumptions

The health risks associated with excess fluoride in water have been known since the second half of the 20th century. The most common side effect is dental fluorosis, which can cause white stains on the teeth in its mildest form. But even this risk is reduced when fluoride levels in water are properly monitored, as is the case in Brazil.

Cases of severe fluorosis, which affect the structure of the teeth, are rare in places where fluoridation is carried out under technical control. “It is important to highlight that the main risk factor for mild fluorosis is not fluoridated water, but rather the inappropriate use of fluoride toothpaste in young children, such as letting the child brush without supervision and swallowing excess toothpaste,” explains Fronza.

In 2024, the National Toxicology Program (NTP) in the United States published a comprehensive review of 74 studies on fluoride exposure and childhood neurodevelopment. The authors identified an association between high levels of fluoride in urine and small reductions in IQ in children.

The result, although statistically significant, has been interpreted with caution by many experts. This is because the review, as well as the Karolinska Institute research, was classified as having low methodological quality. The authors also acknowledged that the correlation does not mean a cause-and-effect relationship and that more studies are needed to substantiate the issue.

“The fact is that the observations from these studies do not change the scientific evidence on the effectiveness and safety of the public health practice of water fluoridation,” Frazão points out. “However, misinterpretations by non-specialists on social media seeking notoriety can be used to garner public attention and contribute to eroding support for public policy, as is the case with immunization programs.”

A study by the Harvard School of Dental Medicine, published on May 30 in the JAMA Health Forum , found that, in just five years, ending water fluoridation could result in about 25.4 million more decayed teeth in children — at an estimated cost of US$9.8 billion in dental care.

The impact would fall mainly on low-income children, increasing inequalities and creating an economic burden with treatments. “Eliminating fluoride from water would not only harm the oral health of the population, but also cause an economic burden and increase inequalities in access to dental care,” warns dentist Letícia Mello Bezinelli, coordinator of the undergraduate program in Dentistry at FICSAE.

Pathways to oral health in Brazil

While recent decisions in the US have called into question the public health policy adopted more than seven decades ago, Brazil has maintained its policy of expanding the scope of fluoridation in water supply systems. However, the challenges are far from being resolved.

According to Frazão, the 1974 law determines that fluoridation must be adopted in all locations in the country that have a water treatment plant. Information about the application of the measure is based on statements from the companies and supply services that serve the municipalities.

In 2018, the USP researcher participated in a study on fluoridation in the country, concluding that just over half of the municipalities monitored water quality and, among these, just over 40% had fluoridation levels considered optimal. “There is significant room to expand and qualify the implementation of public policy in our country,” he says.

According to the Federal Council of Dentistry, studies indicate that approximately 25% of Brazilians still do not have adequate fluoridation coverage. “In some locations, the DMFT rates are higher, which corroborates the importance of adding fluoride to public water supplies,” adds Claudio Miyake. For him, the way forward is to expand public dental services, including not only access to dental treatments, but also prevention and guidance programs – such as Law No. 14,572, of 2023, which instituted the National Oral Health Policy.

Bruna Fronza also reminds us that fluoride does not work alone. It works in conjunction with other essential practices to keep teeth and gums healthy, such as brushing your teeth three times a day, flossing, avoiding excessive sugar consumption between meals, drinking clean water (preferably fluoridated) and visiting the dentist regularly.

“All of these actions require active participation from individuals and the population, which is not always feasible in contexts of social vulnerability,” summarizes the Einstein specialist. “Water fluoridation, on the other hand, is a broad form of protection that does not depend on individual behavior to generate benefits. Therefore, it is considered an effective public health strategy.”

Source: Einstein Agency

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