Why we add fluoride to water—and how it became so controversial
Here's everything you need to know about the benefits and the risks of this common but controversial natural element.
Fluoride is a naturally occurring element that has been added to tap water in some parts of the U.S. since the 1940s.
It’s championed by the American Dental Association, American Academy of Pediatrics, and Canadian Dental Association for its effectiveness in preventing cavities. The U.S. Center for Disease Control lists water fluoridation—adjusting the levels of fluoride in tap water supplies—among the top 10 public health achievements.
Yet, fluoride has a controversial history, including suggestions that adding it to water was a communist plot. So, what is fluoride, why is it added to the water, and should we be worried about possible risks?
What is fluoride?
Fluoride is an element naturally found in soil, rocks, and water.
“Every water source has some fluoride in it,” says Steven Levy, a professor of preventive and community dentistry at the University of Iowa. It even occurs in tea.
Eating foods with refined carbohydrates—like bread, yogurt, or fruit—causes bacteria to create acid which demineralizes the surface of the teeth, creating microscopic cavities.
“Fluoride helps to accelerate and enhance remineralization, so most people don't have cavities ever occur,” he says.
Some countries, including the U.S., U.K., Spain, and Brazil, have local jurisdictions that add fluoride to their water to prevent tooth decay. Germany has added fluoride to salt since 1991.
Adding fluoride to the water
During the early 1900s, researchers realized that people in areas with naturally high levels of fluoride in their water system were more likely to have dental fluorosis—a condition that usually affects children who consume too much fluoride while their teeth are developing.
Dental fluorosis is a “mottling of enamel,” says John Morris, a senior lecturer in dental public health at the University of Birmingham in the U.K.
It “boils down to a cosmetic condition where you get white spots or streaks on your teeth,” says Cheryline Pezzullo, a professor at NYU College of Dentistry.
Yet, people with fluorosis also had much lower cavity rates due to a low level of fluoride naturally present in the water.
U.S. scientists concluded that, at a concentration of about one part per million, fluoride in water protected against tooth decay without causing fluorosis. Today, the recommended level that can be added to water supplies in the U.S. is 0.7 parts per million because people also get fluoride from toothpastes and mouthwash.
Consuming a safe dose
Fluoride is a natural component of bone.
“There is no vertebrate—human, animal—that doesn't have fluoride in its system,” says Howard Pollick, a dental scientist at the University of California, San Francisco.
Long-term exposure to fluoride at concentrations of over four parts per million can cause skeletal fluorosis, weakening the bones.
This significantly exceeds the World Health Organization’s recommended level of 1.5 milligrams per liter as well as the 0.7 mg recommended by the U.S. Public Health Service.
Consuming high enough levels to develop skeletal fluorosis would involve “drinking gargantuan amounts of tea or eating several tubes of toothpaste a day,” says Morris.
Any substance can be toxic at a high level—even water. “It's not the substance but the dose,” says Pollick.
Opposition to fluoride
People have worried about fluoride since it was first added to water. Some opponents were concerned because fluorine gas is deadly, while gum disease advocates “were quite concerned that people would now just rely upon the fluoride in the water and not brush their teeth”, says Pollick.
There were also concerns about cancer risks. A 1990 study showed increased bone tumors in rats given high levels of fluoride. But a Public Health Service report, based on over 50 human population studies, found no “detectable cancer risk to humans” from low levels of fluoride.
There has been recent debate around the link between fluoride exposure and children’s IQ. A 2021 study associated high fluoride exposure with lower IQ among boys while a 2023 meta-analysis found no association. A National Toxicology Program report had “moderate confidence” that fluoride above the WHO’s recommended levels can affect children’s IQ.
Pezzullo highlights that some studies opposing fluoride may have small sample sizes, methodological limitations, or are funded by organizations with a vested interest. Without robust clinical studies, “misinformation, hysteria, and controversy gets perpetuated,” she says.
In 2017, a federal lawsuit was brought against the U.S. Environmental Protection Agency (EPA), calling for it to reduce its recommendation that water should not have more than 4 mg of fluoride per liter (4 parts per million). Areas with naturally higher fluoride than this must be treated.
In September 2024, the judge ruled that, although there is no certainty that fluoridated water harms public health, the EPA must review its regulations.
“EPA is in the process of reviewing the district court’s decision,” EPA spokesperson Remmington Belford said in an email.
Impacts of tooth decay
Levels of tooth decay have fallen but cavities remain “one of the commonest chronic, and remember, entirely preventable diseases of childhood,” says Morris. Rotten teeth impact children’s schooling, education, dietary choices, and their overall ability to thrive, he says.
After Calgary, Canada, removed fluoride from its water, it saw a worsening in children’s dental cavities.
For the most vulnerable in society, who can’t easily access or afford dental care, rotten teeth are a huge burden. “It can lead to infection and pain,” says Pezzullo, “and that infection could spread to the rest of the body.”
Adding fluoride to water is a cost-effective, preventive approach, says Levy. “All you have to do is drink the water.”
The controversy around fluoride
Fluoride can be a heated topic, particularly in the U.S., with some in the 1950s suggesting it was a communist plot to poison America and others saying it was a way for the sugar industry to sell more sweets.
There have been strong reactions to fluoride ever since levels in the water were first adjusted. “I think talk of putting something in the water is quite a visceral issue for some people,” says Morris. “Inevitably, it will attract an extreme opinion from some quarters.”
“People feel it's like an infringement on personal freedoms,” says Pezzullo.
Other public health decisions—such as vaccines and seatbelt legislation—received similar opposition, says Morris, adding that the development of clean drinking water in the 19th century was “similarly riven with controversy.”
While scientists should always continue to ask questions, he says, “we shouldn't be disrupting [public health programs] based on minority opinions.”
Pollick agrees. “Generally, the scientific community speaks with one voice in terms of its support for the appropriate and judicious use of fluoride,” he says.
