More than half of Californians and nearly 75% of U.S. residents live in communities where fluoride has been added to drinking water to prevent tooth decay, an intervention hailed by the U.S. Centers for Disease Control as one of the 20th century’s greatest public health achievements.

Yet should Donald Trump be elected to a second term, water systems will be immediately directed to end this practice, according to supporter Robert F. Kennedy Jr.

“On January 20, the Trump White House will advise all U.S. water systems to remove fluoride from public water,” Kennedy posted Saturday on the social media platform X.

Kennedy described the mineral, which occurs naturally in all fresh water supplies, as an “industrial waste” associated with a variety of health risks.

On Sunday, Trump told NBC News that he had not discussed the fluoride proposal with Kennedy, “but it sounds OK to me.” Reached for comment Monday, a campaign spokesperson told The Times that while Trump “has received a variety of policy ideas, he is focused on Tuesday’s election.”

Kennedy’s statement follows a September ruling from a federal judge in California that could have major effects on the long-standing public health intervention, but that multiple dentists and pediatricians say is based on a flawed reading of the available science.

In his X post, Kennedy tagged Michael Connett, the lead attorney representing the plaintiff in that lawsuit, the advocacy group Food & Water Watch.

In late September, U.S. District Judge Edward Chen ordered the U.S. Environmental Protection Agency to take further unspecified steps to regulate fluoride in drinking water.

In his ruling, Chen cited a literature review by the U.S. National Toxicology Program that found associations between exposure to high levels of fluoride and lower IQ levels in children.

Though he could not “conclude with certainty that fluoridated water is injurious to public health,” Chen wrote, he still determined that it posed an “unreasonable risk” of harm. His ruling did not specify what exactly the EPA should do about that.

The American Dental Assn. and American Academy of Pediatrics issued a joint statement of concern after the ruling, saying that there were major limitations in the study Chen cited. The groups pointed out that the study found low IQ associations only at fluoride concentrations more than double those of the current recommendation of 0.7 milligrams per liter.

“There is nothing about the current decision that changes my confidence in the safety of optimally fluoridated water in the U.S.,” said Dr. Charlotte W. Lewis, a member of the AAP Section on Oral Health, at the time. “Water fluoridation is a public health policy based on a solid foundation of evidence.”

In California, scientifically shaky skepticism of fluoridated water has been around almost as long as the stuff has been coming out of the taps.

In 1952, San Francisco and Antioch became the first communities in California to supplement naturally occurring levels of fluoride in their drinking water, after a raft of studies showing that the mineral made teeth enamel stronger and more resistant to acid. Within a decade, cavities in 6-year-olds’ permanent teeth had dropped by 84%.

The U.S. Public Health Service in 1962 recommended a fluoridation level of 0.7 to 1.2 mg per liter in drinking water supplies. (In 2015 the service revised its recommendation to 0.7 mg per liter — a sweet spot that maximized the dental health advantages while reducing the risk of tooth discoloration associated with excess fluoride exposure.)

Yet as the number of communities in California voting to fluoridate their water grew, so did complaints from skeptics who pushed debunked theories about fluoride’s link to cancer, AIDS and other conditions, according to a history by UC San Francisco oral biology professor Ernest Newburn.

Despite California’s early leadership in water fluoridation, it now ranks 39th among U.S. states in the percentage of the population covered by fluoridated water systems, with 57.5% of residents living in covered communities, according to the United Health Foundation. (Kentucky leads the board with 99.9% coverage.)

An exploratory study published this year found associations between prenatal exposure to fluoride and an increased risk of neurobehavioral problems at age 3 in a cohort of 229 Los Angeles-area children.

The American Dental Assn., the American Academy of Pediatrics and others have pushed back on the study, citing concerns about the methods used to evaluate both child IQ and maternal fluoride levels.

“It is irresponsible of the authors, based on this study, to suggest that recommendations be changed on the beneficial use of fluoride to prevent dental problems,” said Howard Pollick, a professor at the UC San Francisco School of Dentistry, shortly after the study was published.

A larger study published last year followed more than 2,000 children in Australia over five years and found no associations between prenatal fluoride exposure and behavioral development problems.

Both Trump and Kennedy have said publicly that longtime anti-vaccine activist Kennedy will probably have a prominent role in health policy in a second Trump administration.

“I’m gonna let him go wild on health. I’m gonna let him go wild on the food. I’m gonna let him go wild on medicines,” Trump told supporters during a rally at Madison Square Garden last month.

The Associated Press contributed to this report.