Water fluoridation is widely seen as one of the great public health achievements of the 20th century, credited with widely reducing tooth decay. But there has been growing controversy among scientists about whether fluoride may be linked to lower I.Q. scores in children.
A comprehensive federal analysis of scores of previous studies, published this week in JAMA Pediatrics, has added to those concerns. It found a significant inverse relationship between exposure levels and cognitive function in children.
Higher fluoride exposures were linked to lower I.Q. scores, concluded researchers working for the National Institute of Environmental Health Sciences.
None of the studies included in the analysis were conducted in the United States, where recommended fluoridation levels in drinking water are very low. At those amounts, evidence was too limited to draw definitive conclusions.
Observational studies cannot prove a cause-and-effect relationship. Yet in countries with much higher levels of fluoridation, the analysis also found evidence of what scientists call a dose-response relationship, with I.Q. scores falling in lock step with increasing fluoride exposure.
Children are exposed to fluoride through many sources other than drinking water: toothpaste, dental treatments and some mouthwashes, as well as black tea, coffee and certain foods, such as shrimp and raisins. Some drugs and industrial emissions also contain fluoride.
For every one part per million increase in fluoride in urinary samples, which reflect total exposures from water and other sources, I.Q. points in children decreased by 1.63, the analysis found.
“There is concern that pregnant women and children are getting fluoride from many sources,” said Kyla Taylor, an epidemiologist at the institute and the report’s lead author, “and that their total fluoride exposure is too high and may affect fetal, infant and child neurodevelopment.”
Dr. Taylor said that the analysis was meant to contribute to the understanding of the safe and effective use of fluoride. But she said it did not address the benefits and was not intended to assess “the broader public health implications of water fluoridation in the United States.”
Several scientists, including many dentists, criticized the report, pointing to what they said were methodological flaws and emphasizing that the research did not have implications for U.S. drinking water.
The subject is so divisive that JAMA Pediatrics commissioned two editorials with opposing viewpoints to publish alongside the report.
In one, Dr. Steven M. Levy, a public health dentist at the University of Iowa, said that many of the studies included in the analysis were of very low quality. He also warned against concluding that any changes should be made in American fluoridation policies.
“A lay reader or policymaker at a water board in a small community somewhere may see the evidence and think that every way you analyze it, it’s a concern,” Dr. Levy said in an interview. “It isn’t as clear-cut as they’re trying to make it.”
The report’s findings align in some ways with statements by Robert F. Kennedy Jr., President-elect Donald J. Trump’s choice to head the department of health and human services. He had questioned the safety of fluoride and said one of the first acts of the Trump administration will be to advise water systems to remove fluoride.
Criticism of fluoridation has popped up frequently since the practice was initiated in many U.S. communities in the 1950s. But opposition was originally dismissed, as it was strongest among those with extremist or fringe views, and right-wing groups like the John Birch Society, which called fluoridation a Communist plot.
That is changing. Last September, U.S. District Judge Edward Chen in San Francisco ordered the Environmental Protection Agency to strengthen regulations for fluoride in drinking water because of research suggesting that high levels might pose a risk to the intellectual development of children.
In a second editorial published alongside the new study, a public health expert, Dr. Bruce P. Lanphear, noted that as far back as 1944, the editor of The Journal of the American Dental Association expressed concern about adding fluoride, which he termed “a highly toxic substance,” to drinking water. He wrote that “the potentialities for harm far outweigh those for good.”
Some studies have suggested that dental health has improved not because fluoride was added to water, but because of fluoridated toothpastes and better dental hygiene practices. (In some countries, fluoride is added to salt.)
According to this argument, topical application of fluoride to teeth is effective enough to prevent tooth decay, and ingestion is not necessary.
But other studies have reported increases in cavities after public water fluoridation initiatives ceased in some countries.
Currently, the recommended fluoride levels in the United States are 0.7 parts per million, and the study did not find a statistically significant inverse association between fluoride levels and I.Q. scores at below 1.5 parts per million based solely on fluoride levels in water. But nearly three million Americans still drink water with fluoride levels above 1.5 parts per million from wells and some community water systems.
Linda Birnbaum, former director of the National Institute of Environmental Health Sciences, called for more research into the potential effects of fluoride levels below 1.5 parts per million.
But she emphasized that the study had concluded with certainty that a certain amount of fluoride can be damaging to developing brains. “The answer is pretty clear: yes,” Dr. Birnbaum said.
To protect fetuses and babies who are especially vulnerable, she advised parents to avoid drinking fluoridated water during pregnancy and to use fluoride-free bottled water when preparing formula for their infants.
“My recommendation is that pregnant women and infants shouldn’t be exposed to excess fluoride,” said Dr. Birnbaum, who is not an author of the new analysis.
Women who are breastfeeding need not be concerned, she added, as very little fluoride is passed on through breast milk.
“The more we study a lot of chemicals, especially the chemicals that affect I.Q., like lead — there’s really no safe level,” Dr. Birnbaum said.
Some 74 studies from 10 countries, including China, Mexico, Canada, India and Denmark, were examined. Dr. Lanphear noted that the consistent links between fluoride and I.Q. were found in very different populations.
He urged the U.S. Public Health Service to set up a committee, perhaps one that does not include researchers who have studied the subject in the past and can take a fresh look at the topic, to examine two questions seriously: whether fluoride is neurotoxic, and whether it is as beneficial for oral health as it is believed to be.
“If that doesn’t happen urgently, my concern is there will be growing distrust of public health agencies amid the public, and they will have deserved it,” he said.