A US government report says that fluoride at twice the recommended limit is linked to lower IQ in children.

NEW YORK — A U.S. government report expected to spark debate has found that fluoride levels in drinking water that are twice the recommended limit are linked to lower IQs in children.

The report, based on an analysis of previously published studies, marks the first time a federal agency has determined — with “moderate certainty” — that there is an association between higher levels of fluoride exposure and lower IQ in children. Although the report was not designed to assess the health effects of fluoride in drinking water alone, it is a striking confirmation of the potential neurological risks of high levels of fluoride.

Fluoride strengthens teeth and reduces tooth decay by replacing minerals lost through normal wear and tear, according to the U.S. Centers for Disease Control and Prevention. Adding small amounts of fluoride to drinking water has long been considered one of the greatest public health achievements of the last century.

“I think this (report) is pivotal in our understanding” of that risk, said Ashley Malin, a University of Florida researcher who has studied the effects of higher fluoride levels in pregnant women on their babies. She called it the most rigorously conducted report of its kind.

The long-awaited report released Wednesday comes from the National Toxicology Program, part of the Department of Health and Human Services. It summarizes a review of studies from Canada, China, India, Iran, Pakistan and Mexico that finds that drinking water with more than 1.5 milligrams of fluoride per liter is consistently linked to lower IQ in children.

The report did not attempt to determine exactly how many IQ points might be lost with different levels of fluoride exposure. However, some of the studies reviewed in the report suggested that IQ was 2 to 5 points lower in children who had higher exposure.

Since 2015, federal health officials have recommended a fluoridation level of 0.7 milligrams per liter of water, up from 1.2 for five decades. The World Health Organization has set a safe limit for fluoride in drinking water at 1.5.

The report said about 0.6% of the U.S. population — about 1.9 million people — use water systems with naturally occurring levels of fluoride of 1.5 milligrams or more.

“The findings of this report raise questions about how to protect these individuals and what makes the most sense,” Malin said.

The 324-page report does not draw conclusions about the risks of lower levels of fluoride, saying more research is needed. It also does not answer the question of what high levels of fluoride might do to adults.

The American Dental Association, which promotes water fluoridation, has criticized earlier versions of the new study and Malin’s study. Asked for comment, a spokeswoman said in an email late Wednesday afternoon that the organization’s experts were still reviewing the report.

Fluoride is a mineral found naturally in water and soil. About 80 years ago, scientists discovered that people whose diets naturally contained more fluoride also had fewer cavities, prompting pressure for more Americans to use fluoride for better dental health.

In 1945, Grand Rapids, Michigan, became the first U.S. city to add fluoride to tap water. In 1950, federal officials endorsed water fluoridation to prevent tooth decay and continued to promote it even after brands of fluoride toothpaste appeared on the market a few years later. Although fluoride can come from many sources, the primary source for Americans is drinking water, researchers say.

Officials lowered recommendations for fluoride levels in drinking water in 2015 to combat a dental condition called fluorosis, which can cause stains on teeth and is becoming increasingly common among U.S. children.

Separately, the Environmental Protection Agency maintains a long-standing requirement that water systems have no more than 4 milligrams of fluoride per liter. The standard is intended to prevent skeletal fluorosis, a potentially devastating disease that causes bone weakness, stiffness and pain.

But a growing body of research points to another problem, suggesting a link between higher levels of fluoride and brain development. Scientists have wondered about the effects on developing fetuses and very young children who may consume water with formula. Animal studies have shown that fluoride can affect neurochemical cell function in areas of the brain responsible for learning, memory, executive function, and behavior.

In 2006, the National Research Council, a private nonprofit organization in Washington, D.C., said limited evidence from China suggests neurological effects in people exposed to high levels of fluoride. It called for more research on the effects of fluoride on intelligence.

Due to further concerns raised by further research, the National Toxicology Program began work in 2016 to review available research that could provide guidance on whether new measures to limit fluoride use are necessary.

There were earlier drafts, but the final document was repeatedly held up. At one point, an expert committee concluded that the available research did not support the conclusions of the earlier draft.

“Because fluoride is such an important topic for the public and health officials, it was imperative that every effort be made to ensure the scientific results were accurate,” Rick Woychik, director of the National Toxicology Program, said in a statement.

Malin said it makes sense for pregnant women to reduce their fluoride intake, not only from water but also from some types of tea. It may also make sense to have policy discussions about whether to require fluoride content on beverage labels, she said.

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