A US government report expected to spark debate has found that fluoride in drinking water at levels twice the recommended amount is linked to lower IQs in children.

The report, based on an analysis of previously published research, marks the first time a federal agency has determined – “with moderate certainty” – that there is a link between higher levels of fluoride exposure and lower IQ in children. While the report was not designed to evaluate the health effects of fluoride in drinking water alone, it is a striking recognition of a potential neurological risk from high levels of fluoride.

Fluoride strengthens teeth and reduces cavities by replacing minerals lost during normal wear and tear, according to the U.S. Centers for Disease Control and Prevention (CDC). Adding low levels 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 critical to our understanding” of this risk, said Ashley Malin, a University of Florida researcher who has studied the effect of higher fluoride levels in pregnant women on their children. She called it the most rigorously conducted report of its kind.

The long-awaited report, released this week, comes from the National Toxicology Program, part of the Department of Health and Human Services (HHS). It summarizes a review of studies conducted in Canada, China, India, Iran, Pakistan and Mexico, and concludes that drinking water with more than 1.5 milligrams of fluoride per liter is consistently associated with lower IQs in children.

The report did not attempt to quantify exactly how many IQ points might be lost at different levels of fluoride exposure. But some of the studies reviewed in the report suggested that IQ was two to five 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, and five decades before that, the recommended upper range was 1.2. The World Health Organization has set a safe limit for fluoride in drinking water as 1.5.

The report found that about 0.6% of the U.S. population – approximately 1.9 million people – rely on water systems with naturally occurring fluoride levels of 1.5 milligrams or higher.

“The findings of this report raise the question of how these people can be protected and what makes the most sense,” Malin said.

The 324-page report did not reach a conclusion on the risks of lower fluoride levels, saying more research was needed. It also did not answer the question of what high fluoride levels might do to adults.

The American Dental Association, which advocates for water fluoridation, was critical of earlier versions of the new analysis and Malin’s research. Asked for comment, a spokeswoman emailed late Wednesday afternoon that the organization’s experts were still reviewing the report.

Fluoride is a mineral that occurs naturally in water and soil. About 80 years ago, scientists discovered that people whose water supplies naturally contained more fluoride also had fewer cavities, leading to a push to get 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 supported water fluoridation to prevent tooth decay, and continued to promote it even after fluoride toothpaste brands came on the market years later. Although fluoride can come from a variety of sources, drinking water is the most important source for Americans, researchers say.

In 2015, authorities lowered their recommendations for fluoride levels in drinking water to address a condition called fluorosis, which can stain teeth and is becoming increasingly common in children in the U.S.

In addition, the Environmental Protection Agency (EPA) has maintained a long-standing requirement that water systems have no more than 4 milligrams of fluoride per liter. That standard is designed to prevent skeletal fluorosis, a potentially crippling condition that causes weakened bones, stiffness and pain.

But more and more studies point to another problem, namely that there is a link between higher fluoride levels and brain development.

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

After more research raised questions, the National Toxicology Program began a review of available research in 2016. These studies could provide direction on whether new measures were needed to limit fluoride.

There were earlier versions, but the final document was repeatedly blocked. At one point, a committee of experts said that the available research did not support the conclusions of an earlier version.

“Because fluoride is such an important topic for the public and public health officials, it was critical that we do everything we can to get the science right,” 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 just from water but also from certain types of tea. It also might make sense to have policy discussions about whether fluoride content should be required on beverage labels, she said.