NEW YORK (AP) — A U.S. government report expected to stir debate concluded that fluoride in drinking water at twice the recommended limit is linked with lower IQ in children.

The report, based on an analysis of previously published research, marks the first time a federal agency has determined — “with moderate confidence” — that there is a link between higher levels of fluoride exposure and lower IQ in kids. While the report was not designed to evaluate the health effects of fluoride in drinking water alone, it is a striking acknowledgment 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. The addition of 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 crucial in our understanding” of this risk, said Ashley Malin, a University of Florida researcher who has studied the affect 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 Wednesday comes from the National Toxicology Program, part of the Department of Health and Human Services. It summarizes a review of studies, conducted in Canada, China, India, Iran, Pakistan, and Mexico, that concludes that drinking water containing more than 1.5 milligrams of fluoride per liter is consistently associated with lower IQs in kids.

The report did not try 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 IQ was 2 to 5 points lower in children who’d had higher exposures.

Since 2015, federal health officials have recommended a fluoridation level of 0.7 milligrams per liter of water, and for five decades before the recommended upper range was 1.2. The World Health Organization has set a safe limit for fluoride in drinking water of 1.5.

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

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

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

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

Fluoride is a mineral that exists naturally in water and soil. About 80 years ago, scientists discovered that people whose supplies naturally had more fluoride also had fewer cavities, triggering 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 start adding fluoride to tap water. In 1950, federal officials endorsed water fluoridation to prevent tooth decay, and continued to promote it even after fluoride toothpaste brands hit the market several years later. Though fluoride can come from a number of sources, drinking water is the main source for Americans, researchers say.

Officials lowered their recommendation for drinking water fluoride levels in 2015 to address a tooth condition called fluorosis, that can cause splotches on teeth and was becoming more common in U.S. kids.

Separately, the Environmental Protection Agency has maintained a longstanding requirement that water systems cannot have more than 4 milligrams of fluoride per liter. That standard is designed to prevent skeletal fluorosis, a potentially crippling disorder which causes weaker bones, stiffness and pain.

But more and more studies have increasingly pointed to a different problem, suggesting a link between higher levels of fluoride and brain development. Researchers wondered about the impact on developing fetuses and very young children who might ingest water with baby formula. Studies in animals showed fluoride could impact neurochemistry cell function in brain regions 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 pointed to neurological effects in people exposed to high levels of fluoride. It called for more research into the effect of fluoride on intelligence.

After more research continued to raise questions, the National Toxicology Program in 2016 started working on a review of the available studies that could provide guidance on whether new fluoride-limiting measures were needed.

There were earlier drafts but the final document has repeatedly been held up. At one point, a committee of experts said available research did not support an earlier draft’s conclusions.

“Since fluoride is such an important topic to the public and to public health officials, it was imperative that we made every effort to get the science right,” said Rick Woychik, director of the National Toxicology Program, in a statement.

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

___

The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Science and Educational Media Group. The AP is solely responsible for all content.

A Greek-flagged oil tanker traveling through the Red Sea came under repeated attack Wednesday, leaving the vessel “not under command” and drifting ablaze after an assault suspected to have been carried out by Yemen’s Houthi rebels, the British military said.

The attack, the most serious in the Red Sea in weeks, comes during a monthslong campaign by Houthis targeting ships over the ongoing Israel-Hamas war in the Gaza Strip that has disrupted a trade route through which $1 trillion in cargo typically passes each year.

In the attack, men on small boats first opened fire with small arms about 90 miles west of the rebel-held Yemeni port city of Hodeida, the British military’s United Kingdom Maritime Trade Operations center said.

Four projectiles also hit the ship, it added. It wasn’t immediately clear if that meant drones or missiles.

IRAN-BACKED HOUTHI REBELS STRIKE 2 COMMERCIAL SHIPS IN GULF OF ADEN

“The vessel reports being not under command,” the UKMTO said, likely meaning it lost all power. “No casualties reported.”

Later, the UKMTO warned the ship was drifting while on fire in the Red Sea.

The Greek shipping ministry later identified the vessel as the tanker Sounion, which had 25 crew members on board at the time of the attack as it traveled from Iraq to Cyprus.

Later Wednesday, the UKMTO reported a second ship being targeted in the Gulf of Aden by three explosions that occurred in the water close to it, though they caused no damage.

The Houthis did not immediately claim responsibility for the attacks, though it can take them hours or even days before they acknowledge their assaults.

The Houthis have targeted more than 80 vessels with missiles and drones since the war in Gaza started in October. They seized one vessel and sank two in the campaign that also killed four sailors.

Other missiles and drones have either been intercepted by a U.S.-led coalition in the Red Sea or failed to reach their targets.

The rebels maintain that they target ships linked to Israel, the United States or the UK to force an end to Israel’s war against Hamas in Gaza. However, many of the ships attacked have little or no connection to the conflict, including some bound for Iran.

The Houthis have also launched drones and missiles toward Israel, including an attack on July 19 that killed one person and wounded 10 others in Tel Aviv. Israel responded the next day with airstrikes on Hodeida that hit fuel depots and electrical stations, killing and wounding a number of people, the rebels say.

After the strikes, the Houthis paused their attacks until Aug. 3, when they hit a Liberian-flagged container ship traveling through the Gulf of Aden. A Liberian-flagged oil tanker came under a particularly intense series of attacks beginning Aug. 8, likely carried out by the rebels. A similar attack happened Aug. 13 as well.

The last three recent attacks, including Wednesday’s, targeted vessels associated with Delta Tankers, a Greek company.

As Iran threatens to retaliate against Israel over the assassination of Hamas leader Ismail Haniyeh in Tehran, the U.S. military told the USS Abraham Lincoln aircraft carrier strike group to sail more quickly to the area. America also has ordered the USS Georgia-guided missile submarine into the Mideast, while the USS Theodore Roosevelt aircraft carrier strike group was in the Gulf of Oman.

Additional F-22 fighter jets have flown into the region and the USS Wasp, a large amphibious assault ship carrying F-35 fighter jets, is in the Mediterranean Sea.