A federal court in California ruled late Tuesday against the Environmental Protection Agency, ordering officials to take action over concerns about potential health risks from currently recommended levels of fluoride in the American drinking water supply.

The ruling by District Court Judge Edward Chen, an appointee of former President Barack Obama, deals a blow to public health groups in the growing debate about whether the benefits of continuing to add fluoride to the water supply outweighs its risks.

Environmental nonprofit Food & Water Watch and a handful of anti-fluoride groups, like the Fluoride Action Network, have been in court for nearly a decade after the EPA denied their petition against local water utilities adding in the mineral.

While Chen was careful to say that his ruling “does not conclude with certainty that fluoridated water is injurious to public health,” he said that evidence of its potential risk was now enough to warrant forcing the EPA to take action.

“In all, there is substantial and scientifically credible evidence establishing that fluoride poses a risk to human health; it is associated with a reduction in the IQ of children and is hazardous at dosages that are far too close to fluoride levels in the drinking water of the United States,” the judge wrote in his ruling.

The judge’s ruling cites a review by the National Institutes of Health’s toxicology program finalized last month, which concluded that “higher levels” of fluoride is now linked to lowered IQ in children. 

The American Academy of Pediatrics has questioned the validity of the NIH’s report, saying other reviews have come to different conclusions about fluoride’s risks and benefits. The AAP is among the expert groups that continue to recommend using fluoride toothpaste, in combination with fluoridated water, to protect teeth from cavities.

The Centers for Disease Control and Prevention has long hailed the addition of fluoride to drinking water as one of the greatest public health achievements of the 20th century, paving the way for modern use of toothpastes and other dental products that also use fluoride to cut the rate of dental cavities.

While the report said more research was needed into the lower levels of fluoride exposure typically found in U.S. drinking water, Chen ruled that “there is not enough of a margin” of safety at those levels. 

He pointed to previously published studies of pregnant moms finding that their fluoride exposure could be higher. EPA experts had told the court that those higher levels could be in part thanks to the other ways that people are now exposed to the chemical in their food and through toothpaste and other dental products.

“Not only is there an insufficient margin between the hazard level and these exposure levels, for many, the exposure levels exceed the hazard level,” the judge wrote.

Critics have cited near-universal adoption of fluoride toothpaste and other dental products as evidence that the chemical no longer needs to be added to drinking water. Other countries abroad have cut cavity rates without adding it to their water supplies, they argue.

The CDC has argued that continued water fluoridation is still the “most cost-effective method of delivering fluoride to all members of the community regardless of age, educational attainment, or income level.”

Chen said he left it up to the EPA which of a number of options the agency could take in response to his ruling. They range from a warning label about fluoride’s risks at current levels to taking steps towards tightening restrictions on its addition to drinking water.

“One thing the EPA cannot do, however, in the face of this Court’s finding, is to ignore that risk,” he wrote.

Michael Connett, a partner at the law firm Siri & Glimstad and the lead attorney for the groups who brought the lawsuit, said the law now requires EPA to take action to remove the risk of fluoride.

“From our vantage point, the obvious way of eliminating the risk from adding fluoride chemicals to drinking water is to stop adding them,” he told CBS News.

The judge’s ruling stems from a lawsuit brought by the groups under a chemical safety law passed by Congress in 2016, which empowered them to challenge the EPA in court after the agency denied their petition.

Unlike the recent so-called “Chevron doctrine” that the Supreme Court overturned earlier this year, the 2016 law said judges did not need to defer to EPA’s expertise when petitioners challenge the agency’s rejection.

Instead, the law left it up to Chen to decide whether a preponderance of the evidence — if it was more likely than not — showed that fluoride could pose an “unreasonable risk.”

Connett said the ruling marks the first time that a group has been able to use the law to take a citizen petition to trial.

“Clearly, the length of time the judge took to decide this case shows that the court did not rush to make this decision. It took its time, it allowed extensive testimony and evidence. So it was certainly not a rush job, just the opposite of it,” he said.