A federal judge in California has ordered the U.S. Environmental Protection Agency to strengthen regulations for fluoride in drinking water, saying the compound poses an unreasonable potential risk to children at levels that are currently typical nationwide.

U.S. District Judge Edward Chen in San Francisco on Tuesday sided with several advocacy groups, finding the current practice of adding fluoride to drinking water supplies to fight cavities presented unreasonable risks for children’s developing brains.

Chen said the advocacy groups had established during a non-jury trial that fluoride posed an unreasonable risk of harm sufficient to require a regulatory response by the EPA under the Toxic Substances Control Act.

“The scientific literature in the record provides a high level of certainty that a hazard is present; fluoride is associated with reduced IQ,” wrote Chen, an appointee of Democratic former President Barack Obama.

But the judge stressed he was not concluding with certainty that fluoridated water endangered public health.

The ruling was hailed by the environmental group Food & Water Watch, which led a coalition of organizations that sued the EPA in 2017. The agency had denied their so-called citizen’s petition asking it to consider banning fluoridation chemicals from public drinking water.

“The court’s historic decision should help pave the way towards better and safer fluoride standards for all,” Michael Connett, a lawyer for the advocacy groups, said in a statement on Wednesday.

The EPA said it was reviewing the decision.

Water in the U.S. has been fluoridated since 1945, though the recommended levels have since been lowered to address the potential for tooth damage and other risks.

More than 200 million Americans, or about 75% of the population, currently have fluoride added to their drinking water at recommended levels of 0.7 milligrams of fluoride per liter of water.

Communities add fluoride to water on a voluntary basis. Doctors and dentists recommend it because it can help protect children’s teeth that have not yet broken through the gums. The U.S. practice differs from Europe, where fluoridated drinking water is rare.