The current case has put the spotlight on an unpublished assessment by the federal government’s National Toxicology Program (NTP). It reported “moderate confidence” that drinking water containing fluoride at levels at least twice as high as those recommended by the federal government is associated with lower IQ in children. The Fluoride Action Network (FAN) and other groups argue that such data indicate EPA should be regulating fluoride under the Toxic Substances Control Act (TSCA).

This week’s testimony marked the resumption, after a yearslong pause, of litigation that began in 2017. Over 9 days, Judge Edward Chen of the U.S. District Court for the Northern District of California is scheduled to hear the views of seven experts on a range of technical topics, including whether there is a safe threshold for fluoride exposure.

During opening arguments on 31 January, the attorney for the plaintiffs, Michael Connett, highlighted babies fed formula made with tap water as a “critical vulnerable group being exposed to the highest dose of fluoride of any age group in the population. That is a major cause for concern.” He argued that if EPA was aware of a different compound that posed a similar potential threat to newborns, the agency wouldn’t hesitate to regulate it. “I don’t think we need to speculate about what EPA would do with that scenario,” he said.

EPA and others argue there is little strong evidence that the current recommended concentration of fluoride in drinking water—0.7 milligrams per liter—poses a threat. (That level is set to avoid discoloration of teeth in children.) “The dose makes the poison,” said Paul Caintic, a Department of Justice attorney. “Given the current state of science, the court cannot conclude that community water fluoridation presents an unreasonable risk.”

The fluoride lawsuit is the first to reach trial under a 2016 TSCA provision allowing citizens to ask a court to assess a chemical’s risk. FAN turned to that strategy after EPA said its request that the agency ban fluoridation lacked a “scientifically defensible basis.”

The trial began in June 2020 but Chen suspended it months later to wait for publication of the NTP report. That assessment, based on work conducted by NTP experts from 2016 to 2019, initially classified fluoride as a cognitive developmental hazard. But NTP removed that determination after two reviews by the National Academies of Sciences, Engineering, and Medicine in 2020 and 2021 found the evidence was lacking. One flaw, the reviews said, was that the assessment didn’t focus on how people respond to various doses of fluoride.

The report has yet to be formally released, and FAN charges that the repeated reviews have been designed to delay it—a critique supported by some scientists and former NTP leaders. According to emails released to FAN under the Freedom of Information Act, a senior official at the Department of Health and Human Services intervened in 2022 to halt publication just days before the report was scheduled to be released. But in March 2023, as a result of litigation, NTP did release a 393-page draft. And that May, NTP’s science advisory board signed off on the report and sent it to NTP leadership.

Dental health advocates say activists are already spinning the report to their advantage by claiming NTP found no safe dose (a claim not directly stated in the report). The American Dental Association has strongly urged NTP to add a disclaimer to the report highlighting its scientific limitations.

In the meantime, Chen has allowed the lawsuit to resume. He has suggested that EPA must regulate fluoride if FAN proves that fluoride poses an “unreasonable risk” to pregnant women and children. Under TSCA, EPA cannot consider a chemical’s benefits, such as oral health.

Bergeson describes Chen as “a very disciplined jurist and a very demanding taskmaster,” adding that “presumably the data will be looked at correctly and weighed appropriately.”

Lynn Goldman, an epidemiologist at George Washington University and former EPA official, doubts that Chen will order EPA to ban fluoride in drinking water. “I can’t imagine a court coming back and saying, ‘You did a risk assessment wrong, and here’s how you should have done it,’” she says. But the court might order EPA to consider fluoride as a toxic substance under TSCA and make it a high priority for a new evaluation to establish safe exposure limits.