The night before the final day of plaintiffs’ testimony in a trial that could end drinking water fluoridation in the U.S., Canadian researchers published a new systematic review linking fluoride exposure, at very low levels, to lower IQ in children.

Canada’s public health agency, Health Canada, commissioned a team of scientists to study the effects of fluoride on human health, but the agency did not publish the review.

The peer-reviewed journal Critical Reviews in Toxicology independently published the study on Tuesday.

The researchers calculated the toxicological “point of departure” for the effects of fluoride on IQ — also known as the “hazard level,” the lowest point at which a toxic effect is observed — and found it to be 0.179 milligrams per liter (mg/L) in water.

Levels of fluoride found in drinking water in the U.S. and Canada typically are in the higher range of 0.7 mg/L.

The National Toxicology Program (NTP) report linking fluoride exposure and lower IQ in children set the hazard level at 1.5 mg/L, and one of the key studies at the center of the trial set the level even lower than 0.2 mg/L.

The authors of the new study did note the data at that low range had significant uncertainty and would benefit from further data collection.

Food & Water Watch, Fluoride Action Network, Moms Against Fluoridation and other advocacy groups and individuals are suing the EPA in a bid to force the agency to prohibit water fluoridation in the U.S. due to fluoride’s toxic effects on children’s developing brains.

The lawsuit is being brought under the Toxic Substances Control Act (TSCA), which regulates the legal levels of human exposure to toxic substances like lead.

All of the other toxic chemicals regulated under TSCA have acceptable human exposure levels set at least 10 times and up to 79 times the hazard level to account for uncertainty in identified hazard or exposure levels, particularly for the most vulnerable people.

Even at a hazard level of 1.5 mg/L, exposure levels for fluoride carry significant risk under TSCA’s guidelines, but this new level identified by Canadian researchers would set a risk level even further below current exposure levels.

The findings are important to the trial because the identified hazard level was quite low and also because the authors calculated their hazard level in terms of water fluoridation levels, which they extrapolated from the urinary fluoride levels used in most studies. Whether and how that can be done has been a significant point of debate at trial.

The findings also are significant because David Savitz, Ph.D., professor of epidemiology at Brown University and the EPA’s first witness, was part of the expert panel that advised Health Canada on how to interpret this study and other data.

The expert panel concluded there wasn’t enough evidence to lower the amount of fluoride in drinking water based on its neurocognitive effects.

The EPA called Savitz as a witness because he was also part of the National Academies of Sciences, Engineering, and Medicine (NASEM) expert panel that reviewed the NTP report linking fluoride exposure to lower IQs in children. The report is a central piece of evidence in the trial.

NASEM and the NTP authors did not agree on all changes to the report suggested in NASEM’s review, and the final changes were eventually adjudicated by the NTP’s Board of Scientific Counselors, although the report has still not been published.

Savitz will also testify about Health Canada’s interpretation of these findings.

Health Canada’s decision to commission the study and revisit and affirm its existing water fluoridation standards happened while the NTP’s report findings were being debated, and allegedly suppressed, by U.S. public health agencies.

Before Wednesday’s testimony began, plaintiffs moved to put the new study into evidence. EPA attorneys countered, arguing that Health Canada’s expert panel summary report from June 2023 — in which the agency concluded current drinking water levels for fluoride were likely safe — should be entered into evidence instead.

The EPA argued that Savitz had not read this new peer-reviewed document. He had only read and evaluated the unpublished and confidential report submitted to Health Canada, and would not be able to speak to the new publication.

Federal Judge Edward Chen, who will rule on the case, accepted both documents into evidence. He also ordered the EPA to obtain a copy of the unpublished report submitted to Health Canada, if possible.

Study found IQ loss, dental fluorosis were most significant issues

The newly published study systematically reviewed evidence from human, animal and in vitro data on the epidemiological and toxicological effects of fluoride in drinking water using the “Bradford Hill” criteria.

Bradford Hill criteria are a set of epidemiological principles used to identify causal relationships between a toxin and its associated effects.

The review included the studies assessed by the NTP’s systematic review of the literature, along with more recent studies.

The review also examined fluoride’s epidemiological effects beyond the narrow question of IQ loss among children and found strong evidence linking fluoride exposure to dental fluorosis.

It also linked fluoride exposure to other problems, including thyroid dysfunction, kidney dysfunction and hormone disruption, with varying strengths of evidence.

However, the authors established that IQ loss and dental fluorosis were the most significant issues of concern and calculated hazard levels for those two items.

They recommended a 1.56 mg/L hazard level for moderate dental fluorosis and concluded that “precautionary concerns for potential neurodevelopmental cognitive effects may warrant special consideration” for setting acceptable water fluoride levels.