Counsel for the EPA noted that they challenged the plaintiffs’ subpoena of Thayer, but lost that challenge.

Thayer, who previously worked at the National Toxicology Program, said the human brain is more vulnerable to toxic agents at infancy than adulthood because infants don’t have fully developed blood-brain barriers.

Thayer’s testimony came on the third day of a bench trial before U.S. District Judge Edward M. Chen in a lawsuit that Food & Water Watch and other groups filed in April 2017. The suit aims to stop the practice of fluoridating water nationwide under the Toxic Substances Control Act, and it seeks a declaration from the court that fluoride in tap water poses an unreasonable risk of harm to humans.

Fluoride started being added to water in the 1940s to improve people’s dental health, but the plaintiffs say it has since been proven that topical application, not ingestion, is the most effective way to use fluoride and that ingestion actually causes a host of health problems.

During the trial’s opening arguments Monday, the government argued that the studies championed by the environmentalists are unreliable, biased and have too many inconsistencies to form conclusions about fluoride’s health risks.

The nonprofits have called a slew of scientists to the stand to testify on their research on the neurotoxicity of fluoride, including a Harvard Universityepidemiologist on Tuesday who warned that fluoride is a human developmental neurotoxin that is lowering children’s IQ levels.

On Wednesday, plaintiffs also called back to the stand University of Washington professor and physician Dr. Howard Hu, to finish his testimony that he began on Monday regarding an epidemiological study he conducted that looked at the impact of fluoride consumption on newborns and children by testing their mothers’ urine before they were born and then later testing the children’s IQ scores.

Hu said his study “absolutely” found fluoride exposure to have a negative impact on young children, but declined to weigh in on policy recommendations, saying he didn’t want him or his work to be seen as biased. He said he continues to study fluoride exposure and will report on the studies no matter what the results.

Plaintiff’s expert witness Dr. Bruce Lanphear, a professor of health sciences at Simon Fraser University in Canada, also testified Wednesday that higher exposure to fluoride during pregnancy “was associated with diminished IQ scores in children at 3 to 4 years of age.”

Lanphear, whose research on environmental contaminants has been funded in part by the EPA, testified that he was not being compensated for his work in the legal case, but that he believed it was part of his public service duty to participate in it.

“We’ve allowed children with rapidly growing brains to be exposed to toxins,” Lanphear told the court.

Witness Dr. Kathleen Thiessen, a scientist with the Oak Ridge Center for Risk Analysis, whose clients often include federal government agencies, said she worked on the National Research Council’s 2006 scientific review on water fluoridation and human health.

Thiessen testified that the NRC’s review concluded that fluoride exposure can reduce thyroid function, which is especially of concern for pregnant women as thyroid function can reduce the proper development of their offspring.

Thiessen said that in order to set a safe fluoride exposure level, scientists must first identify a level at which there is no observed adverse effect detected.

The nonprofits and individuals, using the Toxic Substances Control Act’s citizen suit provision, are asking the court to overturn the EPA’s rejection of their 2016 petition to ban fluoride in drinking water.

The case will test citizens’ and nongovernmental organizations’ power to force the agency to regulate chemicals it has deemed to be safe based on how a judge assesses the scientific studies and expert testimony that both sides will present at trial. If the plaintiffs are successful, it could embolden others to attempt similar litigation.

The parties did not immediately respond to requests for comment Wednesday afternoon.

Food & Water Watch and the other plaintiffs are represented by Michael P. Connett of Waters Kraus & Paul LLP.

The EPA is represented by Debra Carfora, John Do and Brandon Adkins of the U.S. Department of Justice‘s Environment and Natural Resources Division.

The case is Food & Water Watch Inc. et al. v. EPA et al., case number 3:17-cv-02162, in the U.S. District Court for the Northern District of California.

–Additional reporting by Dorothy Atkins and Juan Carlos Rodriguez. Editing by Jay Jackson Jr.