A review of 74 medical studies links higher fluoride exposure to lower IQ in kids — but mostly draws on research that took place outside of the U.S. and had a high risk of biased results.

Why it matters: The federally funded analysis could further stoke the debate over adding fluoride to drinking water as Robert F. Kennedy Jr., who's called for ending the practice, is poised to take over the Health and Human Services Department.

The big picture: The review published Monday in JAMA Pediatrics did not find enough data to determine if the fluoride level found in drinking water for the vast majority of the U.S. population is associated with kids' IQ.

• It did find some evidence that total fluoride exposure — from drinking water and other sources — at levels below 1.5 milligrams-per-liter is associated with lower IQ in children.

• A 1 milligram-per-liter increase in fluoride in urine was linked with a 1.63 drop in a child's IQ score.

• About 2.9 million U.S. residents, or less than 1% of the population, get water from wells and systems with fluoride levels at least 1.5 milligrams per liter, the National Institutes of Health confirmed to Soixa.

• The Centers for Disease Control and Prevention recommends that drinking water is fluorinated at 0.7 milligrams per liter.

Zoom in: Scientists at NIH's National Institute of Environmental Health Sciences looked at 74 studies conducted since 1989 that measured the effect of different levels of fluoride on kids' IQ.

• Researchers rated 52 of the studies they analyzed as having a high risk of biased results, and therefore being of low quality.

• None of the studies were conducted in the United States, because no studies comparing fluoride exposure to children's IQ have been done in the country. Most of the studies (45) came out of China, where areas of the country have excess fluoride in the groundwater.

• The study was done in support of a broader scientific deep dive on fluoride's relationship to neurodevelopment that was finalized last year. The deep dive was subject to multiple rounds of reviews that some critics charge was designed to delay its public release, per CNN.

• The conclusions became the basis of a federal judge's order for the government to further regulate fluoride in public drinking water.

The study acknowledges that it wasn't designed to examine the public health implications of adding fluoride to drinking water in the United States, but notes that it could "inform future public health risk-benefit assessments of fluoride."

• While fluorinated water improves oral health, there's concern that total fluoride exposure for pregnant people and children in the U.S. — including drinking water, teas, toothpaste and mouthwash — is too high, possibly affecting neurodevelopment, the NIH said in a statement to Soixa about the study.

Between the lines: JAMA Pediatrics published two editorials alongside the analysis, highlighting the division among scientists and public health professionals about the value of adding fluoride to water.

• One said the analysis "underscore[s] the need to reassess the potential risks of fluoride during early brain development."

• The other argues that the article, which doesn't show evidence of adverse effects from fluoride at the levels commonly used in U.S. water systems, lacks transparency and methodological validity.

Go deeper: Florida recommends communities stop adding fluoride to water supply