(NEXSTAR) – On Monday U.S. Health Secretary Robert F. Kennedy Jr. is calling for an end to fluoridation in American communities, but when – and why – did the practice start?

Grand Rapids, Michigan became the first city in the world to add fluoride to drinking water in 1945 as part of a study first sponsored by the U.S. Surgeon General, then taken over by the National Institute of Dental and Craniofacial Research (NIDR).

The 15-year study of nearly 30,000 schoolchildren found that the cavities rate dropped more than 60 percent, according to the NIDR.

“This finding, considering the thousands of participants in the study, amounted to a giant scientific breakthrough that promised to revolutionize dental care, making tooth decay for the first time in history a preventable disease for most people,” according to the NIDR website.

In 1950, federal officials endorsed water fluoridation to prevent tooth decay, and in 1962 they set guidelines for how much should be added to water.

State and local governments decide whether to add fluoride to water and, if so, how much — as long as it doesn’t exceed a maximum set by the EPA, which is currently 4 milligrams per liter.

For decades, there has been some speculation about the safety of fluoride in drinking water – especially at higher levels.

Kennedy, a former environmental lawyer, has called fluoride a “dangerous neurotoxin” and said it has been associated with arthritis, bone breaks and thyroid disease. Some studies have suggested such links might exist, usually at higher-than-recommended fluoride levels, though some reviewers have questioned the quality of available evidence and said no definitive conclusions can be drawn.

On Monday, the U.S. Environmental Protection Agency announced it is reviewing “new scientific information” on potential health risks of fluoride in drinking water. The EPA sets the maximum level allowed in public water systems.

Fluoride can come from a number of sources, but drinking water is the main one for Americans, researchers say. Nearly two-thirds of the U.S. population gets fluoridated drinking water, according to CDC data. The addition of low levels of fluoride to drinking water was long considered one of the greatest public health achievements of the last century.

About one-third of community water systems — 17,000 out of 51,000 across the U.S. — fluoridated their water, according to a 2022 CDC analysis. The agency currently recommends 0.7 milligrams of fluoride per liter of water.

“The primary reason for this current recommended level is that there’s a recognition that people are also being exposed to fluoride in other ways. For example, we have widespread fluoride in toothpaste,” Dr. Bruce Dye, a professor at the University of Colorado School of Dental Medicine, told UCHealth. “So, we want to maximize the benefit for cavity prevention while minimizing dental fluorosis.”

But over time, studies have documented potential problems. Too much fluoride has been associated with dental fluorosis, streaking or spots on teeth. Studies also have traced a link between excess fluoride and brain development.

A report last year by the federal government’s National Toxicology Program, which summarized studies conducted in Canada, China, India, Iran, Pakistan and Mexico, concluded that drinking water with more than 1.5 milligrams of fluoride per liter — more than twice the recommended level in the U.S. — was associated with lower IQs in kids.

The American Dental Association said decades of fluoride in drinking water have been shown to reduce tooth decay. The group said it was willing to help conduct high-quality studies to settle the issue.

“When government officials like Secretary Kennedy stand behind the commentary of misinformation and distrust peer-reviewed research, it is injurious to public health,” said the association’s president, Brett Kessler.

The Associated Press contributed to this report.