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Many people take for granted the addition of fluoride into public drinking water systems that aims to . It’s a . But it’s not nearly as universally accepted as one might think.

At least seven cities or towns across the country debated it just this summer.

For example, , decided to add fluoride back into the water in July after the city council voted two years ago to remove it. Across the country in , voters will revisit a ballot question in November regarding whether to stop adding the mineral to the water supply.

“There has always been periodic discussion,” said , a dentistry professor at the University of Iowa. Levy is involved in an Iowa-based longitudinal study that tracks fluoride intake and its effects on children’s bones. “We are seeing more challenges now because of the communication explosion with the internet.”

The debate started well before 1945 when Grand Rapids, Mich., became the first U.S. city to add fluoride to its water supply. In the decades since, opposition usually stems from studies linking fluoride intake by children with lower IQs, higher rates of attention deficit/hyperactivity disorder and potential toxicity.

Still, fluoridation has become a fairly common practice, with about 74 percent of the population receiving fluoridated water from community water systems, according to the . But the intervention, which is considered by the CDC to be one of the 10 top public health achievements of the 20th century and backed by the American Dental Association and the World Health Organization, also continues to raise grass roots concerns. These range from casting fluoride as unnecessary and ineffective to efforts to paint the mineral as “mass medication” and a “damaging environmental pollutant.”

“Fluoridation is not safe or cost-effective,” said Bill Osmunson, director of the Fluoride Action Network, a national organization against fluoridation of water supplies, adding that people should be given the freedom of choice so they can avoid ingesting excess fluoride.

In Wellington, Mayor often fields angry emails on this issue.

“I watch the videos that they email me, I read the information they send me,” Gerwig said. Gerwig has no background in science, but she read studies and fact-checked the claims being made by the town’s residents. Gerwig said she decided to support fluoridation after she found scientific consensus about the benefits of fluoridation in preventing tooth decay.

The CDC, for instance, water fluoridation to be the most cost-effective method of delivering fluoride to all, reducing tooth decay by 25 percent in children and adults. is still one of the most common chronic conditions among children.

“A big thing about community water fluoridation is that it’s a passive intervention, you don’t really have to do anything other than drink tap water,” said Katherine Weno, oral health director at the CDC. “You don’t have to buy a product or access to a dental professional. It people who don’t have money to go to a dentist or don’t have any insurance.”

But some question the need for continued fluoridation, especially as products such as toothpaste and rinses containing fluoride are available, and because the chemical’s levels vary and indications of harm are not always clear.

, an adjunct professor at Harvard University School of Public Health, has authored a questioning the need for the added fluoride.

“Our dental health is clearly much less dependent on fluoride in drinking water than way back when this important public health intervention was initiated,” Grandjean said.

In a 2016 , Grandjean commented about the need for more research about populations that may be vulnerable to the mineral and the proper dose of it in drinking water. In response, the article drew multiple critical .

“The article misrepresents the current state of the science of community water fluoridation, and does not provide a fair and balanced perspective,” wrote Francis Kim and Scott L. Tomar from the American Association of Public Health Dentistry and Bruce Donoff, dean of the Harvard School of Dental Medicine in .

New studies are published almost every year that bring up concerns about fluoridation in drinking water, linking the intake with various and even , issues that Osmunson also brought up. Weno and Levy said those studies were performed in places where natural fluoride levels are higher and where residents may get fluoride through milk or salt rather than water. Excessive fluoride intake does have health implications —  commonly found in places with high concentrations of natural fluoride such as China, India and Africa. receive water with low natural levels of fluoride.

Health officials also monitor and review what is appropriate. The Department of Health and Human Services in April 2015 released new recommendations for fluoride levels in drinking water, updating and replacing the level in place since 1962 in order to reflect the fact that Americans now have more sources of fluoride in toothpaste, mouthwashes and other products.

But other towns continue to wrestle with the issue. In July, the commissioners of , voted to stop adding fluoride and  city council chose to leave it in. In August, , stopped fluoridation until voters decide in November 2017.

And for some of the local officials involved in these debates, their take on the issue is part of even greater political questions.

“The individuals who benefit the most are poor children,” said , mayor pro tem of Durango, Colo. The town to continue adding fluoride to its water. “If we get national health care for every single person, we could probably eliminate fluoridation in the water because we can ensure that every child is getting dental care.”