It’s been called one of the ten greatest public health achievements of the 20th century by United States Centers for Disease Control and Prevention.

In 2012, 74.6% of the U.S. population had access to fluoridated water.

Fluoridated water reduces the demineralization of tooth enamel and increases remineralization, thereby reducing tooth decay. In low concentrations, fluoride is naturally present in food and drinking water, and the practice of increasing levels of fluoride through water fluordation, which began in 1945, has been shown to smooth inequalities in the health care system by reducing the incidence of cavities by as much as 40%.

But a curious thing is happening in communities across Canada, where the decision to fluoridate or not fluoridate is a municipal decision. The number of Canadians that have access to fluoridated water has now fallen to less than a third. Places like Waterloo, Thunder Bay, Quebec City, and Calgary have all recently decided against the practice.

Opponents of water fluoridation are a vocal and mobilized faction here. The group Canadians Opposed to Fluoridation has successfully lobbied councils across the country, arguing that fluoridation is a dangerous practice that may cause “Alzheimer’s like symptoms”, an “epidemic of hypothyroidism”, Cancer, and a decline in I.Q.

The trouble is, there is no peer reviewed study backing these claims up. Not for hyperthyroidism. Not for Alzheimer’s. And not for Cancer, Down’s Syndrome, mortality, senile dementia, goitre and IQ levels, either.

The anti-fluoride crew has been remarkably successful in spooking municipalities across the country, but not all of them. In Red Deer, Mayor Morris Flewwelling notes that the city spent two years and $75,000 reviewing the issue before deciding to keep the practice of water fluoridation, even though nearby cities bowed under the pressure of special interest groups.

“I think we looked at Calgary and we felt their process was hurried and flawed,” he told Macleans.

The main thing anti-water fluoridation backers play fast and loose with is dosage. Fluoride is toxic, they will tell you. And they are right, at high levels it is toxic. But so is iron. And salt. And oxygen. And water. There is simply no scientific evidence that the concentrations used in water fluoridation, 0.7 to 1.2 ppm, are toxic.

Still, groups like Canadians Opposed to Fluoridation are part of a long history of skepticism behind the science associated with the practice.

In 1958, journalist Gordon Sinclair railed against the practice, referring to fluoride as “rat and bug poison”.

“I’m a writer and a broadcaster, not a medical man, dentist, nutritionist, or anything else that qualifies me as an expert,” declared Sinclair. “But I am a mature citizen who opposes the dumping of this chemical into the water”.