Nearly a half-century after water fluoridation became widespread, a small but growing number of medical officials and environmentalists are again raising concerns over the practice.
Recent research is suggesting that fluoride may be connected to a number of serious conditions, including the development in teenage boys of osteosarcoma, the rare bone cancer that killed Canadian icon Terry Fox, reduced intelligence levels in children, and impaired thyroid function.
In Waterdown, Ont., a suburb of Hamilton, Cindy Mayor has approached her city council and asked that it stop fluoridation for the area's 500,000 residents. "Here we are, mass-medicating with a drug," she says. The activist frets that fluoridation could be one reason for the growing number of people being treated for lowered thyroid hormone levels.
She isn't alone in worrying about fluoride, placed in many municipal water supplies to make teeth more resistant to decay.
While those complaining about fluoride were often portrayed as a kooky fringe - typified by the 1964 movie classic Dr. Strangelove, in which a demented U.S. general feared fluoridation was a communist plot - fluoride criticism has recently gone mainstream.
Although the research linking the chemical with serious conditions is disputed, critics of fluoridation say that at the least it indicates a review of the practice needs to be conducted.
A review is even more pressing, in the view of critics, because scientists now believe that the main protective action from fluoride does not come from ingesting the chemical, with the teeth absorbing it from inside the body, but from direct absorption through topical application to teeth.
This means swallowing water is a far less effective way to fight cavities than brushing with fluoridated toothpaste. That may explain the steep decline in cavity rates observed in industrialized countries since the 1970s, irrespective of whether they fluoridate water. Almost all of Europe does not, and yet has seen a sharp reduction in dental caries.
"I think there is a much broader understanding that there might be some legitimate concerns with fluoride," said Richard Wiles, executive director of the Washington-based Environmental Working Group, a public-interest group that has been lobbying the U.S. National Institutes of Health to provide a second opinion on fluoridation.
About 13.5 million Canadians, or about 43 per cent of the population, live in communities with fluoridated tap water, but almost no fluoridation is done in British Columbia or Quebec, according to Health Canada.
The EWG worries that the public is being overexposed to fluoride, and says water is the easiest source to eliminate.
Fluoridation is based on research from the 1940s, and Mr. Wiles contends that it wouldn't be able to pass a modern risk assessment used for drugs or pesticides.
"We took a look at the science and it was really apparent to us that the current levels of fluoride exposure were unsafe," he said.
The view on fluoride's potential downside is rejected out of hand by Health Canada, as well as the Canadian Dental Association.
"The fluoridation of drinking water supplies is a well-accepted measure to protect public health that is strongly supported by scientific evidence," Health Canada said in an e-mailed statement.
Nonetheless, the department said it is currently studying the recent scientific findings and may adjust the amount it recommends for water.
The Canadian Dental Association also endorses fluoridation. "It's among the greatest public-health measures that has ever been put in place, right up there with vaccination," said Darryl Smith, president of the association.
Dr. Smith worries that if fluoridation critics have their way, it will lead to a loss in the hard-won gains against tooth decay. Currently, about half of the children in Canada younger than 11 don't have cavities.
Although health authorities are confident fluoridation is a good idea, they haven't been very good at picking the optimum dose.
Many jurisdictions have, with little fanfare, recently cut the amount they add, to minimize chances that children will get dental fluorosis, or mottled teeth.
The cuts have been substantial enough to suggest that previous levels to which the public was exposed for more than three decades were too high. Toronto's drinking water, after several reductions, now contains half the fluoride it did before 1999, while the province of Quebec cut the recommended amount by 42 per cent in 2004.
Fluorosis, if severe, causes unsightly staining of teeth, but in mild cases the result is white streaking that many dentists consider cosmetic. During the 1990s, anywhere from 20 per cent to 75 per cent of children were afflicted in fluoridated areas.
Both Toronto and Quebec fluoridate below the level Health Canada believes is optimum: 0.8 to 1 part per million fluoride to water. Toronto is at 0.6 ppm.
Among the recent studies, the most worrisome is the possible association with childhood osteosarcoma.
The disease, the cause of which is unknown, is fatal in about one-third of cases and almost always leads to amputations.
The research indicates that fluoride exposure among boys, but not consistently among girls, during a critical period of bone growth from age 5 to 10 makes them more susceptible to the bone cancer during their teenage years.
Scientists are on the lookout for effects on bones, because they absorb half the fluoride people ingest.
A paper outlining the finding was published in 2006 in the journal Cancer Causes & Control and produced by Harvard University researchers.
Researchers found that boys aged 6 to 8 who were exposed to more fluoridated water were about four times more likely to develop the cancer than those exposed to lower levels. The researchers called their results "remarkably robust."
Although similar findings in young male rodents have been seen in laboratory experiments, other studies that investigated lifetime human exposure to the chemical did not detect any association with osteosarcoma.
The EWG's Mr. Wiles said these findings are the kinds of research clues that should cause governments to consider listing the chemical as a probable carcinogen.
