For 46 years, residents of Vancouver have been downing a fluoride compound with every glass of tap water they drink.

In Vancouver, Camas and Battle Ground, people consume fluoride in infant formula, with bourbon, in instant hot chocolate and tea. They also water their lawns with it, wash their clothes with it and flush it into streams where salmon spawn.

Across the river in Portland, and in the Clark County communities of Washougal, Ridgefield and La Center, the public water supply is fluoride-free.

Nationally, about 70 percent of Americans drink fluoridated water to prevent cavities and promote dental health.

Why the split? Why have some communities chosen to add minute amounts of a potent chemical — 1 part per million, or 1 milligram per liter — to their water supplies? Why have others resisted?

On one side of the great fluoride divide is trust. For more than a half-century, the American Dental Association, the U.S. Public Health Service and the medical establishment have promoted fluoridation of public water supplies as the most efficient and cost-effective way to reduce tooth decay. The federal Centers for Disease Control and Prevention counts it as one of the century’s most significant advances in public health and credits it with a sharp drop in tooth decay since the 1950s.

Vancouver voters approved fluoridation in 1962, ratifying a city ordinance approved in 1961.

“Fluoride is the first line of access to dental care and optimal dental health,” Mary Smith, president of the Washington State Dental Association, told state lawmakers in 2000. The Legislature that year rejected a bill that would have required all public water systems serving more than 1,000 homes to fluoridate.

The argument on the other side has to do with risk — whether fluoridation of drinking water is the most effective way to prevent tooth decay, and whether the potential risk of a fluoride overdose is worth the benefits in an era when fluoride is available in many forms that don’t involve mass medication.

U.S. cities began adding fluoride to their water in the 1950s and 1960s. Back then, that was the only way to deliver its benefits.

A half-century later, fluoride is available in toothpaste, children’s vitamins and topical rinses applied by dental hygienists, as well as in milk, fruit juices, soft drinks and children’s cereal.

Dr. Alan Melnick, Clark County’s health officer, defends fluoridation of public water as the one way to reduce tooth decay that is available to all, even children whose families can’t afford to provide them with regular dental care.

“You have public health methods that rely on behavior change and others that don’t,” Melnick said. “The ones that don’t require behavior changes are the most effective.”

How much risk?

A small but growing number of dentists and scientists, however, question whether it still makes sense to dose the populace with a chemical that has been linked in medical studies to brittle bones, bone cancer and damage to the kidneys and central nervous system. Overdosing with fluoride also can cause a condition called dental fluorosis, which results in the yellowing and mottling of tooth enamel.

Fluoridation advocates once thought that ingesting fluoride made teeth stronger. But the scientific consensus today is that its value as a cavity-fighter comes solely from its ability to inhibit the growth of plaque bacteria on the surface of teeth.

Fluoride coats the surfaces of teeth, said dentist Bill Osmunson, a fluoridation foe who practices in Bellevue and Lake Oswego, Ore., but “it does nothing for the grooves in a tooth. Fluoride is a little like suntan lotion. Putting it on the surface does a little bit of good.”

Last year, Oregon environmental groups and public health advocates succeeded in killing a bill in the Oregon Legislature that would have forced all drinking water systems serving more than 10,000 to fluoridate.

Dr. Alan Bates, an Oregon dentist who is also a Democratic state senator, argued against state-mandated fluoridation.

“What is clear is that water fluoridation is only effective as it passes over the teeth and is not effective as a result of swallowing and absorption,” Bates said in his testimony. Instead of mandating fluoridation, he suggested that the state subsidize topical fluoride rinses for all children who don’t have dental coverage.

“This recommendation is important because for low-income families in fluoridated communities, there is no practical alternative to using fluoridated tap water,” Bates said. “Scientific evidence tells us that any claimed benefits of water fluoridation do not justify the risk.”

Studies raise concern

Fluoride in higher concentrations is a potent poison. In 1952, the Alcoa aluminum plant in Vancouver was found to have illegally dumped up to 7,000 pounds of fluoride compound wastes into the Columbia River each month. The chemical contaminated the surrounding grass and forage and resulted in injury and death to cattle, according to a report in the Seattle Times.

Vancouver uses pure granular sodium fluoride to treat its water supply.

But the fluoride added to most public drinking water supplies, a compound called hydrofluorosilic acid, comes from phosphate fertilizer plants, where it is stripped from smokestack scrubbers that also capture trace amounts of heavy metals such as lead, arsenic and radium. If it weren’t put into drinking water, it would be treated as toxic waste.

Employees who work with fluoride must wear protective clothing and respirators.

A 1998 study by the International Society for Fluoride Research, published in the journal Brain Research, found that laboratory rats given fluoridated drinking water at the same concentrations used in public water systems suffered brain and kidney damage.

In 1999, a union representing 1,500 scientists, lawyers, engineers and other professional employees of the U.S. Environmental Protection Agency called for an end to fluoridation of drinking water, saying the practice “borders on a criminal act” on the part of government.

EPA scientists cited scientific literature documenting “the increasingly out-of-control exposures to fluoride,” the lack of benefits from ingesting fluoride and the potential hazards to human health, including the risk of kidney damage, gene mutations, cancer, reproductive effects, damage to bones and the nervous system, and dental fluorosis.

In 2006, Harvard researchers published a multiyear study reporting that exposure to fluoride at levels considered optimum by the American Dental Association increased the risk of bone cancer in boys by more than 500 percent.

Also in 2006, a report from the National Research Council recommended that the EPA lower its standard for naturally occurring fluoride, which is currently 4 parts per million per liter of water. The council cited adverse impacts including increased bone fractures and damage to neurological, immune and reproductive systems.

Based on that report, the American Dental Association issued a new warning against the use of fluoridated water in infant formula, which was echoed by the U.S. Centers for Disease Control.

Melody Scheer, oral health coordinator for Clark County, defends the safety of water fluoridation, but she says the wide availability of fluoride supplements in other forms does pose a risk.

“If a child swallows toothpaste, or takes too many vitamin pills, that is where you can get an overdose,” she said.

How effective?

Fluoridation opponents also question whether fluoridation of drinking water has been as effective in preventing cavities as the ADA and public health officials claim.

The 2003 National Survey of Children’s Health, conducted by the U.S. Department of Health and Human Services, gathered information on dental health and family income in all 50 states.

It found that the best predictor of good dental health was a history of preventive care in the previous 12 months.

Opponents have used that data to point out that in Washington, where 59 percent of residents drink fluoridated water, 71.5 percent in the 2003 study had very good to excellent teeth. But in Oregon, just 19.4 percent of residents drank fluoridated water, yet 68.4 percent of children in the study had very good to excellent teeth.

“Oregon has as good or better dental health than Washington,” dentist Osmunson said.

Clark County’s Melnick questions the study’s methodology, noting that parents self-reported their children’s dental health information. “My goal is to get the right information out there,” he said.

Osmunson was a believer in water fluoridation for 25 years. He even urged local water districts in Idaho to start adding fluoride.

After moving to Oregon, he started looking at the research. “One of the things that woke me up was the lack of tooth decay in Lake Oswego,” a city that did not fluoridate its water, he said.

Osmunson no longer supports water fluoridation. He doesn’t even recommend fluoridated toothpaste for his patients. There are too many unanswered questions, he said.

“We know fluoride can cause an increase in fractured bones,” he said. “Can it also cause an increase in fractured teeth, and did fluoride contribute? No one has studied that.”

Insert: Did you know?

The fluoride ion comes from the element fluorine, which exists only in combination with other elements. Small amounts of soluble fluoride ions are present in all water sources, including the oceans.

Fluoride attacks bacteria on the surface of teeth by reducing their ability to produce acid.

The U.S. Food and Drug Administration considers fluoride a drug, not a mineral nutrient.

Sodium fluoride is toxic in drinking water at 4 parts per million, making it more toxic than lead and almost as toxic as arsenic, according to the U.S. Environmental Protection Agency. Most public water supplies add fluoride at 1 part per million.