The Centers for Disease Control and Prevention proclaimed it one of the 10 greatest public-health achievements of the 20th century.

Every U.S. surgeon general since the 1950s has endorsed the practice. And the American Dental Association calls it the "single most effective public-health measure to prevent tooth decay."

But despite overwhelming support from the government and medical professionals, the debate over fluoridating public water systems, which largely subsided after the Cold War, has resurfaced.

Phoenix has become the latest Valley city to reassess the controversial cavity-fighting policy as several City Council members are proposing to save money or limit government's reach by discontinuing water fluoridation in the nation's sixth-largest city.

A change would affect the more than 1.4 million Phoenix residents who use the city's fluoridated tap water for drinking, bathing and brushing their teeth.

Anti-fluoride activists have continued to rally against municipal fluoridation of water for decades, worried that overdoses could lead to weight gain and muscle pains from thyroid problems, discoloration of teeth and other yet-to-be-discovered side effects. They also worry that ingesting too much fluoride could damage bones the same way it has been found to eat away at tooth enamel.

Supporters, however, insist that adding fluoride to municipal water supplies provides a major benefit to the oral health of residents, particularly in poor areas, where residents might fail to properly care for their teeth.

Phoenix is one of the largest cities in the nation to reconsider water fluoridation and the first Arizona city to review it since Page rejected the practice in 2006. Before that, Flagstaff rejected the policy in 2001.

The year before Flagstaff rejected fluoride, Gilbert voters went the opposite direction. About 54 percent of 28,000 voters approved adding fluoride to the water system.

Decades of fluoride

Phoenix officials voted to add fluoride to the city's water in 1989.

City Councilman Tom Simplot said it's time for the city to review the scientific and practical aspects of spending money to fluoridate city water. He also questions whether it's a waste of tax dollars if residents are consuming bottled water instead of city tap water.

"Science evolves, and if we haven't studied this for 20 years, we should to make sure science hasn't passed us by," Simplot said. "In light of our budget issues, let's take the time to see if it's a good, continued, wise investment."

Each year, Phoenix spends about $582,000, or about 39 cents per resident, on fluoridating the city's water supply, according to Randy Gottler, deputy water-services director for environmental services in Phoenix.

There already is naturally occurring fluoride in the water, and Phoenix adds more to match the U.S. Environmental Protection Agency's standard of 0.7 part per million, equal to a little more than two drops of water in a 55-gallon barrel, Gottler said.

Arizona Department of Health Services Director Will Humble said 10 cities and towns in the state add fluoride to their water supplies: Bisbee, Chandler, Gilbert, Glendale, Guadalupe, Mesa, Peoria, Phoenix, Tempe and Yuma. Other communities, such as Scottsdale, don't because they may use water sources with naturally occurring fluoride.

Humble said water fluoridation reduces cavities by up to 40 percent for areas with low levels of fluoride, saving an average of $38 per person per year in dental costs. National statistics show it costs 95 cents a person annually to fluoridate water.

"On the savings end, it's about a 40-to-1 payoff," Humble said.

Health questions

Critics say preventing cavities is not enough of a benefit when weighed against the unknown side effects of ingesting too much fluoride.

In years past, public-fluoridation opponents were on the fringe. Some insisted water fluoridation was part of a communist conspiracy to undermine the health of Americans or a government plot to control people's minds.

Today, anti-fluoride activists base their opposition on scientific research linking overdoses of fluoride to thyroid problems, discoloration of teeth, joint pain and bone problems similar to arthritis.

Phoenix resident Jody Clute started researching how fluoride affects the body after doctors diagnosed her with hypothyroidism in 2004. Scientific studies suggest that fluoride inhibits the absorption of iodine, a mineral the thyroid gland needs to regulate the body's metabolism. Those with thyroid problems can experience hair loss or severe weight gain or weight loss.

"Dentists are not endocrinologists," Clute said. "They are telling people to drink something that should be applied topically."

Clute and other anti-fluoride activists, including groups such as the Fluoride Action Network and Mothers Against Fluoridation, became more assertive after the EPA in 2011 announced the maximum threshold for fluoride levels in public water should be reduced to 0.7 milligram per liter instead of 1.2 milligrams per liter. The EPA amended its standards after noticing rising rates of dental fluorosis, a breakdown in tooth enamel that discolors or creates white spots on teeth.

Dental fluorosis results when children younger than 8 consume too much fluoride as their teeth are developing, according to the CDC. The EPA lowered the levels to account for fluoride that people get from toothpaste, mouthwashes and other dental products that weren't available when mass fluoridation of water began in the 1940s.

Reversing course

In the last 20 years, more than 300 communities in the United States and Canada stopped adding fluoride to public water supplies or rejected the practice completely, according to the Fluoride Action Network.

In 2001, Flagstaff voters rejected for the third time attempts by the city to fluoridate water, with 58 percent voting no.

The issue was so heated, turnout was 12 percentage points above what it was in the City Council election the year before, according to the Arizona Daily Sun.

Nationally, smaller cities and towns such as Pottstown, Pa., or Lawrenceburg, Tenn., have stopped fluoridation.

Now, larger municipalities are thinking twice. Last year, Pinellas County, Fla., stopped fluoridating water for nearly 1 million residents by a 4-3 vote of county commissioners. And in New York City, Councilman Peter Vallone has pushed for legislation since 2010 to get rid of fluoride in water serving 8.2 million residents.

Gary O. Jones, a Mesa family dentist and Arizona Dental Association president, said the EPA's latest change shows there is regular scientific review of water fluoridation standards to ensure that it's safe.

The major public-health benefit of fluoridating water comes for lower-income families who can't afford dental care, Jones said. Preventing cavities before they form is the key.

"The concerns are comparable to me as people looking at not immunizing children anymore," said Jones, who was part of the movement to fluoridate water in Mesa in the 1990s. "We have 65 years of research that backs up the fact that it doesn't cause any health problems."

California dentist Bill Osmunson, one of the most vocal anti-fluoridation activists in the country, said someday the practice will be considered "one of the 20th century's greatest public-health blunders."

Osmunson, a spokesman for the Fluoride Action Network and a dentist for 35 years, said he promoted fluoridation for the first 25 years of his career but changed his tune after patients begged him to research the issue. What he found was like a "knee in the gut."

He said people drink different amounts of water daily, which means some people drinking tap water are consuming far more fluoride than others. "How can you give a drug to everybody in a dosage that's not regulated?" Osmunson asked.