A proposal that went before a legislative panel on Monday would outlaw the practice. Filed by Sen. Barbara L’Italien, D-Andover, on behalf of a constituent, it would end a decades-old public health campaign aimed at preventing tooth decay.

“Medical mandates like this are politics pretending to be science,” said Karen Spencer, a Gloucester activist who is involved with anti-fluoridation campaigns. “No government has the right to put something into our water supplies that harms people’s health. It’s immoral.”

Spencer, one of several people who supported the ban during the hearing on Monday, said studies are increasingly linking negative health impacts — such as gastrointestinal ailments, thyroid disorders, kidney diseases and even autism — to fluoride added to the water.

But Dr. John Fisher, a Salem dentist and a trustee of the Massachusetts Dental Society, said the health benefits are indisputable.

“Drinking fluoridated water keeps teeth strong and reduces cavities by about 25 percent in children and adults,” he told the Joint Committee on Public Health. “Community water fluoridation has not only proven to reduce dental decay but also lowered dental costs, leading to outcomes that have important impact on overall health.”

Fisher, chairman of the Better Oral Health for Massachusetts Coalition, said research has shown that fluoridated water protects teeth without posing health risks. The American Dental Association, American Medical Association and National Cancer Institute endorse the practice.

Fisher said the internet is awash with misinformation about water fluoridation.

“There’s a lot of distortions out there,” he said. “I think these are generally good people who believe what they say, but if they actually read the studies they might be more inclined to accept the evidence in support of community water fluoridation as safe, efficient and effective.”

Who has fluoride in water

At least 140 public water systems in the state — serving more than 4 million people — add fluoride compounds, according to the Department of Public Health. Most have done so since the 1950s.  The Massachusetts Water Resources Authority — which serves more than 2.5 million people across 61 communities including Marblehead, Peabody, Swampscott and Lynn — has fluoridated the water for more than 30 years.

Water in Essex has been fluoridated since 1970, Gloucester since 1981 and Manchester since 1983. Half the fluoride in Rockport’s water occurs naturally, according to the state Department of Health; the other half has been added since 1984.

Concerns about the health effects have kept several towns on the North Shore and Merrimack Valley — ­ including Georgetown, Merrimac and Rowley — from doing so.

Other communities, such as Rockport and Gloucester, have voted to keep using it.

The practice dates to the 1940s, when scientists discovered that people whose water supplies had higher concentrations of the naturally occurring chemical experienced less tooth decay.

After World War II, health officials embarked on a public campaign to add fluoride to the public water supply, despite fierce resistance. At one point in the ’50s, some considered such programs part of a communist plot.

Many fluoride treatments these days come from industrial byproducts produced by aluminum companies, which sell fluoride compounds in liquid or powder form wholesale for about $1.10 per pound. It’s approved for human consumption by the Food and Drug Administration.

“This stuff isn’t being manufactured in a pharmaceutical lab and dispensed by a doctor,” said Chris Martel, an anti-fluoridation activist from Amesbury. “It’s an industrial byproduct scrubbed out of factory smokestacks in China and indiscriminately dumped into our water supplies.”

Amesbury stopped adding fluoride in 2009 due to concerns about its quality. Two years later, residents voted to end the practice entirely.

“If you want a fluoride treatment, get it from your dentist,” Martel said. “Introducing this into the municipal water supply makes no sense.”

Limits for fluoride have fallen

Concerns about the detrimental effects of over-fluoridation prompted the U.S. Centers for Disease Control and Prevention in 2011 to lower the recommended level for fluoride in water supplies to 0.7 milligrams per liter.

It lowered the recommended level again four years later.

The changes were prompted, in part, by a federal study suggesting that too much fluoride was streaking children’s teeth and a recognition that many Americans get fluoride from sources other than drinking water.

The MWRA and other communities lowered their fluoride levels.

In 2006, the National Academy of Sciences recommended the U.S. Environmental Protection Agency reduce its maximum fluoride level in drinking water to below 4 milligrams. It warned that a lifetime of drinking water with fluoride at that concentration could raise the risk of broken bones.

At present, 70 percent of Americans get water from systems that add fluoride, according to the U.S. Department of Health and Human Services.

The CDC has called the fluoridation of water supplies to reduce cavities “one of the leading public health achievements of the last century.” But 200 municipalities nationwide have stopped the practice in the past decade.

Most of the country’s major cities still use fluoride, but at least one, Portland, Oregon, voted in 2013 to discontinue its use.

Christian M. Wade covers the Massachusetts Statehouse for North of Boston Media Group’s newspapers and websites. Email him at cwade@cnhi.com.