A group of Marin residents is petitioning the Marin Municipal Water District to remove fluoride from drinking water, but officials said they are bound by state law to fluoridate the county’s water supply.

A loose-knit group, concerned about the health risks of fluoride, has posted a petition on www.change.org asking the district to discontinue fluoridation. So far 766 people have signed the petition, with the goal of 1,000. In March the group packed a water board meeting to speak out against fluoridation; opponents also turned out Friday at the district’s operations committee meeting for an information agenda item on the practice.

The U.S. Centers for Disease Control and Prevention says water fluoridation “has been a safe and healthy way to effectively prevent tooth decay.” The organization has recognized water fluoridation as one of the “10 great public health achievements of the 20th century.”

But opponents say there is cause for concern.

“Anytime we subject somebody to the mandatory use of some compound or drug or poison or whatever you want to call this, that’s where the real impropriety is,” said San Rafael resident John Purdue, who opposes the use of fluoride. “When you look at the science, what has been discovered since the people voted it in is that fluoride does have efficacy if it’s applied topically, but the ingestion of it is detrimental.”

Purdue said it has been linked to thyroid issues and can cause fluorosis, which shows itself in tiny white specks or streaks on a tooth. The latter is a cosmetic condition.

The Marin Municipal Water District serves about 190,000 people between Sausalito and San Rafael. The North Marin Water District, which provides water to Novato and West Marin, doesn’t fluoridate its water and the issue has never come to a public vote.

Fluoridation at Marin Municpal began in December 1973 after voters gave approval in November 1972 with 57 percent approval. Opponents failed to block it in court action and through an appeal to the state Department of Health.

It was taken up again by voters in 1978 after water to five West Marin communities was overdosed accidentally with up to eight times the accepted level of fluoride for about two weeks in late 1977. In that vote, 53 percent of voters gave approval to continue fluoridation.

Opponents would like to see the water board remove fluoride from the system. Removal of fluoride would have to start at the ballot, but even then the district may have no choice but to fluoridate water, officials said.

In 1995 Assembly Bill 733 was passed into law. The law requires public water systems that have more than 10,000 connections to provide fluoridated water as long as they don’t use ratepayer dollars. The district receives about $1 million annually in rental income and uses $140,000 of that for fluoridation.

Because of the law, even if voters asked to stop fluoridation “MMWD would still be required to fluoridate under the health and safety code,” said Mary Casey, the water district’s general counsel, adding that the state attorney general could bring action against the water district to make it comply.

The water district is one of 18,400 U.S. water systems that fluoridate water. In 2010 about 66 percent of residents in the United States received fluoridated water.

Howard Pollick, a University of California at San Francisco dentistry professor and chairman of the Fluoridation Advisory Committee for the California Dental Association Foundation, said fluoridation is a good thing.

“It is because of the epidemic of tooth decay that we have been fluoridating in the United States,” said Pollick, who addressed the committee Friday. “Millions of people are now accessing fluoridated water.”

But the man who helped get water in Marin fluoridated in the early 1970s as an engineer for the water district now has a different opinion.

“We didn’t know any better back then, it was the thing to do,” said Dietrich Stroeh, who went on to serve as general manager for the water district. “There is a lot more known today about it. Why did other countries get rid of it? It should be reconsidered. Why take the risk?”