Federal agencies announced Friday they are planning to lower the recommended concentration of fluoride in drinking water, prompting at least one major Vermont water supplier to follow suit and re-igniting questions about the use of fluoride in the state’s largest city.

Officials with the U.S. Department of Health and Human Services and the U.S. Environmental Protection Agency are proposing to lower the amount of fluoride in drinking water to about 0.7 milligrams per liter of water. For the past 50 years, the federal government had suggested a range of fluoride concentration between 0.7 and 1.2 milligrams per liter.

Fluoride has been in the nation’s drinking water for decades — in some cases since the 1940s — as a way to combat tooth decay. The effort worked. Government scientists said the prevalence of tooth decay in at least one tooth has fallen from about 90 percent to 60 percent, mostly because of fluoridation.

However, people have raised concerns about fluoride. The federal agencies propose lowering the recommended amount of fluoride in water in large part because of an increase in something called fluorosis.

Fluorosis is a spotting and streaking of teeth, especially in children and teenagers, likely from too much exposure to fluoride. While rarely more than a minor cosmetic problem, in extreme cases fluorosis can leave teeth pitted and damaged.

Other health effects are debated much more hotly. Critics say fluoride can cause brittle bones, neurological effects in children and other problems. Federal health officials maintain fluoride is safe and highly effective against tooth decay.

In Vermont, the Champlain Water District, anticipating the new fluoride guidelines, in the past week lowered its concentration to the recommended 0.7 milligrams per liter of water, said Jim Fay, the district’s manager. The previous concentration was about 1 milligram. The organization supplies water to all or parts of nine Chittenden County communities.

Fay said he likes that the recommendation is a number and not a range, because it offers water managers firm guidance. He said he believes the level of fluoride in the federal agencies’ new recommendation is appropriate.

Fluoride, a mineral, naturally occurs in soil and water. Lake Champlain, from which the water district draws its water, has normal background levels of fluoride at 0.1 milligrams per liter, Fay said. So the district adds about 0.6 milligrams per liter, he said.

No decision has been made on the level of fluoride in Burlington’s drinking water, said Laurie Adams, the assistant director of water quality for the Burlington Public Works Department.

In 2005, the city slightly lowered the amount of fluoride in drinking water to 1 milligram per liter from one or two tenths of a percentage point higher than that, Adams said.

Burlington officials are under pressure to reduce the amount further or get rid of it altogether. The city’s Board of Health recommended in January 2009 that Burlington cease fluoridating its water supply. Last month, a subcommittee for Burlington’s Public Works Commission recommended fluoridation be “deferred” until further scientific studies consider health effects of the substance.

In 2006, Burlington residents voted 6,909 to 2,766 to retain fluoridation.

Kevin Hurley, a safe-water advocate living in Burlington, said the new federal guidelines could be a catalyst to lower the amount of fluoridation in the city’s drinking water, or eliminate it completely. Hurley said more studies on fluoride are crucial, given what he said are indications the substance could make bones brittle and even depress children’s IQ.

Fluoride can become an emotional topic because of its strong adherents and equally passionate opponents, Hurley said.

“You get a lot of crackpot statements coming from both sides, frankly,” he said. That emotional response is all the more reason why expert scientific assessments of fluoride are key, he said.

Hurley said at the least, people should be given the choice on whether to ingest fluoride. “We don’t add chemicals to the water to treat heart attacks or strokes,” he said.

Fluoride is widely available in toothpaste, mouthwash and supplements. One of the reasons federal officials said they want to lower levels of fluoride in drinking water is because toothpaste and other products are adding to the amount of fluoride consumers ingest.

The Associated Press contributed to this report.