The regional agency that supplies most of the South Bay’s drinking water plans to mix in fluoride this fall, despite lingering debates over the additive’s health benefits.

The Metropolitan Water District board of directors voted in 2003 to fluoridate the water it supplies in a six-county service area – a move cheered by advocates who fought for the change as a method of preventing tooth decay.

After years of planning, the agency is prepared to fluoridate the water at its Riverside treatment plant in late October.

The South Bay will be affected by the planned Nov. 12 rollout at the F. E. Weymouth Treatment Plant in LaVerne, which services areas from Los Angeles Harbor to Santa Monica, said agency spokesman Denis Wolcott.

“Our board made a policy decision back in 2003. At this point in time the board’s not planning any action to reverse it,” Wolcott said. “The equipment’s been bought. It’s been installed. It’s been tested.”

Still, opponents of fluoridated water Monday urged a board committee to reconsider.

Members of the Environmental Working Group point to an American Dental Association recommendation that parents of formula-fed infants mix the powder with non-fluoridated water to reduce the risk of fluorosis, or the brown staining of teeth.

The group also cites a March 2006 National Research Council report that recommended lowering the Environmental Protection Agency’s ceiling on fluoride in drinking water – now 4 milligrams per liter – and a controversial Harvard study linking fluoride consumption with bone cancer in boys.

Today, the EPA also recommends that children under 9 not drink water that has more than 2milligrams per liter of fluoride because of the threat of fluorosis.

“It is known that fluoride is harmful in certain doses,” said Bill Walker, vice president for the group’s west coast region.

He described the board’s decision to mix it into the water supply as “overkill,” as many people also consume fluoride daily in products such as toothpaste.

But proponents say fluoride has long been held up as a safe and effective way of preventing tooth decay.

Edgar Dymally, a senior environmental specialist with the MWD’s water quality section, argues the Harvard research study “was not peer-reviewed and the instructor (determined) it was a premature release.”

“There’s no information that states categorically that drinking fluoridated water is anything but safe and effective,” he said.

The California Dental Association, which lobbied for fluoridated drinking water several years ago, continues to support the change, said policy development manager Gayle Mathe.

The Metropolitan Water District’s fluoride levels will fall below recommended limits, at 0.7 to 0.8 milligrams per liter, she added.

“It reaches the most underserved populations (with) dental decay being the most significant problem for children,” Mathe said.

“The key is the dose.”