Most water agencies in Southwest County plan to go along with regional plans to fluoridate water starting in October, but only passively.

Metropolitan Water District, which distributes hundreds of millions of gallons each day to about 18 million customers in the southern half of the state, plans to begin in early October to add fluoride compounds, which most dental-health advocates say help to prevent tooth decay but which critics say are toxic and harmful over time.

Most naturally occurring groundwater contains up to 0.3 parts of fluoride per 1 million, and the American Dental Association recommends that public drinking water contain fluoride at two to three times that level.

According to the American Water Works Association, a consortium of utilities, the water used by about two-thirds of Americans contains added fluoride. “Fluoride” refers to a group of compounds containing the element fluorine. Fluoridated drinking water typically contains a compound of fluorine, hydrogen and silicon.

Critics, particularly in western states, have argued that excess fluoride can build up in the brain, possibly contributing to Alzheimer’s disease. One frequently cited study found that fluoridated water was associated with elevated levels of bone cancer among men who were born in areas served by fluoridated water, though most doctors say it is safe if measured and delivered properly.

Eastern Municipal Water District, which serves French Valley and Menifee, expects to spend about $900,000 for its own equipment to add fluoride up to the dental association’s standard.

The district said several of its customers had written in to protest its plans to use fluoridated water. Southwest County’s other three large agencies said complaints had been minimal.

Those three agencies have little alternative to accepting Metropolitan’s new supply. None plan to add more fluoride but none plan to filter out Metropolitan’s newly added fluoride, either. Water agencies that serve Temecula and Lake Elsinore have no immediate plans to add fluoride beyond what’s in the water from Metropolitan.

Eastern extracts its own groundwater and buys both treated and “raw” —- untreated and unfluoridated —- water from Metropolitan that is drawn from the aquifer supplied by Northern California’s snowmelt and from the Colorado River. Eastern already treats the raw water at two separate plants, where the new equipment will add fluoride to the same level as the fluoridated water from Metropolitan, spokeswoman Betty Gibbel said.

Eastern plans for its entire drinking water supply to contain about 0.7 parts fluoride per 1 million, the standard that Metropolitan uses.

Western Municipal Water District and Rancho California Water District have no plans to add fluoride. Those agencies buy from Metropolitan and mix it with their own treated water before sending it on to customers. As a result, customers from Lake Elsinore to Wine Country will receive water with fluoride levels only slightly higher than what they have now.

Officials with the Rancho California district, which serves households in Temecula and southern Murrieta, and large agricultural customers in Wine Country and De Luz, don’t plan to add fluoride, either.

“It’s a controversial subject,” district spokeswoman Meggan Reed said.

Adding fluoride remains a vague possibility for Western, which serves a sliver of northern and western Murrieta, spokeswoman Tedi Jackson said.

Like Eastern and Rancho California, the district uses both imported water from Metropolitan and locally extracted groundwater with a fluoride content slightly above 0.3 parts per 1 million. The resulting mix should have a fluoride content between that and the 0.7 parts per 1 million that Metropolitan expects to achieve, Jackson said.

A spokesman for the Elsinore Valley Municipal Water District said critics have focused on a handful of cases from districts that fluoridated their water far beyond the recommended range of 0.7 to 0.8 parts per 1 million.

Spokesman Greg Morrison said the district buys about 55 percent of its water from Western. That water’s fluoride content will be diluted further when it’s mixed with local groundwater, which won’t be additionally fluoridated. The district serves residents of Lake Elsinore, Wildomar, and northwestern Murrieta.

“With the dilution factor, it’s pretty much a nonissue,” spokesman Greg Morrison said.