SAN JOSE — San Francisco has had it since 1951, Oakland since 1976. Los Angeles and San Diego, along with Contra Costa, Marin and San Mateo counties, have it too.

And starting Monday, large sections of San Jose — the nation’s biggest city without fluoride in its drinking water — finally will begin to receive the additive.

The move comes several years after a push by dentists, the Santa Clara County Public Health Department, the Silicon Valley Leadership Group and Lucile Packard Children’s Hospital, who contend that fluoride can help reduce high rates of cavities, particularly in low-income children with limited access to dental care. In 2011, they persuaded directors of the Santa Clara Valley Water District, the area’s wholesale water provider, to vote 7-0 for the $6.2 million project to retrofit the district’s three drinking-water treatment plants.

“It’s a social justice issue. The richest kids in Palo Alto have had fluoride for years, but the kids in East San Jose haven’t,” said Fred Ferrer, CEO of the Health Trust, a nonprofit organization in San Jose that funds Silicon Valley health programs, from diabetes testing to Meals on Wheels.

“We see kids in elementary school coming in with rampant cavities, abscessed teeth and root canals,” said Ferrer, whose organization built two dental clinics in East San Jose and Sunnyvale that see 30,000 children a year, many of them low-income. “When you have that kind of pain, usually you can’t eat well, or sleep well. You can’t pay attention in school.”

Exactly which parts of the South Bay will begin receiving fluoridated water is somewhat complex. That’s because the area is served by 13 different government and private water agencies, and drinking water comes from multiple sources, including reservoirs, wells and imported water from the Delta. That’s also a big part of the reason San Jose was the last major U.S. city to receive fluoride.

To help pay for upgrading the drinking water plants, the Health Trust donated $1 million, while $900,000 came from First 5 Santa Clara County, which receives money from the state tobacco tax. The California Dental Association Foundation donated $500,000.

On Monday, 230,000 people in East San Jose, Almaden Valley and Santa Teresa who are customers of San Jose Water Company will begin receiving fluoridated water. That’s when the Santa Clara Valley Water District will start adding fluoride at its Santa Teresa Water Treatment Plant. Also affected will be parts of Campbell, Milpitas and Los Gatos.

By 2020, after the water district has finished upgrading its two other water treatment plants, Penitencia and Rinconada, another 520,000 people will receive fluoride, including residents of West San Jose, Cupertino, Saratoga and other parts of Los Gatos.

About 415,000 people in the county already have fluoride, according to the water district. They include residents of Palo Alto, Mountain View and parts of Sunnyvale whose fluoridated water comes from San Francisco’s Hetch Hetchy system, as well as 100,000 or so residents of Evergreen, Alviso and North San Jose who are customers of the San Jose Municipal Water System, which has provided fluoride for years.

By 2020, roughly 1.16 million county residents will have fluoride. But that will still leave about 700,000 without it. They are in places such as downtown San Jose and Willow Glen, which are served mostly by groundwater wells from San Jose Water Co.

Other parts of the county, such as Gilroy and Morgan Hill, rely entirely on city-owned well water, which is not fluoridated.

John Tang, a spokesman for San Jose Water, said that the cost to fluoridate downtown, Willow Glen and other parts of the company’s service area served by wells would be about $23 million. The Health Trust is raising money toward that goal.

“We’ve been neutral on this issue since Day One,” Tang said. “We are going to follow the law. If funding becomes available, we are going to have to do it.”

A state law signed by former Gov. Pete Wilson in 1995 required water providers — both governments and private companies — with more than 10,000 connections to add fluoride. But it didn’t mandate that water agencies pay for fluoridation. They are required to fluoridate only if somebody else puts up the money.

Over the years, various state and federal grants, along with money from charitable foundations, have paid the capital costs to install fluoridation equipment in cities around California. Roughly 64 percent of California residents now receive fluoridated water. According to the U.S. Centers for Disease Control and Prevention, 66 percent of Americans — 211 million people — do.

A naturally occurring mineral, fluoride was first added to drinking water in Michigan in 1945 as a way to strengthen teeth and reduce cavities. The CDC has called it “one of the 10 greatest public health achievements of the 20th century,” citing studies that it reduces tooth decay by 25 percent.

But as in years past — when the John Birch Society denounced fluoride as a communist plot to poison America — some critics say fluoride isn’t safe. Detractors say it causes fluorosis, a pitting of the teeth, aggravates thyroid problems and causes other ailments.

“The water district is acting as a doctor giving out drugs,” said Arlene Goetze, an anti-flouride activist from Sunnyvale. “The only people who can mandate a drug are medical doctors.”

Goetze, who said she also believes vaccines and Wi-Fi also are unsafe, is not moved by the fact that fluoride is endorsed by the American Dental Association, American Medical Association, CDC, California Department of Public Health, American Academy of Pediatrics and the World Health Organization.

Dr. Howard Pollick, a professor of dentistry at UC San Francisco, said studies show fluorosis can occur, but only when fluoride levels are much higher than the U.S. standard of 0.7 parts per million, which is what will be delivered starting Monday in San Jose.

He said that the issue has been studied extensively and that fluoride is safe. In 2011, for example, scientists at the California Environmental Protection Agency reviewed the research and decided not to add fluoride to the state list of carcinogens and harmful chemicals that must be posted by businesses under Proposition 65, a 1986 ballot measure.

“The science is robust on the evidence to support the benefit of fluoridation,” Pollick said. “Over the next five or 10 years, and in generations to come, children, adults, especially the poor and most vulnerable, will experience less tooth decay. And it will save a lot of grief and pain.”

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For more information, call the California Department of Public Health fluoride line at 1-844-435-8420, the Santa Clara Valley Water District at 408-630-2020, or the San Jose Water Co.at 408-279-7900. Or go to www.valleywater.org/services/fluoridation.aspx#EN11