Seven decades since American cities began fluoridating public water supplies to prevent tooth decay, the South Bay remains something of a holdout. It’s the largest metropolitan region in the nation to not fluoridate its water.

That’s about to change, but not without some pushback. Activists have threatened to sue the Santa Clara Valley Water District to prevent it from adding the mineral, which it planned to start doing this December.

The plaintiffs, groups called Safe Pure Water and Healthy Alternatives to Pesticides, plan to protest fluoridation at a water district meeting Tuesday night.

“Water district officials should stick to the agency’s legal charter and not masquerade as dentists prescribing fluoride to patients they have never even met,” said Brandi Madison, the group’s spokeswoman.

Two-thirds of the country drinks tap water treated with the chemical, which the U.S. Centers for Disease Control and Prevention call one of the top public health achievements of the 20th century.

Although fluoridation is common, it has never been universally accepted. Detractors criticize the practice of adding it to the public tap as mass medication. Others take issue with the chemical itself, which they see as an environmental pollutant. A 2015 study linked fluoride with the prevalence of attention deficit disorders in the U.S.

Public health officials, however, say fluoridation is a boon for low-income communities who are unable to afford regular dental care.

Santa Clara Valley’s water district, which provides wholesale drinking water for 1.8 million people, debated the issue for years before deciding to jump on board with the other 75 percent of the country.

Marty Grimes, a spokesman for the district, said the agency has been raising money with help from local nonprofits. The Santa Clara County Health Trust donated $1 million for the cause, with another $900,000 coming from the First 5 California and $500,000 from the California Dental Association Foundation.

The district has created a landing page—which you can find here—about fluoridation, which includes a list of frequently asked questions.

Jennifer Wadsworth is the news editor for San Jose Inside and Metro Silicon Valley. Email tips to [email protected]. Follow her on Twitter at @jennwadsworth. Or, click here to sign up for text updates about what she’s working on.