The Tacoma-Pierce County Board of Health doesn’t have the authority to order water systems in the county to fluoridate their water, the state Supreme Court ruled yesterday.

Individual water districts are empowered by state law to decide whether to include the dental-cavity-preventing chemical in their water supplies, the justices said in the 6-3 decision.

The court did not mention the dental-health benefits of fluoridation. The Centers for Disease Control and Prevention (CDC) and most dental experts have endorsed use of the chemical in drinking water.

A state health official said many local health boards that support fluoridation were watching the case with great interest. They wondered if they, too, could mandate fluoride, said Denise Clifford, director of the state Department of Health’s office of drinking water.

Two years ago, the Tacoma-Pierce County board voted unanimously to require water districts serving more than 5,000 people to fluoridate their water. The ruling affected about 240,000 Pierce County residents in 14 water districts. Tacoma, University Place, Fircrest and the military bases already had fluoridation.

The board said it had the responsibility to protect the public’s dental health, just as it adopts measures to stop the spread of infectious diseases. And it offered to help pay for each affected water district to establish a fluoridation system.

Four sets of plaintiffs soon sued the board, saying that it couldn’t require fluoridation, that it was imposing an illegal tax and that it violated due-process rights.

The opponents included five private water companies, the Lakewood Water District, an anti-fluoridation group and the city of Bonney Lake. A Pierce County Superior Court ruled in favor of the health board, and the case was appealed.

In its opinion, the Supreme Court cited a state law that says either local water-district commissioners or water customers can vote to fluoridate. They said this law trumps another state law that gives health boards broad powers to protect the health of citizens.

Dr. Federico Cruz-Uribe, director of the Tacoma-Pierce County Health Department, called the decision very disappointing. “We were optimistic,” he said. “We have such a clearly defined problem that needs to be addressed. And fluoridation is a step to address it.”

Cruz-Uribe said at least 50,000 low-income children in Pierce County don’t receive the dental care they need, partly because many dentists are unwilling to accept the low fees they receive under Medicaid.

Clifford said about 57 percent of the state population has fluoridated drinking water. Nationwide, about two-thirds of the population has fluoridation, according to the CDC. Controversy over the chemical has raged for years.

Opponents of fluoridation claim the chemical causes problems ranging from allergies to cancer, but health authorities from the CDC to the World Health Organization have said there is no risk to drinking the water. Anti-fluoride groups also say fluoridated water supplies take away personal choice and if parents want their children protected by the chemical, they should see a dentist.

Opponents of the Pierce County mandate, however, were focused on the autonomy of water districts.

“We never took a side for or against fluoridation. … We just wanted our customers to be able to vote on it,” said Randy Black, general manager of the Lakewood Water District, serving 71,000 customers. A vote will be taken in November, he said.