SACRAMENTO – San Diego’s days as the biggest national holdout against water fluoridation could be numbered.

Gov. Arnold Schwarzenegger yesterday signed legislation designed to remove the perceived threat of legal action that San Diego officials cited to explain why the city is not adding the protective treatment to its water.

The measure, SB 96, by Sen. Dede Alpert, D-San Diego, could close a long, colorful history of resistance to fluoride in San Diego, one of a few California cities that do not treat their water with it.

San Diego voters, amid conspiracy theories about plots to poison the water, twice approved measures in decades past that prohibited adding fluoride to the local water system.

The chemical is widely considered a deterrent to tooth decay, although skeptics continue to question its health benefits. The Centers for Disease Control and Prevention declared fluoridation “one of the 10 great public health achievements of the 20th Century.”

“It’s a huge step forward,” Alpert said yesterday. “Reputable scientists all agree that this is a very effective way to improve public health.”

It’s also a “relatively inexpensive way to make major strides in dental health, especially for children,” she said. As such, Alpert said, the legislation may be one of the most far-reaching accomplishments of her political career.

But the cost, estimated at $8 million a year for San Diego, may present an obstacle. City officials could not be reached late yesterday, but they have indicated in the past that the ongoing annual expense could be a problem. Mayor Dick Murphy, however, did send a letter of support for Alpert’s bill, and the City Council voted four years ago to fluoridate the city’s water.

“It’s an important public health issue and one that just isn’t hard to deal with,” Alpert said. “Hopefully the city of San Diego can get a grant to do this and we can finally do what should have been done a long time ago.”

The measure indemnifies the city against any lawsuit that attempts to to invoke the citywide rejections of fluoridation in 1954 and 1968. It declares that the state can use public health concerns to pre-empt local fluoride bans.

Alpert said she was stunned when she moved from the East Coast to San Diego in the 1960s and heard the talk about a communist conspiracy to poison the local water supply. Those fears may have waned, but fluoridation remains controversial locally.

A split Oceanside City Council narrowly voted to fluoridate its water in 2001. The decision has emerged as an issue in the upcoming city election. Elsewhere, Escondido’s decision to add fluoride to its water this year has drawn a lawsuit.

Alpert carried the bill at the request of a local fluoridation coalition and the California Dental Association, which offered a large grant to San Diego.

The measure sailed through both houses with lopsided, bipartisan votes during the final week of the session. The bill had no serious opposition, although the Sierra Club weighed in late, objecting that the legislation was another hasty, late-session rewrite of what had been a completely different bill.