ESCONDIDO, Calif. -- Fluoride will be added to the Escondido water supply beginning Monday, despite a lawsuit intended to prevent it.

Escondido would be the first city or water district in San Diego County to fluoridate its water under a 1996 state law requiring large suppliers to begin the process when funds are available.

Public health experts say fluoridation, used for decades in much of the country, has been proven safe and effective in preventing tooth decay.

Fluoride opponents question its safety and warn of harmful effects.

Critics of Escondido's plan say a chemical containing trace amounts of arsenic and lead will be used to add fluoride to water, which they say could result in higher cancer risks.

"If they really take a look at what is going into the water, this type of fluoridation, there will be no detectable amounts of arsenic or lead," Kay Stuckhardt said of the San Diego Fluoridation Coalition.

A lawsuit was filed in 2001 to block Escondido and the state from using hydrofluorosilic acid, which contains trace amounts of arsenic and lead, as the fluoride additive. After several delays, the trial begins Oct. 12.

About 75 percent of Escondido will receive the fluoride-treated water.

"Residents will know exactly when we are going to fluoridate. Because we have two different water districts in the city of Escondido, we have a map, so everyone will be able to tell if their home is going to be fluoridated or not," Escondido Mayor Lori Pfeiler said