One of the nation’s leading activists against fluoridating public water supplies will speak in Petaluma later this month on a controversial county plan to add fluoride to the drinking water supply as a technique for reducing tooth decay in children.

Dr. William Hirzy, a professor of chemistry at American University, questioned the dental health benefits of fluoride, and said the chemical could pose other health threats.

“Adding fluoride to the drinking water at any level from any source is a bad idea,” he said in an interview. “There is no relation between tooth decay and non-fluoridation of the water supply. But there is substantial evidence that exposure to high levels of fluoride in the drinking water can lead to attention deficit hyperactive disorder, hyperthyroidism and even loss of IQ.”

For more than three years, the Sonoma County Board of Supervisors has been considering the idea of fluoridating the county’s water supply. Several cities within the county have formally opposed the concept, including Cotati, Sebastopol and Sonoma.

Anti-fluoride activists have held meetings at Aqus Café in the past week and have lobbied Petaluma Mayor David Glass and members of the City Council to join in opposition to fluoridating public water supplies.

Hirzy is expected to appear before the City Council on April 20.

“It has been estimated that placing fluoride into Petaluma’s drinking supply would cost over $1 million,” said Hirzy. “For that money you could provide everyone in the county with fluoride pills, or just buy them toothbrushes and teach them proper hygiene.”

Hirzy said that fluoride only provides benefits for dental health when it is applied topically, such as in toothpaste.

He added that fluoride toothpaste provides “a much higher fluoride content” when applied to the teeth than when ingesting it.

“One can spit out most of the fluoride and avoid as much internal exposure as possible,” he said.

During a meeting at Aqus Café on March 31, local activist Stephen Fuller-Rowell, who leads Clean Water Sonoma, said swallowing fluoride through the water supply to gain its benefits was similar to “swallowing sun-screen to protect yourself from sunburn.”

“Dental care in Sonoma County is pathetic, especially for the poor,” he said. “Prevention is the key to tooth decay. Fluoride toothpaste works very well and you don’t have to swallow it.”

Supervisor David Rabbitt has long supported adding fluoride to the local drinking water supply.

“The science is there,” he said. “Fluoridating the water is good for dental health, and oral health is important to overall health. The main issue is how do you make it happen?”

When the issue was being debated in 2012, Rabbitt noted that the Surgeon General’s office had estimated that every $30 spent on putting fluoride into local water supplies resulted in $38 in health benefits.

He also noted that 69 percent of counties in the United States fluoridate their water.

The county’s Fluoridation Advisory Committee, which was formed by the Board of Supervisors to study the issue, has said fluoridating the water is one part of the county’s strategic goals to prevent dental disease.