"The question is do you want to play Russian roulette with your brain," she said in an interview that she did from her cell phone. "I don't know that cell phones are dangerous. But I don't know that they are safe."

Of concern are the still unknown effects of more than a decade of cell phone use, with some studies raising alarms, said Davis, a former health adviser in the Clinton Administration.

She said 20 different groups have endorsed the advice the Pittsburgh cancer institute gave, and authorities in England, France and India have cautioned children's use of cell phones.

Herberman and Davis point to a massive ongoing research project known as Interphone, involving scientists in 13 nations, mostly in Europe. Results already published in peer-reviewed journals from this project aren't so alarming, but Herberman is citing work not yet published.

The published research focuses on more than 5,000 cases of brain tumors. The National Academy of Sciences in the U.S., which isn't participating in the Interphone project, reported in January that the brain tumor research had "selection bias." That means it relied on people with cancer to remember how often they used cell phones. It is not considered the most accurate research approach.

The largest published study, which appeared in the Journal of the National Cancer Institute in 2006, tracked 420,000 Danish cell phone users, including thousands that had used the phones for more than 10 years. It found no increased risk of cancer among those using cell phones.

A French study based on Interphone research and published in 2007 concluded that regular cell phone users had "no significant increased risk" for three major types of brain tumors. It did note, however, that there was "the possibility of an increased risk among the heaviest users" for one type of brain tumor, but that needs to be verified in future research.

Earlier research also has found no connection.

Joshua E. Muscat of Penn State University, who has studied cancer and cell phones in other research projects partly funded by the cell phone industry, said there are at least a dozen studies that have found no cancer-cell phone link. He said a Swedish study cited by Herberman as support for his warning was biased and flawed.

"We certainly don't know of any mechanism by which radiofrequency exposure would cause a cancerous effect in cells. We just don't know this might possibly occur," Muscat said.