“The question is do you want to play Russian roulette with your brain,” she said in an interview from her cell phone while using the hands-free speaker phone as recommended. “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 Research Council 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.

The Asociated Press contributed to this story.