The largest study to date on the potential health effects of mobile phone use concludes that the devices do not increase the risk of brain cancer among average cell phone users. But it finds a slightly greater risk of cancer among heavy users.

The massive international research project, conducted by the World Health Organization's Interphone Study Group, surveyed more than 13,000 mobile phone users in 13 countries over the past decade. Overall, members of the panel concluded that radio-frequency energy emitted by mobile phones does not increase the risk of the two main types of brain tumors, glioma and meningioma.

However, when researchers looked at the 10 percent of subjects who identified themselves as heavy users, they found a 40 percent increased risk of glioma. Heavy users included those who talked on a mobile phone for at least a-half hour a day, seven days a week, for ten years.

But panel member Daniel Kewski, of the Center for Population Health Risk Assessment at the University of Ottawa in Canada, says the group was unable to prove that heavy mobile phone use increased the risk of cancer because the findings were based on the sketchy memories of some participants.

"If we asked you to be a participant, we would sit down with you and ask you to try and remember every cell phone that you had used in your entire life," said Daniel Kewski. "And then we would ask you how many times a day you made phone calls, and how long you were on the phone for each of those calls on average. So your recall of your cell phone utilization patterns may not be perfect."

Kewski says the remaining 90 percent of mobile phone users were on the device an average of two to two-and-a-half hours per month.

Otis Brawley, chief medical and scientific officer of the American Cancer Society, says there is little evidence that even heavy cell phone use causes a significant increase in brain cancer. And if it did, says Brawley, the problem would be dwarfed by other more serious public health concerns:

"If you use the worst case studies that we have to date, it's about 300 additional brain tumors per year in the United States," said Otis Brawley. "Compare that with the fact that we have definite data to show that 3,000 deaths are caused by auto accidents due to cell phones in the United States every year."

Panel member Jack Siemiatycki, a professor of environmental epidemiology at the University of Montreal agrees, saying there is no reason for unnecessary panic or concern among average cell phone users.

"For people who are heavy duty users, who really do use 1,000 minutes a month or 2,000 minutes a month, or something like that, then you are getting into a zone where we may have seen a risk," said Jack Siemiatycki.

In that case, Siemiatycki says, users should consider operating their mobile phones without holding them to their ears by using wired earplugs or by using a wireless Bluetooth device, which emits less radio frequency energy than the mobile phone's electronics.

Christopher Wild is director for the International Agency for Research on Cancer based in Geneva:

Wild says the Interphone Study Group will convene a meeting next year to review data from all studies that have been conducted looking at cell phone use to see whether the devices increase the risk of brain cancer among heavy users.

"We need to look in more depth at that finding in the context of all the other information that's available to us before we are confident to conclude that there's no associated risk," said Christopher Wild.

The paper by the Interphone Study Group on mobile phone use and the risk of brain cancer was published in the International Journal of Epidemiology.

by Jessica Berman