The radiation emitted by mobile phones has been classified as "possibly carcinogenic to humans" by a World Health Organization (WHO) scientific working group.

The May 31 announcement, however, doesn't imply that cell phones cause cancer. It suggests that there are still enough unknowns not to rule out long-term health effects of the devices, which are now used by billions of people around the globe.

The International Agency for Research on Cancer (IARC) based its new conclusions on previous studies of humans, animals and lab work. The group called evidence for a link between cell phone use and glioma and acoustic neuroma cancers "limited"—and a link with other types of cancers "inadequate." Evidence for a link between cancer and exposure to other radiofrequency electromagnetic sources—including microwaves, radar, and television and radio transmission signals—was also found to be insufficient.

Over the years, the findings on cell phones and cancer have been about as spotty as mid-'90s cellular service coverage. A decades-long study in some European countries found no increase in the occurrence of brain cancer despite a huge jump in mobile phone use. A study earlier this year concluded that cell phone emissions did have an affect on brain metabolism near where callers held the devices against their heads. But as Scientific American columnist Michael Shermer explained last year, "Physics shows that it is virtually impossible for cell phones to cause cancer." (In short: unlike UV radiation, the radiation from cell phones is too weak to destroy the bonds inside cells.)

But the 31 scientists involved in the new assessment seem to want to play it on the safe side. "Given the potential consequences for public health," IARC Director Christopher Wild said in a prepared statement, "it is important that additional research be conducted into the long-term, heavy use of mobile phones."

A more detailed explanation behind the IARC's recommendations will be published in the July 1 issue of The Lancet Oncology.

Image courtesy of iStockphoto/Yuri_Arcus