Cellphones are possibly carcinogenic" to humans, according to [an international panel of experts] organized by the World Health Organization. But an exhaustive, eight-day review of hundreds of studies concluded that the existing evidence is insufficient to know for sure. And because cellphones are so popular, further research is urgently needed, the experts said. "Possibly carcinogenic" is the WHO's third-highest rating, falling below "carcinogenic" and "probably carcinogenic". Other substances that the group has categorized as "possibly carcinogenic" include talcum powder, which has been possibly linked to ovarian cancer, and low-frequency magnetic fields, which are emitted by power lines and appliances and have been possibly associated with childhood leukemia. The cellphone classification marks a departure for the WHO, which previously said there were no risks from exposure to radio frequency electromagnetic fields emitted by the devices. "The conclusion means that there could be some risk, and, therefore, we need to keep a close watch for a link between cellphones and cancer risk," said Jonathan M. Samet of the University of Southern California, who chaired the WHO's International Agency for Research on Cancer panel.