A World Health Organization (WHO) advisory panel has concluded that extensive cell phone use may increase the risk of a rare type of brain tumor, but the panel says that the evidence is not definitive and urged that more research be conducted into the issue. The WHO’s International Agency for Research on Cancer, composed of 31 scientists from 14 countries, said a review of existing studies into cell phone use and brain tumors shows that long-term use is “possibly carcinogenic.” The panel said that cell phone use should be classified as a Category 2B risk, a designation the WHO has given to 240 agents, including the pesticide DDT, lead, and other chemicals. The study is certain to intensify the debate over cell phone use and cancer; a 13-country study released last year, called Interphone, reported that the heaviest users of cell phones had a 40 percent higher risk of contracting a rare brain tumor known as a glioma. Other groups, such as the American Cancer Society and the National Cancer Institute, have said there is currently no evidence linking heavy cell phone use to an increased incidence of brain cancer. And the cell phone industry noted that other substances — including coffee and talcum powder — also are classified as a Category 2B risk.