The use of cell phones and other wireless communication devices are \"possibly carcinogenic to humans\", the World Health Organisation\'s cancer research agency said Tuesday. The radio frequency electromagnetic fields generated by such devices are possibly cancer-causing \"based on an increased risk for glioma, a malignant type of brain cancer,\" the International Agency for Research on Cancer (IARC) said in a statement. A group of 31 experts meeting in the French city of Lyon over the past eight days \"reached this classification based on its review of the human evidence coming from epidemiological studies,\" said Jonathan Samet, president of the work group. Two studies in particular, the largest conducted over the last decade, provided evidence that cell phone use was associated with higher rates of glioma, \"particularly in those that had the most intensive use of such phones,\" Samet said. A number of individuals tracked in the studies had used their phones for 10 to 15 years. \"We simply don\'t know what might happen as people use their phones over longer time periods, possible over a lifetime,\" he said. There are about five billion mobile phones registered in the world. Both the number of phones in circulation, and the average time spent using them, have climbed steadily in recent years, the working group found.