The World Health Organization, International Agency for Research on Cancer, has classified radiofrequency electromagnetic fields as possibly carcinogenic to humans (group 2B).  

IARC Monograph 102, Non-Ionizing radiation part 2: radiofrequency electromagnetic fields    http://monographs.iarc.fr/ENG/Monographs/vol102/mono102.pdf

Press Release here

Lancet Oncology article here.