The classification was issued  in Lyon, France, by the International Agency for Research on Cancer after a review of dozens of published studies. The agency is an arm of the World Health Organization and its assessment now goes to WHO and national health agencies for possible guidance on cellphone use.

Classifying agents as "possibly carcinogenic" doesn't mean they automatically cause cancer and some experts said the ruling shouldn't change people's cellphone habits.

"Anything is a possible carcinogen," said Donald Berry, a professor of biostatistics at the M.D. Anderson Cancer Center at the University of Texas. He was not involved in the WHO cancer group's assessment. "This is not something I worry about and it will not in any way change how I use my cellphone," he said - speaking from his cellphone.

The same cancer research agency lists alcoholic drinks as a known carcinogen and night shift work as a probable carcinogen. Anyone's risk for cancer depends on many factors, from genetic makeup to the amount and length of time of an exposure.