OTTAWA - The cancer agency of the World Health Organization says using cell phones and other wireless devices may put people at greater risk of brain cancer, but more study is needed to be certain.

A group of 31 scientists -- including two Canadians -- with the WHO's International Agency for Research on Cancer met in France for a week to study whether exposure to the type of electromagnetic radiation emitted by wireless devices could cause cancer.

On Tuesday, the group classified radio frequency electromagnetic fields as "possibly carcinogenic to humans," based on a higher risk of glioma, a rare but often deadly form of brain tumour.

Other substances in the same category include pesticides and gasoline engine exhaust. The other categories include carcinogenic, probably carcinogenic and probably not carcinogenic.

The findings are based on data from case studies in multiple countries.

The chair of the working group, Dr. Jonathan Samet of the University of Southern California, said it's too soon to draw conclusive links between cancer and wireless devices. The studies done so far haven't tracked people for longer than a decade or so. That's not long enough for scientists to rule out possible cancer links, since many cancerous tumours take a long time to develop.

"We still have much to learn about how these fields interact with biological material," Samet told reporters on a conference call. "We have to leave open the possibility that there are things for us to learn."

IARC's director also stressed the need for more research into the long-term, heavy use of cell phones.

"Pending the availability of such information, it is important to take pragmatic measures to reduce exposure such as hands-free devices or texting," Christopher Wild said in a news release.

The agency estimates five billion people -- almost three quarters of the world's population -- use cell phones.

A public-relations firm working on behalf of Waterloo, Ont.-based Research In Motion, maker of the popular BlackBerry wireless handheld device, was not immediately available for comment.

Canadian members of the cancer agency's working group include Jack Siemiatycki of the Universite de Montreal and James McNamee of Health Canada.

The agency has published more than 100 papers since 1970 on the cancer-causing properties of asbestos, X-rays and the human papillomavirus, among others.

Last year, a decade-long study of thousands of people wasn't able to find a clear link between cell-phone use and cancer. That study, called Interphone, found very heavy phone users were at greater risk of glioma than those who never used cell phones. But the Interphone findings weren't considered sufficient to make the case.