The largest study to date of the safety of mobile phones has found no clear link to brain cancer, although it said further study is merited given their increasingly intensive use.
"The study doesn't reveal an increased risk, but we can't conclude that there is no risk because there are enough findings that suggest a possible risk," the study's chief author, Elisabeth Cardis, told AFP.
The results of the Interphone study, which included 2,708 cases of glioma tumours and 2,409 meningioma tumours in 13 countries over a 10-year period, is due to be published on Tuesday in the International Journal of Epidemiology.
It found no increased risk of glioma or meningioma tumours after 10 years of using a mobile phone, although it found "suggestions of higher risk" for the heavyest users.
The heavyest users who reported using their phones on the same side of their heads had a 40 percent higher risk for gliomas and 15 percent for meningiomas, but the researchers said "biases and errors" prevent making a causal link.
Given that the heavyest users in the study talked an average of half an hour per day on their mobile phones, a figure which is not heavy by today's standards, the researchers recommended further research.
They also cited the need for the study of the impact of mobile phone use among young people, who have rapidly become intenstive users, and who were not included in the Interphone study.
"Observations at the highest level of cumulative call time and the changing patterns of mobile phone use ... particularly in young people, mean that further investigation of mobile phone use and brain cancer is merited," said Christopher Wild, director of the International Agency for Research on Cancer, which coordinated the study.
The researchers noted, however, that the latest mobile phones have lower emissions, and the popularity of hands-free devices and texting reduce exposure to the head.
Cardis said the European Union is funding a new study of risks of brain tumours from mobile phone use during childhood and adolescence.
Explore further: Identifying a better message strategy for dissuading smokers: Add the positive
More information: International Journal of Epidemiology: http://ije.oxfordjournals.org/
International Agency for Research on Cancer: http://www.iarc.fr/