People who use mobile phones intensively seem to have a higher risk of developing certain types of brain cancer, French scientists observed on Tuesday, reviving questions about phone safety.
Individuals who used their cellphone for more than 15 hours each month over five years on average had between two and three times greater risk of developing glioma and meningioma tumours compared with people whose used their phone rarely, they found.
The study, appearing in the latest issue of British journal Occupational and Environmental Medicine, is the latest foray in a long-running exploration of mobile-phone safety.
Over the last 15 years, most investigations have failed to turn up conclusive results either way, although several have suggested a link between gliomas and intensive, long-term use.
"Our study is part of that trend, but the results have to be confirmed," said Isabelle Baldi, of the University of Bordeaux in southwestern France, who took part in the probe.
In 2011, the International Agency for Research on Cancer (IARC) said radiofrequency fields used by mobile phones are possibly carcinogenic.
But research faces several challenges. They include clear proof in the lab that these fields are harmful to human cells.
Another is getting an accurate picture of phone use in real life, filtering out lifestyle factors such as smoking which amplify cancer risk and taking into account changing phone technology.
The new study looked at 253 cases of glioma and 194 cases of meningioma reported in four French departments (counties) between 2004 and 2006.
These patients were matched against 892 "controls," or healthy individuals drawn from the general population, in a bid to spot any differences between the two groups.
The comparison found a higher risk among those who used their phone intensively, especially among those who used it for their work, such as in sales. The duration of use in this category ranged from between two and 10 years, averaging at five years.
But study also found several inconsistencies with other investigations that have suggested a link between heavy phone use and brain cancer.
For instance, in contrast with previous work, it found that cancer occurred on the opposite side of the brain, rather than on the same side, of where the phone was customarily used.
"It is difficult to define a level of risk, if any, especially as mobile phone technology is constantly evolving," the study acknowledged.
"The rapid evolution of technology has led to a considerable increase in the use of mobile phones and a parallel decrease of [radiowave intensity] emitted by the phones."
"Studies taking account of these recent developments and allowing the observation of potential long-term effects will be needed."