But for most people, its still not clear if theres added risk, the authors say. Plus, the devices and the way people use them keeps evolving so that more research is needed going forward, they add.
This isnt the first study to point to a tumor risk with heavy cell phone use, said Dr. L. Dade Lunsford, a distinguished professor of neurosurgery specializing in brain tumor management at the University of Pittsburgh.
But these kinds of studies rely on people to recall how much they have used cell phones in the past with no indication of their actual use, said Lunsford, who was not involved in the French research.
The new results found no difference between regular cell users and non-users, which suggests that if there is a link, it is only applicable for people who claim to use their cell phone the most, he noted by email.
Cell phones emit radiofrequency electromagnetic fields in the microwave spectrum, which may be cancer causing, although thats not yet proven, said Dr. Seung-Kwon Myung of South Korea's National Cancer Center.
Myung led a large analysis of all the previous studies of cell phone use and brain tumors.
In 2011, the World Health Organizations International Agency on Cancer classified this radiation as possibly carcinogenic, based on existing studies.
The French team, led by Dr. Gaelle Coureau of the Universite Bordeaux Segalen, used a cancer registry to identify adults with meningiomas or gliomas, the two most common types of adult brain tumors.
Brain tumors are generally rare relative to other types of cancer. Less than eight in 100,000 people in the U.S. each year are diagnosed with meningiomas, and 85 percent of those tumors are benign, according to Johns Hopkins Medicine and the Cleveland Clinic.
Malignant brain tumors represent only two percent of all cancers.
The new analysis included 253 cases of glioma and 194 cases of meningioma in four French regions, and twice as many people from the same areas of France who had never had a brain tumor, for comparison.
Researchers interviewed the participants about their past cell phone use, with questions about the model of phone they had used, how long they had used it, the average number and length of calls made and received each month and whether the phones were used for work.
Regular users, who had used a mobile phone at least once a week for at least six months at a time, were no more likely to have a tumor than those who had never used a cell phone, according to the results published in Occupational and Environmental Medicine.
People with the longest cumulative duration of calls, or more than 896 hours on the phone, were about twice as likely to have a glioma or meningioma than people who had talked less.
Case-control studies use questionnaires to ask people to recall how often they used their phones for periods up to 10 years and more, said Dr. Michael Repacholi, former coordinator of the World Health Organization Electromagnetic Field (EMF) Project. Most people could not accurately remember how many calls or for how long they used their phone each call 10 years ago, and so they give best estimates.
Cancer registries have not shown a significant increase in brain cancers since mobile technology was introduced 20 years ago, which is reassuring, Repacholi said.
Mobile phone users shouldnt worry too much about this problem until larger, better studies have been done, and those will take at least another 10 years, Myung told Reuters Health by email.
The new French study will not affect the worlds conversion to mobile phone use, which has saved more lives across the world than probably any other technology in the last 100 years, Lunsford said.
The ability to communicate without land line access to report illness, injury, impending weather disasters, to access 911, fire, police has undoubtedly saved more lives than any conceivable risk of the late and as yet unverified risk of exposure to non ionizing radiation from mobile phones, he said.
To minimize the risk, if there is any, Myung recommends following five safety tips issued by the Environmental Working Group: use a headset or speakerphone when possible, when in use hold the phone away from your body, text instead of calling, dont store the phone in your pocket or under your pillow and try only to use it when you have a strong signal.
Fewer signal bars means the phone must try harder to broadcast its signal, he said. Research shows that radiation exposure increases dramatically when cell phone signals are weak.