PARIS, France - The largest study of its kind found no link between long-term use of mobile phones and increased risk of brain tumors, the British Medical Journal (BMJ) reported on Friday.
Danish researchers found no evidence of enhanced risk among more than 350,000 mobile-phone subscribers whose health was monitored over 18 years.
Earlier research on the possible link between cell phone use and cancerous tumors has been inconclusive, partly due to lack of long-term data.
In June, the World Health Organisation's International Agency for Research on Cancer (IARC) classified the radio-frequency electromagnetic fields emitted by mobile phones as "possibly carcinogenic to humans."
The new study follows up an earlier investigation that compared the cancer risk faced by all mobile phone subscribers in Denmark -- some 420,000 people -- with the rest of the adult population.
Patrizia Frei, a postdoctoral research fellow at the Danish Cancer Society, and colleagues examined health records from 1990 to 2007 for 358,403 cell phone subscribers.
Overall, 10,729 tumors of the central nervous system were diagnosed.
But among people with the longest mobile phone use -- 13 years or more -- cancer rates were nearly the same as for non-subscribers.
"The extended follow-up allowed us to investigate effects in people who had used mobile phones for 10 years or more, and this long-term use was not associated with higher risks of cancer," the study concluded.
The findings, however, could not rule out the possibility of a "small to moderate increase in risk" for very heavy users, or people who have used cell phones for longer than 15 years.
"Further studies with large study populations where the potential for misclassification of exposure and selection bias is minimised are warranted," the researchers said.
In a commentary, Anders Ahlbom and Maria Feychting from Sweden's Karolinska Institute said the new evidence was reassuring but called for continued monitoring of health registers.
There are about five billion mobile phones registered in the world, a figure that continues to rise sharply along with the average amount of time spent using them.
The IARC does not issue formal recommendations, but its experts pointed in June to a number of ways consumers can reduce possible risk.
Texting and using hands-free sets for voice calls lower exposure to potentially harmful radiation, compared to device-to-ear voice calls, by at least 10-fold, they said.