GENEVA (AP) -- Cell phone users worried about getting brain cancer aren't off the hook yet.
A major international study into the link between cell phone use and two types of brain cancer has proved inconclusive, according to a report due to be published in a medical journal Tuesday.
A 10-year survey of almost 13,000 participants found most cell phone use didn't increase the risk of developing meningioma -- a common and frequently benign tumor -- or glioma -- a rarer but deadlier form of cancer.
There were "suggestions" that using cell phones for more than 30 minutes each day could increase the risk of glioma, according to the study by the World Health Organization's International Agency for Research on Cancer. But the authors added that "biases and error prevent a causal interpretation" that would directly blame radiation for the tumor.
Longer call times appeared to pose a greater risk than the number of calls made, the study found.
Among the factors that weren't examined were the effects of using handsfree devices during calls or the risk of having cell phones close by while not making calls -- such as in a pocket, or next to the bed at night.
The authors acknowledged possible inaccuracies in the survey from the fact that participants were asked to remember how much and on which ear they used their mobiles over the past decade. Results for some groups showed cell phone use actually appeared to lessen the risk of developing cancers, something the researchers described as "implausible."
The authors said further investigation is necessary before they can conclude with certainty that there is no link between cell phone radiation and brain cancer, partly because people's use of the devices has changed considerably since the start of the study in 2000.
Scientists are also planning to examine whether cell phone use increases the risk of tumors in the ear's acoustic nerve and the parotid gland, where saliva is produced. A separate study will look into the effects of cell phone use on children, who are believed to be more susceptible to the effects of radiation.
The paper, which will be published in the International Journal of Epidemiology, was compiled by researchers in 13 countries including Britain, Canada, France, Germany and Japan, but not the U.S. Scientists interviewed 12,848 participants, of which 5,150 had either meningioma or glioma tumors.
Almost a quarter of the €19.2 million ($24 million) required to fund the study was provided by the cell phone industry, though WHO said measures were taken to ensure the scientists' independence was protected.
Network operators and handset companies had keenly anticipated the results, which could have threatened the rapid development of their business. There were an estimated 4.6 billion mobile phone subscriptions at the end of last year, compared with about 1 billion in 2002, according to the International Telecommunication Union.
In a statement Sunday, the Mobile Manufacturers Forum welcomed the study.
"The mobile phone industry takes all questions regarding the safety of mobile phones seriously and has a strong commitment to supporting ongoing scientific research," the industry group said.
The study's lead authors are due to present their findings to the media in Geneva on Monday.