GENEVA (AP) – Cellphone users worried about getting brain cancer aren't off the hook yet.

A major international study into the link between cellphone use and two types of brain cancer has proved inconclusive, according to a report due to be published in a medical journal today.

A ten-year survey of almost 13,000 participants found most cellphone use didn't increase the risk of developing meningioma – a common and frequently benign tumour – or glioma – a rarer but deadlier form of cancer.

There were "suggestions" that using cellphones for more than 30 minutes each day could increase the risk of glioma, according to the study by the World Health Organisation'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 tumour. 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 cellphones 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 cellphone 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 cellphone 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 cellphone use increases the risk of tumours in the ear's acoustic nerve and the parotid gland, where saliva is produced. A separate study will look into the effects of cellphone 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 US Scientists interviewed 12,848 participants, of which 5,150 had either meningioma or glioma tumours. Almost a quarter of the €19.2 million ($24 million) required to fund the study was provided by the cellphone industry, though WHO said measures were taken to ensure the scientists' independence was protected.