The largest-ever study into the possible health risks of using a mobile phone has proved inconclusive.br br The World Health Organisation said that in most cases, phone users did not increase their risk of brain tumours.br br There were "suggestions of higher risk" - 40 per cent higher for some tumours - for those who spoke on their phones for an average of half an hour a day on the same side of their head over a 10-year period. br br But the researchers said "biases and errors" prevented them from making a causal link between mobile phones and tumours. br br Al Jazeera's Nadim Baba reports.