Thousands of people are to be studied over the course of 20 or 30 years to see if the long-term use of mobile phones increases the risk of developing brain tumours and other medical disorders, as part of the largest ever study into the subject.

At least 250,000 mobile phone users will be monitored as part of the multi-million-pound Cohort Study on Mobile Communications (Cosmos), which aims to settle the debate over whether there are any serious health risks from using the devices once and for all.

Previous research has relied on studying people who have already developed illnesses, which has led to suggestions that they suffered from an inherent "recall" bias, such as people having more vivid memories of holding their mobile phones to the side of their head which developed a brain tumour. Cosmos will pick up diseases and symptoms as they arise and changes in people's health will be compared with their usage of mobile phones, taking into account both the number and duration of calls and the positioning of handsets.

Volunteers taking part in the study will be aged 18 to 69 and recruited through co-operating network operators. Between 90,000 and 100,000 people are expected to participate in the UK, with others joining from Finland, the Netherlands, Sweden and Denmark. The total cost of the study for the first five years is estimated at £5-7 million. The UK arm alone will cost £3.1m, jointly funded by the Government and industry.

Dr Mireille Toledano, one of the principal investigators from the School of Public Health at Imperial College London, said that Cosmos is the largest study of its kind to date and has the power to detect illnesses linked to mobile phone radiation or usage.

"Previous studies have looked at usage for less than 10 years and focused mainly on retrospective use. They've had a short observation period from the start and only been able to focus on brain cancers. We'll be prospectively monitoring mobile phone use and prospectively looking at any health developments," Dr Toledano said.

The scientists will be analysing trends for brain, head and neck cancers, but also multiple sclerosis and neurodegenerative disorders such as Alzheimer's, Parkinson's and Motor Neurone Disease, as well as strokes and heart conditions. They will be watching out for less serious problems such as sleep disorders, headaches, tinnitus and depression.

Data on fertility will not be included since this is one aspect of health that does not lend itself to a long-term prospective study. Findings will be released at periodic intervals. A report focusing on cancer will be published after 10 years.

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