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The most exhaustive investigation ever undertaken into brain cancer risk from cell phones suggests that heavy users may be at an elevated risk of developing the tumors, a finding that is likely to continue fueling health concerns over the popular electronic gadgets.

The study, dubbed Interphone, found that that people who reported chatting on the phones the equivalent of a half an hour a day over 10 years had an elevated risk of a rare and often deadly brain cancer known as glioma, the type that last year felled U.S. Senator Ted Kennedy, although the researchers concluded their evidence is not strong enough to link the devices to the disease.

The frequent users had a 40 per cent higher risk of glioma, compared to people who never used the phones, as well as about twice the risk of developing tumors on the same side of their heads where they normally held their mobiles while talking, or where most of the energy emitted by their phones would be absorbed.

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The results were based on a review of cell phone use among more than 5,100 people diagnosed with brain cancers in 13 countries, including Canada, Japan, Germany and Israel from 2000 to 2005, well before the recent explosion in mobile usage, a factor that worries researchers.

"I think these results are of concern because the study subjects were light users compared to today," commented Elisabeth Cardis, the study's lead author and a professor at the Centre for Research in Environmental Epidemiology in Barcelona.

While Dr. Cardis said the research on possible harm isn't conclusive, she said taking a precautionary approach of reducing exposure to cell phone radiation "might be a reasonable course of action until stronger conclusions can be drawn around the risks."

The peer reviewed research was coordinated by the International Agency for Research on Cancer, the UN's respected cancer watchdog, and is being published Tuesday in the International Journal of Epidemiology. Dr. Cardis, a Canadian who headed IARC's radiation group during much of the period of the study, is one of the first prominent epidemiologists to suggest caution when it comes to cell phones.

But IARC concluded that the study didn't confirm a link between cell phones and brain cancer, although the finding of increased risk among heavy users and the rapid growth in the amount of time people spend on their phones since the research began indicates more investigation is warranted.

"An increased risk of brain cancer is not established from the data," said Christopher Wild, IARC's director, in a statement "However, observations at the highest level of cumulative call time and the changing patterns of mobile phone use since the period studied by Interphone, particularly in young people, mean that further investigation of mobile phone use and brain cancer risk is merited."

The study cost about 19.2 million Euros, with about a quarter of the funding from the mobile industry and much of the rest from government sources. Cell phone companies had no role in designing the research or in vetting its results.

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Researchers have been investigating the cancer hazard from phones because using them amounts to placing a small radio transmitter next to the head, exposing the brain and ears to a small dose of microwave radiation.

A paradoxical result found in the research is that moderate or infrequent cell phone users had a reduced risk of about 20 per cent in developing brain cancer, compared to people who shun the devices.

Dr. Cardis said the unusual results - some cell phone use seems to protect against cancer while high use seems to be associated with cancer - might be due to the difficulty researchers had in recruiting controls. These are people without cancer who were used in the study as a comparison group to see how they differed in their cell phone usage from those with the tumors.

According to spot checks by the researchers, the people volunteering to be controls appear to have had higher cell phone use than the population at large, suggesting the results could be skewed towards underestimate the risks from the mobile devices.

Because the study began 10 years ago and cell phones have become much more popular since then, many of the people in the high use category wouldn't be considered particularly heavy mobile chatters by today's standards. Most people reported only about 2 and a half hours of phone use a month, or a lifetime total of 100 hours. The heaviest users were classified as those who had a lifetime tally of more than 1,640 hours on the phones.

The average Canadian has usage of 6.7 hours a month, and would end up in the high risk group after about 20 years. There are approximately 23 million wireless subscribers in the country, or 70 per cent of the population, according to figures provided by the Canadian Wireless Telecommunications Association, an industry, trade group.

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Given that even IARC is calling for more research in the wake of the results, the study hasn't changed the highly polarized views on cell phone safety. The wireless industry maintains that there are no reasons for any concern. But independent analysts aren't convinced because there has also been a hint in some previous studies that long term users face elevated brain cancer risks, although other studies haven't found any harm.

"There is no way" this will end the controversy over cell phones, contends Louis Slesin, editor of Microwave News, an independent newsletter that tracks research into the biological effects of radiation. "I don't think that anyone challenges that there is uncertainty here."

But that isn't the view in the mobile industry. With cell phones operating below radiation safety standards set by governments "there is no risk for human health issues," says Marc Choma, spokesman for the Canadian Wireless Telecommunications Association.

Besides glioma, the researchers also tracked the incidence of meningioma, a slow-growing brain tumor that is often benign. Heavy users also had a 15 per cent elevate risk of contracting it, compared to people who don't use cell phones.

Based on the reasoning that other cancers in the head and neck might be caused by exposures to cell phones, the scientists also looked at the incidence of acoustic neurinoma, a cancer of the inner ear, and salivary gland tumours. The studies on them are to be released later.

Critics of cell phone safety have likened the devices to holding a small microwave oven up to the head, but government safety standards limit the amount of energy discharged to such low levels that people can't feel their heads being warmed slightly by the radiation.

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Health Canada has played down safety worries and according to its Web posting on the issue says the radiation from cell phones "poses no confirmed health risks."

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