Cellphones have never been more popular, but questions about their safety are resurfacing.

On Tuesday the issue grabbed the spotlight on CNN's Larry King Live, where most experts brought on the show expressed concern about the potential health impacts of using cellphones beyond a decade.

Dr. Keith Black, the neurosurgeon who treated high-profile U.S. lawyer Johnnie Cochran before Cochran died of a brain tumour in 2005, was among them. "There are a billion people using cellphones and they'll be using cellphones for many years," he told King. "We don't know if this is going to be a safe practice."

In fact, three billion people around the world, or nearly half the global population, are now cellphone users. And they're beginning to use them at earlier ages and more frequently. Some even rely on the wireless devices as their primary phone, racking up hundreds – even thousands – of hours of talk time a year. Parents give them to their kids as a means of staying in touch.

A lot is at stake. Worldwide cellphone sales reached nearly 300 million units in the first quarter of 2008, while telecom carriers earn billions in profit, despite stepped-up competition. In an economy dependent on mobility, cellphones are becoming a sort of Swiss Army Knife of communications, offering a phone, email device, digital camera, MP3 player and Web browser in one convenient package.

Some studies have found no risk from long-term use. Mobile phone providers, citing the authority of the World Health Organization, maintain the technology poses no health risks and that radiation from cellphones falls well below government safety guidelines.

"They don't feel there's any need for precaution," said Peter Barnes, president and chief executive of the Canadian Wireless Telecommunications Association.

Indeed, most wireless customers either don't believe there's a health risk, or they've weighed past inconclusive studies with the convenience of wireless communications and chosen to keep their cellphones.

But behind the scenes, cellphone makers and service providers are closely following the long-awaited Interphone research project, a massive 13-country study co-ordinated by the WHO's International Agency for Research On Cancer. Canada is a participant, along with Australia, Japan, Israel, New Zealand and eight European countries.

The project, considered the most comprehensive look at the potential link between cellphone use and cancer, involves the pooled analysis of thousands of cases of tumours in the head and neck area: gliomas, meningiomas, acoustic neuromas and parotid gland tumours. It was originally supposed to be completed in 2004 and published in 2006. Years later the final report has still not been released, a source of frustration among some scientists who say such a delay would never be tolerated in the context of a drug study.

"Certain people are embarrassed it's taking so long," said Dr. Louis Slesin, who has studied the issue extensively as founder and publisher of New York-based scientific newsletter Microwave News. "At some point, it becomes a public health scandal that they're not releasing it."

Elisabeth Cardis, the Canadian-born researcher who led the WHO-backed project, said the study will be submitted for publication "soon." Interpretation of the results has not been easy, she said, pointing to the difficulty of satisfying the more than 50 different researchers who must approve the final manuscript.

The delay, however, hasn't stopped individual countries from releasing parts of their own studies. Israel, Sweden, Denmark and France are among the participants that have gone public with certain results. Slesin said analysis of those results, in the context of heavy cellphone use of 10 years or longer, has raised some troubling red flags.

One 2007 research paper, co-authored by Finnish scientist Anna Lahkola, looked at glioma cases from studies conducted in five North European countries, including some data from Interphone. The paper found a significantly increased risk of developing glioma after cellphone use of more than 10 years.

A 2008 analysis, led by Swedish oncologist Dr. Lennart Hardell, looked at 10 European studies published between 2001 and 2007 – most of them country-specific studies from Interphone – and also found a heightened risk from long-term use. "We conclude that this meta-analysis gave a consistent pattern of an association between mobile phone use and ipsilateral glioma and acoustic neuroma," after 10 years or more of use.

"Clearly, it's time to take tumour risk very seriously," said Slesin, adding that the results of the Lahkola study convinced him that such a risk could no longer be discounted.

Australia, like Canada, has yet to release its Interphone results. Bruce Armstrong, a professor of public health at the University of Sydney who is leading Australia's contribution to Interphone, warned there's nothing conclusive yet. But he agreed the patterns emerging from recent reports do suggest a possible health risk.

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In February, Israeli scientist Dr. Siegal Sadetzki and her team of Interphone researchers published a study showing that heavy cellphone users had a 50 per cent greater risk of developing a tumour of the main salivary gland – or parotid – on the same side of the head where they hold the phone, compared with those who didn't use cellphones.

"I would say our results are in line with previous results that are showing something is going wrong here," said Sadetzki, speaking from Israel in a telephone interview with the Star. "After 10 years or more we do see something there."

Sadetzki said it's true that the vast body of global research on cellphone use still supports the claim that the wireless technology is safe, but most of those studies were done too early to have any meaning. "It takes at least 10, 20 or 30 years to see exposure to cancer," she said, explaining that while Hiroshima was bombed with atomic weapons in August 1945, the first study showing brain tumours emerged only decades later.

Combining studies done in the 1990s with those carried out over the past few years only dilutes the outcome, she added, because just recently has data been collected on people who have been heavy cell users for 10 years or more. "The main argument is that if you take the results from the last three years, you do see an indication."

Even Barnes from the Canadian wireless association acknowledges some of the recent findings. "Clearly, they have suggested a possible increased risk over 10 years."

But Barnes warned about jumping to conclusions. He said recent research related to Interphone indicates that study subjects considered long-term cellphone users prone to what's called "recall bias." In other words, when they're asked to think back 10 years or more they often overestimate how much they used their cellphone. This, Barnes explained, can cause a positive bias in the estimates of disease risk.

Dr. Daniel Krewski, head of Canada's participation in Interphone, co-authored the latest study on recall bias and found that it had potential to skew the outcome of the 13-country project. Such biases must be factored into the final Interphone analysis, said Krewski, otherwise "if you're recalling your phone incorrectly we're not going to get accurate results."

The need to get a better grip on recall and other possible biases is why the final draft of the Interphone study has been several years delayed, said Krewski. "This is such a high-profile study we want to make sure we get it right before it becomes public."

But some scientists, including Sadetzki, are already looking beyond Interphone and toward new research aimed at teenagers and pre-teens. "They have a long way ahead of them, so the exposure will accumulate," she said, recognizing that the technology – so integrated into our daily life – is here to stay regardless of what the studies suggest.

"Nobody will stop using this technology. There are car accidents and still we keep driving cars. The question is what precautions do we take?"

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