You're going to see a lot of headlines today boasting that cellphone use probably won't increase your chances of cancer. That would be some great news, indeed, but here's the harsh truth: like studies we've seen before, this one isn't ruling anything out.
The Danish study pits data from the country's national cancer registry against nearly 360,000 mobile phone contracts in Denmark (more on that in a minute) and spans 17 years, from 1990 to 2007. Culling data from a history of cellphone users to form a conclusion, rather than doing what many other studies (such as the one linked above) are left to do — start the research now, and hope that in ten or twenty years we'll have a definitive answer — is a novel approach, but one with some drawbacks.
First, though, the conclusion that's causing all the buzz: "there were no increased risks of tumours of the central nervous system, providing little evidence for a causal association." Read: cellphones probably won't give you cancer.
While the study pulls from the largest sample size to date (358,403, up from as much as 250,000 in a multinational study last year), what's being seen as a weakness here is a lack of texture in the data.
In an critique titled "Not enough data excluding cellphones' morbidity," French doctors Philippe Charlier and Luc Brun state that "further studies with more precise data are necessary" before anything can be decided:
[This study] takes into account only individual subscription holders, without including those who have only one professional cellphone. More, the daily duration of use of the cellphone is not known, the subscribtion holders not having been questioned on that point. These two limits are important, as it is the intensive use of a cellphone that could cause an increased risk of central nervous system tumour. [Source]
So, we're more or less in the same place right now as when we all woke up this morning. A definite link between increased cancer risk and cellphones has never been disproved (or proven), it's believed that cellphones probably don't increase one's cancer risk, and more studies still have to be done.
Cancerous tumors may not be the only worry, besides. From the New York Times:
Last year, a 13-country study called Interphone also found no overall increased risk but reported that participants with the highest level of cellphone use had a 40 percent higher risk of glioma, an aggressive type of brain tumor. (Even if the elevated risk of glioma is confirmed, the tumors are relatively rare, and thus individual risk remains minimal.)
You can read the Danish study for yourself here, published in BMJ, a British medical journal. It's worth mentioning another editorial in the same journal here, one which praised the study's methods (in that it didn't rely on retroactive information from cell users themselves — a questionnaire-style approach that's dubious at best), but concluded that further study is needed.