Despite numerous studies indicating that cell phones pose no health risk to their users, a few studies have been released that suggest prolonged use might contribute to brain cancer. For the World Health Organization, that was enough to declare the phones "possibly carcinogenic" and to call for further studies on the link.

At least one of these studies was already in the works. Some specific features of how Denmark tracks its citizens have made that nation a convenient laboratory for long-term population studies. Now, one study has looked at almost the entire Danish adult population and found that having a cell phone doesn't seem to be associated with any additional risk of brain cancers.

Since 1968, any adult citizen of Denmark has had a national ID number that's used for a variety of identification purposes. Conveniently, cell phone companies have used the numbers to register subscribers, meaning that "the whole Danish adult population [can be] subdivided into subscribers and non-subscribers of mobile phones and followed up for incidence of cancer and other diseases." Bad for privacy, but good for science!

Of course, following up with that many people can be a bit of a hassle, so the authors took advantage of another national resource: the country's Institute of Cancer Epidemiology was already tracking everyone who was born after 1924 and still alive in 1990. Basically, any cancer that occurred in a Danish citizen living in Denmark was registered, and, conveniently, the cancer patients were tracked by the same national ID number.

By processing all this data, a research team was able to identify nearly 360,000 people who had cell phone subscriptions during the study period, along with a few million controls. Collectively, the subscription holders logged over 3.8 million person-years of cell phone time. The only significant weakness to the study is that the authors couldn't associate corporate accounts with individual users, so they weren't considered in the analysis.

An association with cancer was measured through what's called the "incidence rate ratio," which is exactly what it sounds like: it's the ratio between the rate at which the experimental population ended up with cancer and the rate at which the controls ended up with cancer (the calculations used a log linear Poisson regression model, for those statistically inclined). Things like age, income, and the number of years that a person had a cellphone plan were all adjusted for.

In the end, almost nothing stood out; most ratios were essentially 1, meaning that there was no observed link between the phone users and cancer. There was a slight but insignificant rise in gliomas in men, but rates were highest in the most recent subscribers. There was almost no change among women. For meningiomas, male cell phone users actually had some cancer protection (not statistically significant, but a larger change than in gliomas). Women again saw almost no difference.

In none of the cases was there any indication of a dose effect related to longer ownership of a cell phone. Where information about the site of the tumor was available, there was no indication that tumors were more likely to occur where the phone was held.

In short, there's nothing here that indicates a risk associated with cell phone use. Right now, the study is limited to 15 years of exposure, based on the year that the phones were introduced in Denmark. But researchers will undoubtedly be tracking the group as it continues to age, so we'll have to wait for any truly longer-term risks to be determined.

BMJ, 2011. DOI: 10.1136/bmj.d6387 (About DOIs).