A group of researchers at the Danish Cancer Society’s Institute of Cancer Epidemiology in Copenhagen and at the International Agency for Research on Cancer in Lyon have concluded the first serious prospective study of a large population of both cell phone users and non-users over a long span of time. Put simply, the study finds that long-term use of cell phones does not cause brain cancer. It’s published in the current issue of the British Medical Journal. Here’s the abstract:

Objective: To investigate the risk of tumours in the central nervous system among Danish mobile phone subscribers.

Design: Nationwide cohort study.

Setting: Denmark.

Participants: All Danes aged ≥30 and born in Denmark after 1925, subdivided into subscribers and non-subscribers of mobile phones before 1995.

Main outcome measures: Risk of tumours of the central nervous system, identified from the complete Danish Cancer Register. Sex specific incidence rate ratios estimated with log linear Poisson regression models adjusted for age, calendar period, education, and disposable income.

Results: 358,403 subscription holders accrued 3.8 million person years. In the follow-up period 1990-2007, there were 10,729 cases of tumours of the central nervous system. The risk of such tumours was close to unity for both men and women. When restricted to individuals with the longest mobile phone use—that is, ≥13 years of subscription—the incidence rate ratio was 1.03 (95% confidence interval 0.83 to 1.27) in men and 0.91 (0.41 to 2.04) in women. Among those with subscriptions of ≥10 years, ratios were 1.04 (0.85 to 1.26) in men and 1.04 (0.56 to 1.95) in women for glioma and 0.90 (0.57 to 1.42) in men and 0.93 (0.46 to 1.87) in women for meningioma. There was no indication of dose-response relation either by years since first subscription for a mobile phone or by anatomical location of the tumour—that is, in regions of the brain closest to where the handset is usually held to the head.

Conclusions: In this update of a large nationwide cohort study of mobile phone use, there were no increased risks of tumours of the central nervous system, providing little evidence for a causal association.

The researchers didn’t have information about minutes of use, so they leave the door open to investigating that dimension in the future, but they didn’t find either of the correlations in their data that you would expect to find if cell phones did cause cancer: An increased risk of cancer based on years of use,and an increased incidence of tumors close to the ear. While some will say “Aha! They didn’t measure the minutes!”, they’ll be clutching at straws.

The article points out that studies that have tried to draw a causal connection between cancer and cell phone use have suffered methodological flaws: They start with people who have brain cancer, and try to work backward to cell phone use based on the patient’s recollections:

[Previous] studies are based on few cases. In addition, most studies have been retrospective case-control studies with self reported data on mobile phone use, which are prone to bias, particularly random reporting bias and differential recall bias for cases and controls, which hampers the risk estimation and precludes firm conclusions.

To do this sort of thing right, you have to do a “cohort study” that takes a group of non-users and tracks them over time as some become users and others don’t. Then you adjust for known risk factors in the two populations to make sure you’ve isolated cell phone use as the operative variable and crunch away. Cohort studies have only been done in Denmark, where there are national, non-anonymous registries of both cell phone use and cancer. The previous Danish cohort study actually found that long-term cell phone users had a decreased risk of brain cancer, but the sample size was very small:

There was, however, a decreased risk (standardised incidence ratio 0.66, 0.44 to 0.95) of developing a tumour of the brain or nervous system in people who had had a subscription for more than 10 years, but this
result was based on only 28 cases.

The conclusion was very clear:

In conclusion, in this update of a nationwide study of mobile phone subscribers in Denmark we found no indication of an increased risk of tumours of the central nervous system. The extended follow-up allowed us to investigate effects in people who had used mobile phones for 10 years or more, and this long term use was not associated with higher risks of cancer. Furthermore, we found no increased risk in temporal glioma, which would be the most plausible tumour location if mobile phone use was a risk. As a small to moderate increase in risk for subgroups of heavy users or after even longer induction periods than 10-15 years cannot be ruled out, however, further studies with large study populations, where the potential for misclassification of exposure and selection bias is minimised,
are warranted.

I doubt this will slow down the fear-mongers who peddle books and collect speaking fees for claiming that cell phones will kill you, but it’s nice to have some hard data on the books.