A new study casts doubt on the possible link between cell phones and brain cancer, but experts say the risk can't be ruled out.
The study of 358,403 Danish cell phone plan subscribers over 17 years -- the largest study of its kind -- found subscribers of 13 years or more faced the same cancer risk as non-subscribers.
"In general, our findings are in line with most of the epidemiological research that has been conducted to date," said Patrizia Frei of the Danish Cancer Society's Institute of Cancer Epidemiology, lead author of the study published today in the journal BMJ. "They are also in line with in vitro and in vivo studies that show no carcinogenic effects on the cellular level."
The results come just five months after a panel of experts from the World Health Organization's International Agency for Research on Cancer deemed cell phones a possible cause of cancer -- a statement that sparked fear in many of the world's 5 billion cell phone users.
While Frei's findings offer some comfort for communicators on-the-go, experts say further studies are still warranted.
"Frei and colleagues' results may seem reassuring, but they must be put into the context of the 15 or so previous studies on mobile telephones and cancer," Anders Ahlbom and Maria Feychting of the Karolinska Institute of Environmental Medicine in Stockholm, Sweden, wrote in a BMJ editorial. "Although most of these studies were also negative, there are a few exceptions."
One exception is a 2010 study that found a slight, statistically insignificant increase in risk in a rare form of brain cancer called glioma among cell phone users.
"Most of data that shows an association between cell phones and brain cancer is very weak," said Timothy Jorgensen, associate professor at Georgetown Lombardi Comprehensive Cancer Center.
Some such studies are limited by recall bias -- the tendency for people with cancer, desperate for answers, to over-report certain behaviors like cell phone use. Frei and colleagues avoided recall bias by using Denmark's central population register, a mammoth database containing health records as well as cell phone plan details for every resident from birth to death. The register also allowed the researchers to control for education and socioeconomic factors.
"No single study is definitive," said Dr. Peter Shields, deputy director of Ohio State University Medical Center's Comprehensive Cancer Center. "You can't say, based on this, that we never have to worry. But this may end up being the best study out there."
But the study has limitations. In particular, cell phone subscriptions were used as a surrogate for use. And "having a mobile phone subscription is not equivalent to using a mobile phone," Ahlbom and Feychting wrote.
Conversely, some users might be non-subscribers.
"In all of these studies, you have to get information from somewhere," said Jorgensen. "They assumed that people who subscribe to cell phone plans are using their phones, and I think that's a reasonable assumption. The alternative is to talk to people and ask them to tell you about their cell phone use. But people are notoriously inaccurate."
Even Frei admits the study doesn't close the book on cell phones and brain cancer.
"We didn't have any information on the amount of use, so we couldn't do any subanalysis on people with heavy phone use," she said. "There are still some open questions, about greater amounts of use, and about the effects on children."
Some experts think research resources should start to be shifted towards other possible carcinogens.
"At some point, we need to start prioritizing research on things we think might cause cancer," said Shields. "We might not have a definite 'no' for cell phones, but we can certainly have a definite 'Let's move on.'"
In June, ABC News reached out to 92 physicians about their own cell phone use. Sixty-five said they hold their cell phones up to their ear, and 27 said they use hands-free devices to minimize their cancer risk.
MedPage Today reporter Kristina Fiore contributed to this report.