The WHO has classified cell phones as "possibly carcinogenic," but there are still many open questions.
Most research suggests that cell phones do not pose an increased cancer risk.
If worried, people can text or use headphones to reduce radiation exposure from wireless phones.
With cell phones classified as "possibly carcinogenic" this week, the World Health Organization put the devices smack dab in the middle of its groupings for cancer risk and raised new concerns about how wireless devices might be affecting our health.
A closer look, however, revealed just as many unknowns as there were before the announcement came out. For the scientists who recommended the new classification, concern emerged from some evidence that heavy cell phone use might raise the risk of certain kinds of brain tumors, such as gliomas and acoustic neuromas.
Still, the new WHO report was peppered with uncertainty. The researchers acknowledged that the evidence linking wireless phones with brain tumors was "limited." For other kinds of cancer, the evidence was "inadequate."
A WHO press release quoted working group chairman Jonathan Samet of the University of Southern California as saying simply: "There could be some risk, and therefore we need to keep a close watch for a link between cell phones and cancer risk."
For now, experts say, it's up to individuals to decide how much to use their cell phones. There are ways to minimize radiation exposure, and there's no need to panic.
"In my view, the evidence has been increasingly reassuring about the absence of a hazard," said David Savitz, an epidemiologist at Brown University in Providence, R.I. He is part of a panel that has been monitoring the issue. "I'm surprised, looking at the research up to this point, that they would even be that supportive of a possible hazard."
"Studies have gotten better and better, and there has been more time for problems to develop if they were going to develop," he added. "And yet, as the research grows in quality and quantity, there continues to be a fairly consistent absence for evidence of harm."
To cut through a pile of conflicting research and news reports, the WHO's International Agency for Research on Cancer brought together 31 international experts from 14 countries last week to consider hundreds of studies, both published and not-yet published, regarding cell phones and cancer risks.
After eight days in France, the group put radiofrequency electromagnetic fields (the kinds that come out of cell phones) into group 2B.
That makes mobile phones less risky than the 107 substances in Group 1 that are known to be "carcinogenic to humans," such as tobacco, arsenic, radium and plutonium. Cell phones are also safer than the 59 agents in Group 2A, which includes diesel engine exhaust, certain insecticides and shift work that disrupts circadian rhythms. These are considered "probably carcinogenic."
But cell phones are now officially more of a concern than the hundreds of agents listed in groups 3 and 4, which are "not classifiable" or "probably not" carcinogenic. Those include caffeine, extremely low-frequency electric fields, saccharin and personal hair-coloring products.
Along with cell phones, other items in Group 2B include lead, traditional Asian pickled vegetables, coffee, certain food dyes and working as a firefighter or dry cleaner.
The WHO's working group did not say how much cell-phone exposure might be too much. But they pointed to new data suggesting a 40 percent increase in glioma risk among people who averaged 30 minutes of cell-phone exposure a day for 10 years.
That would increase the risk of developing a glioma from six out of every 100,000 people every year to about eight out of every 100,000, said Henry Lai, a bioengineering professor at the University of Washington, Seattle. There are an estimated five billion mobile phone subscriptions globally.
"If you multiply that, it would be quite a lot of people," Lai said. He mentioned a recent study out of China that linked heavy cell-phone use to a 13-fold increase in risk for salivary gland cancer.
Other studies have shown a boost in activity in adjacent regions of the brain when people hold cell phones up to their ears and changes in sperm when men keep cell phones in their pockets.
"Cell phones are voluntary," Lai said, and he chooses not to use one. But he added that if people are worried, they can greatly reduce their radiation exposures by texting or using ear buds instead of holding their phones up to their ears. Most cell phones actually come with warnings for users to hold the phone a few millimeters away from their heads.
Despite concerns by Lai and others, most evidence so far is reassuring, Savitz said. For one thing, scientists have not measured an uptick in cases of brain cancer or other problems since cell phones began their path toward ubiquity more than 20 years ago.
Meanwhile, a long-term, ongoing, 13-nation study called Interphone has found that "overall, cell phone users have no increased risk of the most common forms of brain tumors," according to a National Cancer Institute statement. "In addition, the study revealed no evidence of increasing risk with progressively increasing number of calls, longer call time, or years since beginning cell phone use."
With so many people using cell phones so frequently these days, perhaps the main point of the 2B classification is not to alarm people but to encourage researchers to keep their eyes open. It's possible, after all, that related health problems just haven't emerged yet.
"We've been wrong before, and when an exposure is this widespread, we want to be as vigilant as possible," Savitz said. Cell phones are "probably without harm. That lingering uncertainty is what people have to make judgments about.