The panel's decision, which it based on a review of published studies on the topic, lands cellphones on a list of possible carcinogens that includes the pesticide DDT and gasoline engine exhaust. But it has surprised experts outside the panel, who say the data on cellphone use and brain cancer is still inconclusive.
"In general, rating agencies such as this tend to be conservative by nature. They have to be," said Dr. Henry Friedman, deputy director of the Preston Robert Tisch Brain Tumor Center at Duke University Medical Center in Durham, N.C. "The reality is that all of the epidemiological studies that have been done to date have basically been negative."
An estimated 5 billion cellphone users represent nearly three-quarters of the world's population.
The cellphone-cancer conundrum has been a hot topic since shoe-size phones hit the scene in the late 1970s. Roughly 30 studies have failed to link the devices to cancer. But a large study last year found a slight, statistically insignificant increase in risk in a rare form of brain cancer called glioma among cellphone users. Another study out of the National Institutes of Health Research linked cellphone use to increased brain activity.
"While experimental evidence and very limited human studies suggest that we should be cautious, people should realize there are many things we are exposed to every day that also is classified by IARC as possibly carcinogenic," said Dr. Peter Shields, chief of Georgetown University Hospital's cancer genetics and epidemiology program in Washington, D.C. "The classification used by IARC for cellphones is the lowest of all the carcinogenic classes, and no one should think that cell phones pose the same risk as smoking and asbestos."
Working as a hairdresser is considered riskier than using a cellphone, according to the IARC's classification system, achieving "probable carcinogen" status. Other possible carcinogens include working as a dry cleaner or a firefighter. But the cellphone industry was quick to point out other possible carcinogens.
"IARC conducts numerous reviews and in the past has given the same score to, for example, pickled vegetables and coffee," John Walls, vice president of public affairs for CTIA: the Wireless Association, said in a statement. "This IARC classification does not mean cellphones cause cancer."
Epidemiological research on environmental factors and cancer risk is plagued by methodological pitfalls, such as subjects' bias in recalling previous exposures and insufficient follow-up time for a disease that can develop slowly, like cancer. A study examining cellphone use in the decade leading up to a cancer diagnosis, for example, might be unable to detect a connection if the disease were triggered 15 years ago.
"The only thing one could say is that maybe we need studies with a much longer follow-up period," Duke's Friedman said.
Nevertheless, some experts believe the evidence, inconclusive as it is, warrants caution. ABC News reached out to 92 physicians, 65 of whom said they would continue to hold their cellphones up to their ear, but 27 said they will use hands-free devices to minimize their risk.
In light of the IARC decision, the World Health Organization and national health agencies, such as the U.S. Centers for Disease Control and Prevention, might decide to provide similar guidance on cellphone use.
"[The] IARC is saying that we should be cautious and think through what we do when we regulate exposures from cellphones," Georgetown's Shields said. "They follow the precautionary principle and want to maximally protect public health."
The Federal Communications Commission (FCC) requires cell phones to meet safety standards based on the advice of federal health and safety agencies, an agency spokesman said in a statement.
"We support the IARC recommendation for more research to clearly identify any potential health risks and, as appropriate, consider whether further actions may be required."
The Associated Press contributed to this report.