The finding, from the agency's International Agency for Research on Cancer, adds to concerns among a small but growing group of experts about the health effects of low levels of radiation emitted by cellphones. The panel, which consisted of 31 scientists from 14 countries, was led by Dr. Jonathan M. Samet, a physician and epidemiologist at the University of Southern California and a member of President Obama's National Cancer Advisory Board.
The group didn't conduct any new research but reviewed numerous existing studies that focused on the health effects of radio frequency magnetic fields, which are emitted by cellphones. During a news conference, Dr. Samet said the panel's decision to classify cellphones as "possibly carcinogenic" was based largely on epidemiological data showing an increased risk among heavy cellphone users of a rare type of brain tumor called a glioma.
Last year, a 13-country study called Interphone, the largest and longest study of the link between cellphone use and brain tumors, found no overall increased risk, but reported that participants with the highest level of cellphone use had a 40 percent higher risk of glioma. (Even if the elevated risk is confirmed, gliomas are relatively rare and thus individual risk remains minimal.)
Most major medical groups, including the American Cancer Society and the National Cancer Institute, have said the existing data on cellphones and health has been reassuring. For years, concerns about the health effects of cellphones have been largely dismissed because the radio frequency waves emitted from the devices are believed to be benign. Cellphones emit nonionizing radiation, waves of energy that are too weak to break chemical bonds or to set off the DNA damage known to cause cancers. Scientists have said repeatedly that there is no known biological mechanism to explain how nonionizing radiation might lead to cancer or other health problems.
The W.H.O. panel ruled only that cellphones be classified as Category 2B, meaning they are possibly carcinogenic to humans, a designation the panel has given to 240 other agents, including the pesticide DDT, engine exhaust, lead and various industrial chemicals. Also on the list are two familiar foods, pickled vegetables and coffee, which the cellphone industry was quick to point out.
"This I.A.R.C. classification does not mean cellphones cause cancer,'' John Walls, vice president for public affairs for CTIA-The Wireless Association, an industry group, said in a statement. Mr. Walls noted that both the Federal Communications Commission and the Food and Drug Administration have concluded that the weight of the scientific evidence does not link cellphones with cancer or other health problems.
This year, The Journal of the American Medical Association reported on research from the National Institutes of Health, which found that less than an hour of cellphone use can speed up brain activity in the area closest to the phone antenna. The study was among the first and largest to document that the weak radio frequency signals from cellphones have a measurable effect on the brain. The research also offers a potential, albeit hypothetical, explanation for how low levels of nonionizing radiation could cause harm without breaking chemical bonds, possibly by triggering the formation of free radicals or an inflammatory response in the brain.
"We looked carefully at the physical phenomena by which exposure to such fields might perturb biological systems and lead to cancers," said Dr. Samet. But he said the result was inconclusive, adding, "We found some threads of evidence about how cancer might occur but have to acknowledge gaps and uncertainties."
The panel made no comment on how large or small a risk cellphone radiation may pose to human health. "Our task was not to quantify risk,'' said Dr. Samet.
Although the panel did not make specific recommendations to consumers, a representative did note that using a hands-free headset during a conversation or communicating via text message would be options for lowering radio frequency exposure. (Read: Tips to reduce mobile radiation exposure)
The panel's recommendation is unlikely to have any immediate effect, but is expected to be used as guidance by the World Health Organization, which may make recommendations about cellphone safety.
Dr. Meir Stampfer, a professor of epidemiology at the Harvard School of Public Health who is a paid adviser for the cellphone industry, said it was important to remember that the panel's decision to rank cellphones as a "possible" carcinogen was very different from saying that they pose a real health risk.
"It's a very thoughtful group, but the important thing is putting it into the perspective of what 'possible' means, and the likelihood that this is really something to be concerned about,'' Dr. Stampfer said. "The evidence doesn't support that. Comparing this to going out in the sun or any number of normal everyday activities that we're not really concerned about, I would put cellphones in the lower part of that category.''
Still, Louis Slesin, editor of Microwave News, a newsletter that focuses on nonionizing radiation, said in an e-mail that the fact that the W.H.O.'s cancer panel had expressed concern had the potential to change the debate about the health risks of cellphones. "It's a wake-up call for the telecom industry and for the U.S. government to take cellphone radiation seriously," he said. "The first step should be limiting the use of cellphones by children."
Henry C. Lai, a professor of bioengineering at the University of Washington in Seattle and an expert in electromagnetic fields, said the credibility of the W.H.O. panel made it difficult to dismiss the findings.
"The debate will go on, except this is the first statement from the W.H.O. saying we should be careful with exposure to this kind of radiation,'' he said. "It's quite a mixture of people, and some very respectable researchers. If someone says this panel isn't good, I don't know who else we should be listening to."
Dr. Lai added that the solution to concerns about cellphone risks is relatively simple. "A precautionary approach is the best policy," he said. "If people use cellphones, they should consider using an earpiece. Just keep the phone away from the head."