Breaking with its previous reports, the World Health Organization said Tuesday cell phone radiation is possibly carcinogenic to humans, although the group cautions not enough evidence is available to conclusively prove cell phones pose a danger to the nearly 5 billion people worldwide who use them.

After reviewing hundreds of articles, a working group of 31 scientists from 14 countries upgraded the risk of radiofrequency electromagnetic fields to Group 2B, which means there's limited but not sufficient evidence to establish a link between cancer and the microwave-level radiation produced by cell phones, microwaves and radar, according to the AP. Car exhaust and DDT are also in the 2B category, the AP said.

"The conclusion means that there could be some risk, and therefore we need to keep a close watch for a link between cell phones and cancer risk," Dr. Jonathan Samet, the working group chairman and a professor with the University of Southern California in Los Angeles, said in a statement.

The scientists, who met with the International Agency for Research on Cancer in Lyon, France, based their findings on a possible increased risk for glioma, a type of brain cancer. The scientists said they had access to new studies currently accepted for publication but not yet in print.

However, CITA, a wireless communication lobby, dismissed the findings, saying IARC didn't conduct any original research and that the group had previously given the same hazard rating to "pickled vegetables and coffee."

“This IARC classification does not mean cellphones cause cancer," John Walls, vice president, public affairs for the group, said in a statement.

Cell phones emit low-frequency, non-ionizing radiation, and many researchers previously have thought there wasn't a plausible mechanism to link this type of radiation with cancer, as the radiation was too weak to scramble a cell's DNA. The Food and Drug Administration has previously stated that there was no evidence cell phones caused health problems.

However, a study published in the Journal of the American Medical Association in February found regular cell phones users had more brain activity (glucose metabolism) on the side of the head nearest the cell phone's antenna. But it's not clear if this translates into an actual health risk.

And a large study released last year found no connection between cell phone use and cancer, except for a small subset of patients who used cell phones at least 30 minutes per day over a 10-year period. These patients had a 40 percent increase risk for glioma, the IARC said. But the number of patients in this subset was small, and the rate of cell phone use was based on patients' recall years later.

Identifying the risks from cell phones is hard, because it takes decades for cancers to show up and because, as so many people use mobile phones, it's hard to find a suitable control group.

Reinforcing how widespread cell phones are, Twitter users noticed the irony of how they received this scary bit of information.

"World Health Organization acknowledges cell phones may cause cancer. Unfortunately we're all reading about that...on our cell phones," the user foxdavebriggs‎ tweeted.

The IARC's report will be published in The Lancet Oncology's July 1 issue.

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