Or they're completely harmless. It depends which physicist or epidemiologist you ask, which research you cite, and even which section of a particular study you believe.
It's enough to keep you awake at night. (That insomnia, by the way, probably has nothing to do with yakking away on the phone before bedtime. Then again, it might, according to one study).
"The solution here is not hard. This is not rocket science. Just keep it away from your brain," said epidemiologist Devra Davis, author of "Disconnect: The Truth About Cell Phone Radiation, What the Industry Has Done to Hide It, and How to Protect Your Family," due out next month.
Davis, who argues that the $153 billion wireless industry is blocking Americans' access to reliable information on cell-phone radiation, said that consumers should change their habits now - even if the science is inconclusive - to prevent a "catastrophe" years down the road.
"In my professional opinion, there are a number of cases of people with brain tumors today for which cell-phone radiation was a major contributing cause," she said.
Experts who believe that the radiation could be harmful recommend using a speaker phone or a hands-free device - preferably a wired headset - instead of holding the phone to your ear, and texting instead of talking.
"A Bluetooth [hands-free device] does reduce your exposure significantly," Davis said. "But if you have the Bluetooth on all the time and the phone is in your front pocket where it's radiating your gonads, that really is not a good idea. You want to keep the phone off of your body."
Bruce Stutz, whose report in Yale Environment 360 last week examined a decade of peer-reviewed studies, said it's difficult to assess the dangers of cell phones because brain cancers are slow-growing and the use of wireless devices is evolving.
"There's no real way to know at this point. There are indications both ways," Stutz said. "Most of the scientists that I spoke with are of the opinion that there is more research that needs to be done."
Inaccurate data on radiation exposure can wreak havoc on a study's results. The World Health Organization's 10-year, 13-country survey published this year actually found a reduced cancer risk among some cell-phone users, which researchers described as "implausible" and attributed to a flawed methodology.
Want reassurance that your phone is safe? Talk with Robert Park, a physics professor and former chairman of the Department of Physics at the University of Maryland. Even though he sets off a firestorm of controversy every time he says it, Park states unequivocally: "Cell phones don't cause cancer."
Park, who seeks to debunk pseudoscience, said the radiation emitted by cell phones is not nearly powerful enough to alter DNA, and therefore, is not harmful to humans.
"There's nothing else to say about it," he said. "There is no reason to do these damned studies."
Park speculates that the legal industry might be pushing the cancer connection, hoping to cash in on a "mass tort blitz."
"If they can show, however remote the possibility, just the tiniest evidence that every once in a great while a cell phone causes cancer, then everyone [who] uses a cell phone . . . that gets cancer can sue," Park said.
Kenneth Foster, a University of Pennsylvania bioengineering professor who studies the health and safety of electromagnetic fields, said that if phone radiation were dangerous, the studies probably would be more conclusive by now.
"People have been arguing about the health effects of electromagnetic fields for over 100 years," Foster said. But, he admits, cell phones are relatively new, so "realistically, nobody knows what's going to happen in the long run."
"It'll never die down," Foster said of the dispute. "Anyone who wants to worry can pick and choose some bits of evidence and say the world is coming to an end."
The research to date is not damning enough to concern heavy cell-phone users like Diane Rossi, 33, of Ridley Park. She burns through up to 3,000 minutes a month, and another study isn't going to slow her down.
"It's always something," said Rossi, who works at a lithium-battery research and development company. "I'm not really sold on the fact that a cell phone is going to up my risk. I feel like I'm exposed to these waves every day all the time anyway. I can't imagine a lot of people in my age group changing their habits."
But Davis, the epidemiologist, said independent studies of cell phones have shown a higher risk of cancer and other diseases than studies supported by the wireless industry. She's worried that the public is being misled by the "he said, she said" nature of the debate and the conflicting data.
In June, San Francisco passed a cell-phone "Right-to-Know" ordinance that would require stores to inform customers of how much radiation each cell phone emits. The industry group CTIA-The Wireless Association, which insists that cells phones are safe, filed a lawsuit last month seeking to block the law.
John Walls, CTIA's vice president of public affairs, said the law would mislead consumers by creating "the impression that there's a difference in safety between one device and another," even though all the phones on the market meet Federal Communications Commission safety standards.
"The FCC and science tells us they're all safe," Walls said.
Davis said the wireless industry appears to be "following the playbook" of big tobacco, which denied the risks of smoking until the evidence was incontrovertible. Walls calls the comparison "outrageous.
"It's a little frustrating, to be honest with you," Walls said. "We're being painted in some circles as being the bad guys in this scenario, when all we do is defer to the scientific conclusions of world-renowned public-health organizations and agencies."
So, should you be taking steps to limit your cell-phone radiation exposure, like using a headset or speaker phone or turning it off when you're not using it?
No need, said Walls.
"Science tells us those types of precautions aren't necessary," he said. "But if they provide a higher comfort level for people, certainly that's their prerogative and their right to do that."
But Davis said if you ignore her advice, "you're treating yourself and your children like lab rats."
"From a public-health point of view, we don't want to be in the position, 20 years from now, of proving that cell phones were dangerous," she said. "We want to avoid what could become a catastrophe by taking simple steps to reduce exposure now."