NEW YORK (CNN/Money) - You thought your cell phone bill was enough to worry about.

Then there are all those horror stories. Car wrecks caused by distracted drivers. The executive with a malignant brain tumor.

And all the warnings: Turn the cell phone off on the airplane. Don't call from inside a hospital.

But how real are the risks?

Fears of brain cancer from cell phone usage gained currency with a few high profile lawsuits alleging a connection.

In one suit against Motorola and several major cell phone carriers a Maryland neurologist claimed that cell phone use had caused his brain cancer. The plaintiff's attorneys presented research by a Swedish cancer researcher indicating a possible connection. But defense attorneys had plenty of research too, and in September 2002, the suit was thrown out of court.

Still, people are worried. The reason is that cell phones produce electromagnetic radiation which penetrates the brain a short distance from the phone's antenna. But electromagnetic radiation from cell phones is different from the ionizing radiation from, say, an X-ray or a hunk of uranium, which damages DNA and is clearly linked to cancer.

The radiation from cell phones falls in a frequency range somewhere between what you're exposed to when you stand next to your television and what comes out of a leaky microwave oven. It could, theoretically, damage brain cells by heating them. But the heat from cell phones is slight. It isn't like your brain is being baked.

"The research is unequivocal that this type of radiation doesn't cause cancer. But it hits a bunch of emotionally resonant buttons, so we're all afraid of it," said David Ropeik, director of risk communications at the Harvard Center for Risk Analysis.

If you take a call in the waiting room at a hospital, could you be causing someone to go to code blue on the floor above you?

The possibility that anything really awful could happen is small, according to the limited research that's been done. In a study released in January 2001, researchers at the Mayo Clinic in Rochester, Minn., tested cell phones with hospital devices that monitor heart and lung activity. The phones did cause interference, they found, but in most cases the interference would not have been cause for concern.

These tests were done "in vitro" said Dr. David Hayes, one of the study's authors, meaning patients weren't connected to the machines. Tests still need to be done with patients actually connected to the devices.

"Until that's done we really need the [no cell phone] signs up," he said.

It seems silly: Doesn't a jumbo jet have safeguards to protect the airplane from being knocked off course by your little Nokia phone?

Really, there are two reasons you're required to turn off your phone on an airplane. The Federal Communication Commission bans the use of cell phones on airplanes because they could wreak havoc with cell phone systems on the ground.

Signals from your cell phone, when you use it on or near the ground, reach just a few cell phone nodes near you and the node that's getting the strongest signal picks up your call. If you move, while driving your car or walking, the next node picks up the call.

From the air, however, your phone's signal could reach miles, hitting many nodes at once, all with equal strength. Plus, you're moving at several hundred miles an hour. Cell phone systems weren't designed to handle that.

The Federal Aviation Administration, for its part, supports the FCC ban for its own reasons. They fear cell phones may interfere with navigation and other aircraft systems.

Incident reports submitted by airline crews also demonstrate the potential for trouble. NASA's Aviation Safety Reporting System's "Passenger Electronic Devises Database Report Set" -- which could be subtitled "passengers behaving badly" -- contains several reports of incidents involving passengers whose "personal electronic devices" seemed to create disturbances in aircrafts' electronic systems.

In some cases, whether or not a device caused a problem depended on specific location within the airplane. In one report, moving a passenger with a wireless hearing aid to a different seat solved the problem.

In controlled tests done in February 2000, Britain's Civil Aviation Authority showed cell phones can, indeed, interfere with avionics equipment on airliners and that the exact position on the aircraft makes a big difference. Interefence levels varied signicantly as cell phones were moved throughout the fuselages of test aircraft.

Many cities and one state now require drivers to use a hands-fee headset when talking on the phone while driving. That at least leaves both the driver's hands free, if not his entire brain.

Which brings up the real problem with talking on the phone while driving. It's not the lack of one hand that's the trouble. Simply having a telephone conversation while driving actually hampers the ability to drive, according to published research.

A study published in the New England Journal of Medicine in February 1997 highlighted the problem. Researchers cross-checked the cell phone billing records and accident records of 699 cell-phone-owning drivers who'd been in wrecks over a 14-month period.

They found that a driver's risk of crashing while using a cell phone was four times greater than the risk without the phone. They also found that hands-free units didn't help a bit. Chatting drivers crashed just as often when they had both hands free.

There's plenty of research showing that doing things like memory, reasoning and arithmetic tasks while driving seriously hampers the driving part of the equation. It's no surprise then that, when David Strayer, a researcher at the University of Utah, put college undergrads in driving simulators and asked them to talk on the phone while driving, their driving suffered. They missed traffic signals and reacted more slowly to events whether they were using hands-free cell phones or not. Listening to the radio, or even to an audio book, didn't present nearly the same level of difficulty.

"Can you imagine your pilot calling in to reserve theater tickets while flying your plane?" said Strayer.

Peter Valdes-Dapena is a CNN/Money producer. You can e-mail him at peter.valdes-dapena@cnnmoney.com.