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Are those full-body scanners at the airport giving passengers dangerous levels of radiation? An expert in radiology says, not even close.

Dr. James O'Donnell, a professor of radiology at Case Western Reserve School of Medicine and the Director of University Hospitals Division of Nuclear Medicine, says you're naturally exposed to higher levels of radiation every day than what you'd get in an airport scanner.

He says radiation is all around us in different forms, from sunlight, to cell phones, computer screens, lights, bricks and other natural occurring sources.

In a year, the average Clevelander receives the radiation equivalent of about 20 chest x-rays, or 200 millirems. Someone from Denver, a mile above sea level, is exposed to double that level.

Dr. O'Donnell says the level in an airport full-body scanner is about .005 of 1 millirem, meaning you'd need to go through it about 10,000 times to get the equivalent of a year of exposure in Cleveland.

A TSA officer demonstrates what the images from the Advanced Imaging Technology unit looks like at John F. Kennedy International Airport. Michael Nagle/Getty Images

Can They Cause Cancer?

The likelihood of these machines being the direct cause of cancer is very slim. Of the two technologies, the mmw scanners, which beam non-ionizing radiation, are less dangerous. There is no denying back-scatter technology exposes the inspected individual to small doses of radiation, which can cause cancer when the doses add up. The Rapiscan Secure 1000 serves up about 3 microrems per scan, according to the TSA.

Francis Masse, director of the MIT Radiation Protection Office considers 5000 millirems (equal to 5,000,000 microrems) of annual radiation exposure adequately safe.

However, while scientists are aware of the dangers of large doses of radiation at once, Dr. O'Donnell says there's no real data over how much is too much over a lifetime. Cancer patients who are treated with radiation may be good examples of how much the human body can tolerate and many go on to live years longer cancer-free.

How Do the Machines Work?

There are two kinds of scanning devices:

• Back-scatter—Back-scatter scanning machines send narrow, low-density X-ray beams over the subject, front and back, at high speeds, which just barely penetrate the clothing. The X-ray radiation reflects off the body—and off of any objects hidden on that person. The process takes between 6 and 8 seconds.
• Millimeter wave—Millimeter-wave (mmw) machines are typically composed of twin vertical arrays of extremely high-frequency transmitters that circle the passenger and create a three-dimensional image. The transmitters emit beams of radio frequency (RF) energy, which bounce off sub–clothing surfaces and expose hidden items. Like back-scatter machines, the mmw scan requires several seconds before an image is produced.

Where Are the Machines?

These technologies are now in use at 65 U.S. airports—including all of the major ones, such as JFK in New York and Chicago's O'Hare—with more on the way. As of October, 189 back-scatter units and 152 mmw scanners were operational, and the TSA is looking to push the total number of machines to more than 1000, with the goal to scan two out of every three air travelers by 2011.

What Can the Scanners Actually See?

According to the TSA, only what it requires to keep weapons and explosives off airplanes. The back-scatter device and the mmw machine each produce different kinds of images. While the former creates an almost cartoonish two-dimensional snapshot of the inspection subject, the latter produces a holographic, 3D image.

Conscious of privacy concerns, the TSA has given numerous assurances that every machine used in airports will feature face-and-genitalia-obscuring checks installed—the so-called "modesty algorithm." TSA officials also stress that the body scanners are incapable of data storage and that once an inspection ends and the image is cleared, it disappears forever.