And now we come to the most controversial and hotly debated topic regarding whole-body scanners: their safety. The question about safety comes down to whether a scanner uses ionizing radiation or not. Ionizing radiation has enough energy to remove electrons from atoms and therefore alter the structure of biological molecules, such as proteins and nucleic acids. X-rays are a form of ionizing radiation; radio waves, visible light and microwaves aren't.
Backscatter machines use X-rays, so the question then becomes one of intensity and duration. The manufacturers of the scanners insist that a single scan exposes a person to minuscule levels of radiation. In fact, a Rapiscan executive has said, "You would have to go through [a backscatter] scanner 1,000 times to equate to one medical X-ray. You get twice as much radiation when eating a banana than when going through the scanner" [source: Paur].
But other studies have come to more troubling conclusions. One, from the Marquette University College of Engineering, found that backscatter X-rays do penetrate the skin and strike deeper tissues. In a second study, researchers from the Columbia University Medical Center estimated that 1 billion backscatter scans per year would lead to 100 radiation-induced cancers in the future.
Millimeter wave scanners don't carry these risks because they use non-ionizing radiation. To date, no known safety issues have been found with this type of scanner.
Here's another biggie: privacy. Both types of scanners are capable of producing images that reveal intimate details about travelers. With that said, the TSA has gone to great lengths to protect the privacy of those being scanned. The software of backscatter machines, for example, includes a privacy algorithm to blur genitalia and faces while highlighting potential threats.
Most (but not all) millimeter wave machines use automated target recognition (ATR) software that renders every subject as a generic outline, with suspicious areas highlighted. And if it doesn't detect anything suspicious in a scan, it displays the word "OK" with no image at all. For scanners without ATR software, the security operator viewing the resulting image sits at a remote location and communicates wirelessly with the agent operating the machine.
Supposedly, neither type of machine is capable of storing images -- each image is deleted automatically as soon as the security team completes its inspection -- but there have been reports of U.S. marshals in Florida downloading and storing thousands of images [source: McCullagh].
That's it. That's all we have. You may now consider yourself an expert on advanced imaging technology machines.