Dear Cecil: I work as a consultant and am required to travel by air several times per week. I recently began encountering the new screening devices being installed at all airports. My question is, how do these things work? Have there been any long-term health studies? I'm sure they are probably safe for casual travelers, but what about someone who flies much more frequently, like me? I already have 200,000 miles and 130 flights this year. That's a lot of exposure. Rick Jette

Cecil replies:

I wouldn’t worry about the health danger, for reasons we’ll get to. The real risk is to your privacy. Remember those “X-ray specs” once advertised in comic books that supposedly would allow you to see people naked beneath their clothes? Well, the technology has finally been perfected. At many U.S. airports, federal security personnel now get an intimate look at you every time you fly.

Although the Transportation Security Administration had been studying scanners for several years, things got more urgent after the Nigerian “underwear bomber” allegedly tried to blow up a transatlantic flight bound for Detroit last Christmas. Since then the TSA has been rushing full-body scanners into service. The agency expects to have 450 units operating by year’s end and is seeking funding for another 500 in 2011.

There are two types of full-body scanner:

Backscatter scanners use low-level X-rays to look under your clothes. Hidden items reflect some of the radiation and show up on a display. Your skin absorbs the rest.

Millimeter-wave scanners bounce low-energy radio waves off your body, producing more detailed images than backscatter technology.

The two devices, which have been deployed in roughly equal numbers, use fundamentally different types of radiation. Millimeter-wave scanners use a type of microwave radiation — it’s right next to police radar-gun emissions on the electromagnetic spectrum. Some fear that microwaves can be dangerous — you’ll recall those stories about microwaves from cell phones causing brain cancer. But further research hasn’t borne out such fears, and the low-power microwaves in a scanner are generally thought to be harmless.

That’s not true of backscatter technology. The X-rays in these machines are a form of ionizing radiation — the kind emitted by nuclear weapons, which causes cancer in large doses. The standard assumption is that even tiny amounts of ionizing radiation present some risk. But in this case it’s slight. Under the worst-case scenario, one in 200 million backscatter scans could trigger a fatal cancer. Frequent flyers are at much greater risk simply from exposure to cosmic rays while aloft — a scan exposes you at most to 10 microrems of radiation, a high-altitude flight to several thousand. Nonetheless, the Inter-Agency Committee on Radiation Safety, which helps coordinate international policy, has recommended that children and pregnant women not be scanned. U.S. travelers have the option of instead going through a metal detector and then getting patted down by hand.

Why take a chance with X-rays at all? Why not just use millimeter-wave scanners? The TSA dances around on this issue, but evidently in part the idea is to promote competition among vendors. (The two types of machine are made by different companies.)

Now to the question of privacy. Let’s be blunt: a full-body scan means whenever you pass through airport security, you’re going to have a total stranger look at you naked. Millimeter-wave scans in particular are luridly detailed. True, faces are purposely blurred, the scan inspector is in a remote locked room, never sees you in person, and isn’t allowed to carry a cell phone with a camera, and the images are discarded immediately after inspection. But remember we’re dealing here with the TSA, the outfit whose agents made a nursing mother drink her own breast milk, mistook a Congressional Medal of Honor for a ninja throwing star, and forced a woman to remove her nipple rings with pliers. In March of this year a British Aviation Authority employee got a harassment warning from police after he captured an image of a female colleague passing through a full-body scanner at Heathrow airport. In May a TSA employee in Miami took a baton to a coworker who’d made fun of his genitalia after he passed through a scanner.

You may think that’s a small price to pay if it means bad guys can never sneak weapons onto planes. But scans don’t guarantee that. They can’t detect items concealed in body cavities or by folds of flesh. “These technologies can be evaded relatively easily,” a radiation safety expert tells me. “It’s a money-making invasion of privacy.”

To date, full-body scans have met with little public outcry. Then again, the equipment is still being rolled out. A lot of people will encounter scanners for the first time during the upcoming holiday season. If you think airport security has been a barrel of laughs up till now, just wait.

Cecil Adams

Send questions to Cecil via cecil@straightdope.com.