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Can full-body airport scanners harm you?

Big Brother may be watching air travelers more closely than ever, but at least he's not subjecting you to as much radiation as what you choose to be exposed to once up in the air.

The scanners are supposed to be the high-tech version of a physical pat-down. TSA

Since explosive materials were sneaked onto a U.S. domestic flight on Christmas Day, full-body scanning machines are far more likely to make their way to security lines at your local airport, even though they might not have detected said materials.

While the Transportation Security Administration already has 40 such devices in place, it just bought 150 to be placed in U.S. airports and says it plans to buy 300 more (they go for $170,000 apiece). On Wednesday, the Netherlands announced that these scanners would be used on passengers for all flights out of Amsterdam to the U.S., and there is talk of scanners in Nigeria as well.

So, setting aside the non-health-related question of whether the scanners will work in detecting explosive materials, are they safe?

These full-body scanners fall into two main categories: millimeter wave and backscatter. The first directs radio waves over a body and measures the energy reflected back to render a 3D image. The latter is a low-level X-ray machine that creates 2D images.

The scanners are supposed to be the high-tech (and energy-inefficient) version of a pat down, and can detect items such as nonmetallic weapons and explosives not picked up by metal detectors. (They only scan surfaces, so body cavity stashing may soon get all the more popular.)

Millimeter wave scanners produce 30 to 300 gigahertz electromagnetic waves, and reveal explosives if they are denser than other materials. This means that these scanners emit less radiation than a typical cell phone, according to TSA. Whether cell phones are harmful is of course the topic of many debates.

The backscatter machines, meanwhile, are low-level X-ray machines that expose bodies to as much radiation as about two minutes of flying in an airplane does. In other words, if you already use a cell phone and you already fly, you are already exposing your body to more radiation than these scanners will.

David Brenner, professor of radiation biophysics at Columbia University and co-author of a report on radiation scanning systems (PDF here), tells me that the risks associated with these low-level radiation scanners are extremely small.

"As far as we know there are no risks associated with the millimeter wave scanners, so my own view is that if you have a choice, you'd want to use the millimeter wave scanners, not the X-ray scanners," he said. "But that being said, the risks, if they exist, are very small. The issue is that small risk multiplied by a very large number of people gives you some population concerns."

As for privacy concerns, TSA writes:

This state-of-the-art technology cannot store, print, transmit or save the image. In fact, all machines are delivered to airports with these functions disabled...Each image is automatically deleted from the system after it is cleared by the remotely located security officer.

According to TSA, six full-body scanners are being used at airports in Albuquerque, N.M.; Las Vegas; Miami; San Francisco; Salt Lake City; and Tulsa, Okla. The other 34 are used for secondary screening of people who set off a metal detector in Atlanta; Dallas/Fort Worth; Denver; Detroit; Indianapolis; Jacksonville and Tampa, Fla.; Los Angeles; Phoenix; Raleigh-Durham, N.C.; Richmond, Va.; and two airports in the Washington, D.C., area: Baltimore/Washington and Reagan National.

If you'd rather not give TSA officials a sneak peek at your body's personal contours, you'll have to put up with a physical pat-down instead. Hey, at least somewhere along the way there's a personal choice on precisely what form one's invasion of personal space will take.