Sweating Bullets: Body Scanners Can See Perspiration as a Potential Weapon

December 19, 2011

Pro Publica

By Michael Grabell and Christian Salewski

“If you’re sweating, you could be a terrorist.  Going through the body scanners make us nervous – hence the sweat.”  –KTRN

While X-ray body scanners used in airports face concerns about potentially increasing cancer cases, a safer type of scanner has been plagued by another problem: a high rate of false alarms.

The scanner, known as the millimeter-wave machine, uses low-level electromagnetic waves that, unlike X-rays, have not been linked to cancer. The Transportation Security Administration already uses the millimeter-wave machine and says both types of scanners are highly effective at detecting explosives hidden under clothing.

But two of Europe’s largest countries, France and Germany, have decided to forgo the millimeter-wave scanners because of false alarms triggered by folds in clothing, buttons and even sweat.

In Germany, the false positive rate was 54 percent, meaning that every other person who went through the scanner had to undergo at least a limited pat-down that found nothing. Jan Korte, a German parliament member who focuses on homeland security, called the millimeter-wave scanner “a defective product.”

While it’s difficult to know for sure if the millimeter-wave machine has a worse false-alarm rate than the X-ray machine, recent tests suggests that it does. The TSA wouldn’t release its results, citing national security. But a British study found the X-ray machine had a false-alarm rate of just 5 percent.

For the millimeter-wave machines, a complicating factor is new privacy software that was installed in many countries after a public outcry over the scanners’ graphic images. The software automates detection and no longer creates an image of a passenger’s body. While false alarms were reported before automation when human screeners interpreted images, the software appears to have made the problem worse.

The privacy safeguards are also an obstacle to lowering the false-alarm rate, researchers say. The machines do not save images or data, which could be used to teach the software how to distinguish real threats from false ones.

The problem of false alarms comes down to fundamental physics. Millimeter waves penetrate clothing and reflect off objects. But because of their frequency, millimeter waves also reflect off water, which can cause the scanner to mistake sweat for a potentially dangerous object, said Doug McMakin, the lead researcher who developed the millimeter-wave scanner at the Pacific Northwest National Laboratory. (X-rays, which operate at a higher frequency, pass through water more easily.)

Click here for the full report.