by Michael Grabell and Christian Salewski, ProPublica
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.)
In addition, millimeter waves penetrate clothing materials differently, and layers of clothing can create a barrier, triggering a false alarm.
"These are known as clutter issues in the imaging," McMakin said.
The manufacturer, L-3 Communications, said that in the United States the scanners have not experienced a high rate of false alarms caused by either clothing or sweat. L-3 executives noted that the millimeter-wave machine is installed in airports in some of America's most humid cities, including Houston, New Orleans and Miami.
But as late as last November, the head of the TSA told Congress that false alarms were too frequent to deploy the privacy software. The TSA said the rate has improved since then and now meets its standards, which it would not disclose.
"As with many types of technology, there will be an anticipated amount of false alarms that are considered acceptable, and we continue to work with industry vendors to improve both the detection and operational capabilities for all of our technology," spokesman Greg Soule said.
But results from other countries, as well as tests conducted in the United States before 9/11, show false alarms occurred between about a quarter and half of the time. Moreover, dozens of U.S. travelers told ProPublica they had to get a pat-down despite passing through the body scanners.
Only one report of the false alarm rate for the X-ray body scanners could be found. At Manchester Airport in the United Kingdom, where 13 machines have been tested on more than 2.5 million people, the rate has been less than 5 percent -- and that includes passengers who left items such as keys in their pockets, said airport spokesman John Greenway.
Referring to the false alarm rate, Peter Kant of the manufacturer, Rapiscan Systems, said, "Our numbers internally are in the very low single digits." The company, as well as several physicists, said sweat does not cause false alarms with the X-ray scanners.
In an effort to close a gaping hole in its ability to catch explosives, the TSA in 2009 began installing body scanners alongside metal detectors for routine screening. The deployment ramped up quickly after a Nigerian man tried to blow up a plane that Christmas with explosives hidden in his underwear.