The folks at ProPublica.org, the online investigative news website, have looked into the high number of false positive readings being made by a type of airport scanner. http://www.propublica.org/article/sweating-bullets-body-scanners-can-see-perspiration-as-a-potential-weapon

The millimeter-wave machines are considered safer than the X-ray body scanners, which are also widely used at airports. Unlike X-rays, however, the low-level electromagnetic waves are not linked to cancer.

They are blamed for so many errors, however, that France and Germany have banned them.  The false positives have been triggered “by folds in clothing, buttons and even sweat.”

According to the ProPublica story, “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.”

The Transportation Security Administration has installed 250 millimeter-wave machines at airports here, and plans to add another 300 this spring.