As of Tuesday, 29 U.S. airports have 93 advanced imaging technology units. Ninety airports are home to 930 advanced technology x-ray devices and over 7,300 chemical detectors known as explosive trace detection systems are in use in airports, according to the Transportation Security Administration.

This array of machines peeks under travelers clothing, into their baggage and tastes the chemicals from their hands more than ever before.

“If terrorists are going to put bombs in embarrassing places, we have to look in embarrassing places,” said Carey Rappaport, deputy director of the Northeastern University Awareness and Localization of Explosive-Related Threats Center in Boston.

But all the attention  paid to what can and can’t be seen and what images will and won’t be saved by full-body scanners detracts from the real question of what are these things looking for and what can they see.

The scanning technologies the TSA uses can be broken down into two major types: imaging systems and chemical detection systems.

Imaging systems use a variety of technologies –  among them are low-power x-rays and millimeter rays, which are like weak microwaves and similar to the radar detectors police use to catch speeders, – to do something similar to sonar and radar. Each bounces a wave off of a target and gives information about the target. Sonar and radar can say where something is, how big it is and how it is moving.

The waves used by airport scanners go through clothing and hair, but bounce off of tissue and other objects. A computer interprets the way the waves interact with those objects and return to the sensors, telling the human machine operators some information about what the object is. The waves interact differently with tissue, plastic, metal, liquid and other chemicals, giving the systems’ operators a good idea of the shape of any concealed objects and a vague impression of what any concealed objects may be made from.

“All the things that are man-made are anomalous,” Rappaport said. “You’re expecting to just see flesh.”

More advanced scanners still being tested and not yet in commercial use different types of waves and are even more sensitive to the different types of disruption made by different substances, and so give a more refined view of what hidden objects are made of.

“One of the things about millimeter rays and x-rays is they look for shapes and some chemical composition,” Rappaport said. “But they can’t say ‘this is RDX’ or ‘this is sugar.’” RDX is the explosive in C-4. It was used in the 2006 Mumbai train bombings that killed 209. “What are they trying to look for are threats,” Rappaport said. “A navel ring isn’t a threat. A gun is. A piece of plastic in the shape of knife is.”

Explosive trace detection systems work on an entirely different principle. These machines measure the chemicals swabbed off of hands, zippers, handles and anything else that gets swabbed, looking for tiny amounts– sometimes only a few molecules – of explosives or explosive precursors.  One type of chemical detector looks for specific substances (although many of them, including the most common used for homemade bombs and the most common narcotics), while another type gives a yes or no only for so-called energetic chemicals, which are necessary components of any explosive.

But as the sophistication and sensitivity and, likely, invasiveness of detectors grows, so will the creativity of people seeking to do harm, Rappaport said. “What do you do with people who have taken explosives internally?” he said. “This is tough stuff to write about; tough stuff to consider.”