There are two types – the Millimeter Wave Scanner and the Backscatter scanners.
Millimeter Wave Scanner From a health perspective, this is the less controversial of the two scanners (from a privacy perspective, they’re both equally invasive).
Regular searching procedures, used by the Transportation Security Agency (TSA) staff include patting down, baggage x-ray screening for suspicious items and explosives, screening with a metal detector, and comparing passengers’ names submitted by airlines to a watch-list.
In 2010, the TSA had implemented full-body x-ray scanning, which was meant to guarantee that no suspicious or threatening items transported by would-be terrorists could be carried on board (Council on Foreign Relations).
Backscatter X-Ray Scanners This one is a bit confusing, and it doesn’t work in the same way traditional x-rays do (which go straight through an object).
Instead, it hits the surface of an object with x-ray photons (it’s a small dose, but they are carcinogenic); the radiation that is reflected back then produces an image of the object.
Unlike the MW Scanner, it creates only a two-dimensional picture, so a traveler’s front and back must be x-rayed.
In either case, the machines perform a “virtual strip search” essentially creating a monochromatic photo of the traveler’s naked body, revealing the naked body and anatomy in detail.
Mechanical (and sometimes manual) inspection seems to be the most effective way of revealing hidden threats, especially compared to the majority of other existing methods.
For example, specially trained dogs can smell explosives or armory lubrication, but it is doubtful passengers would like to be examined by such dangerous beasts.