Three types of the system first microwave basically RADAR, limited penetration images detailed, health effects unknown may interfere with DNA. Second backscatter x-ray penetrates 4" [10cm] image's very detailed dose to skin/breast/testicals small but significant. Third system is forrwardscatter/transmission x-ray VERY penetrating looks through your body. X-rays are a well known cancer helth risk no safe dose has been establised. Children, elderly, expecting mothers and people with DNA repair issues should avoid.