Figure 1: Millimeter-wavelength radiation has a frequency of roughly hundreds of gigahertz, which falls in between microwaves and infrared radiation, the same frequencies as some wifi devices.
Figure 2: TSA's information sheet on Backscatter X-Ray
First of all, we should clarify some things. Most of the TSA's "Advanced Imaging Technology" (AIT for short) scanners operate using millimeter waves, not x-rays. These millimeter wave scanners work by bouncing electromagnetic radiation off your body and detecting the reflected waves as a 3-D image (the reflected waves will be phase-shifted based on how far away you were and the material the waves are striking, thus creating the "image"). Millimeter-wavelength radiation has a frequency of roughly hundreds of gigahertz, which falls in between microwaves and infrared radiation, the same frequencies as some wifi devices. A three-second-long stint in the millimeter wave scanner is no worse for you than standing in front of your wireless router at home, and because they only need just enough intensity to create an image (and not broadcast streaming video across the house), the overall dose is much, much smaller.
There are a few AIT scanners which operate using low-intensity x-rays ("backscatter technology"). X-rays are considered worse for you mainly because they are higher energy (note, however, that energy and intensity are not the same thing!). This higher energy allows for more penetration into matter, which is why x-rays are used to see inside your body. But just like the millimeter wave scanners, we only need a quick image of the body, and not a high-resolution picture of your skeletal system (like you'd get if you went to the doctor with a broken arm), so the intensity - and thus the overall dose - is very small. The normal amount of background radiation that you receive each day - everything from cell phones to naturally occurring radiation in the ground - is equivalent to nearly 100 screenings from the new TSA scanners. In fact, you'll get a lot more radiation being on the plane than going through the scanners - you're higher in altitude, so there's less atmosphere protecting you from the natural cosmic radiation (it's still not enough to warrant being afraid, though!). The x-ray scanners are safer than x-rays from the doctor or dentist (because they're lower intensity), and certainly far safer than cigarettes (yes, cigarettes do give you a dose of radiation!) or tanning beds. As a frequent traveler, your worry should be catching somebody else's flu on the flight, not the minuscule amount of radiation you'll get from the security screening beforehand.
As it states on the TSA's website:
Advanced imaging technology is safe and meets national health and safety standards. Backscatter technology was evaluated by the Food and Drug Administration’s (FDA) Center for Devices and Radiological Health (CDRH), the National Institute for Standards and Technology (NIST), and the Johns Hopkins University Applied Physics Laboratory (APL). For comparison, a single scan using backscatter technology produces exposure equivalent to two minutes of flying on an airplane, and the energy projected by millimeter wave technology is thousands of times less than a cell phone transmission.
Kelly Chipps (AKA nuclear.kelly)
Department of Physics
Colorado School of Mines
John from Atlanta, Georgia