Do backscatter X-ray systems pose a risk to frequent fliers?

The Z Backscatter system made by American Science and Engineering works differently from standard x-ray systems. Instead of relying on images created by the absorption and pass-through properties of the objects being X-rayed, the backscatter system picks up images produced when materials scatter X-ray photons. An X-ray scatter pattern is more specific than an absorption pattern when it comes to identifying organics. A scatter pattern changes depending on the element it's interacting with -- carbon, hydrogen or lithium, for instance. Elements with lower atomic numbers (fewer protons) on the periodic table scatter X-ray photons very powerfully, while elements located farther down on the periodic table tend to absorb more photons than they scatter. Most organics are located closer to the start of the periodic table. So backscatter systems are very good at imaging organic material -- much better than dual-energy systems. They easily pick up the scatter patterns of drugs and explosives and body parts. This ability to detect and identify organic material, along with a technology called "Flying Spot" that lets the machine pinpoint the location of a particular X-ray beam at any given moment in time, allow backscatter images to be incredibly accurate and lifelike.

It's also why some people object to incorporating the technology into airport security checkpoints. Most of us don't want strangers viewing incredibly accurate and lifelike pictures of our bodies. And yes, it's possible for backscatter X-raying to produce photo-quality images of what's going on beneath our clothes. But because of privacy concerns, for the time being, the peep show has been distorted: The trial system in Phoenix produces cartoon-like outlines of the passengers being X-rayed, not photo-quality images, and the private areas are blurred.

Some wonder how, then, the system can actually boost security. And the manufacturer, American Science and Engineering, Inc., admits that distorting the image does decrease the machine's usefulness. What if someone tapes a vial of liquid explosives to his scrotum? This is one of the issues the trial run in Phoenix hopes to resolve. Can a backscatter system still increase security if its images are obscured?

The other issue besides privacy that has surrounded backscatter X-raying has to do with radiation exposure. Most of us do not get X-rayed on a regular basis; and when we do get X-rayed in a hospital or doctor's office, we've got a lead vest thrown over our vital organs. But at airports, there's no lead vest. So are people who travel a lot going to be subjected to dangerous levels of radiation if they get backscattered too often? Most experts say no. According to the Health Physics Society (HPS), a person undergoing a backscatter scan receives approximately 0.005 millirems (mrem, a unit of absorbed radiation). American Science and Engineering, Inc., actually puts that number slightly higher, in the area of .009 mrem. According to U.S. regulatory agencies, 1 mrem per year is a negligible dose of radiation, and 25 mrem per year from a single source is the upper limit of safe radiation exposure. Using the HPS numbers, it would take 200 backscatter scans in a year to reach a negligible dose -- 1 mrem -- of radiation. You receive 1 mrem from three hours on an airplane, from two days in Denver or from three days in Atlanta. And it would take 5,000 scans in a year to reach the upper limit of safety. A traveler would have to get 100 backscatter scans per week, every week, for a year, in order to be in real danger from the radiation. Few frequent flyers fly that frequently.

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