Tuesday, January 5, 2010

Radiation Safety

A question arose this morning about the risks of radiation from backscatter imagers.

Background radiation exposure in the US is ~3mSv (milli-Sieverts, equivalent to ~300 mrem). The lifetime risk of a lethal malignancy from all causes in the US is ~20%. A single 10mSv exposure at age 25 years increases the lifetime risk to 20.05%; however, the cumulative risks of repeated exposures are unclear.

A CT scan of the abdomen and pelvis exposes a person to 4.5 – 18 mSv of radiation; a plain film of the same area involve 0.7 mSv (See Adv Chronic Kidney Dis 16:48, 2009 for more on these types of studies).

So what is the danger from these new scanners? According to a posting at How Stuff Works:

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.

backscatter-1These scanners appear to offer little danger from radioactivity. The posting referenced above is from 2007 when these devices underwent initial testing in the Phoenix airport. The scanner program generated the cartoon image to the left, although its capabilities are more dramatic as shown on the right-hand portion of the image.

Regarding the diminished image used, the following comment was raised in the article:

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?

Like deja vu, huh?

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