Sunday, August 24, 2014

Research I missed: Radiation doses of the old naked scanners

I'm currently going through the recent paper by Mowery, et al on the effectiveness and security of the "old" (x-ray backscatter) naked scanners. Along the way, I came across this paper by the American Association of Physicists in Medicine (AAPM) [pdf], which was given access to the same machines by TSA and Rapiscan in order to assess radiation doses.

Their findings (in brief):
ANSI and the HPS have issued a standard (ANSI/HPS N43.17 2009) that “applies to the manufacture and operation of security screening systems that are intended to expose humans to primary beam x-rays, gamma radiation, or both.” Our measurements indicate that the effective dose from a single screening exam is well below the screening limit of 0.25 μSv per screening for a general use, full-body scanner.5 The standard also states that the effective dose (computational adult model) shall not exceed 250 μSv over a 12-month period. For our stimated effective dose of 11.1 nSv to a standard man from a single screening, an individual would need to go through more than 22,500 screenings in a year to reach this limit.
Which is to say, the effective dose (akin to an overall radiation dose) is quite small and is, indeed (as TSA and Rapiscan said), well below the safety standards that were already in place for medical radiation exposure. It is also well below the threshold of what has been advanced for acceptable levels of non-medical radiation exposure - specifically for the purpose of security screening, commissioned by the FDA post-9/11 - which largely draws on the medical standards. However, it is not as if there is a quantifiable individual health costs vs health benefits that can be made for mandatory screening as a condition of air travel. Of course, such a calculation can statistically be made in medicine, the cost/benefit analysis can be presented to the patient by a doctor, and medical scans are voluntary. I'm not saying it's unsafe, rather that it ought to be up to the individual to weigh the risks with benefits without having to forfeit such fundamental rights such as freedom of movement and freedom of contract.