Thanks. There is also a rather extensive FDA letter stating their position that it is safe (they say for up to 1000 scans per year). They give slightly different doses but all are in the same range. Oddly, they use sieverts, and give allowed doses of 0.25 µSv/screening and cumulative 0.25 mSV/annum.
They give an individual's increased (fatal) cancer risk from one screening as 1 in 80 million, whcih is obviously low, but if every passenger is screened on each trip, that is many millions and statistically, some cancers are being caused. So "perfectly safe" might be a stretch. It adds to the "noise" everyone receives, but not very much. As to energy, I saw one article mention 28 kev x-rays (~ 44 pm) in a discussion of the low energy nature of it. The argument was that penetration is not deep enough to warrant a "whole body" radiation model and dose should be evaluated at the skin surface and immediately below. The FDA letter responded to this and indicated the skin exposure was about twice their "whole body" exposure, but still not a concern. The implication of the original article was that the risk was primarily melonoma, rather than deep body tumors. The FDA wasn't buying it. I will still make the argument that if the same job can be done with either ionizing or non-ionizing radiation, why would you choose ionizing? TSA has some of both kinds of scanners. I have not seen them take a position on relative cost or effectiveness. I think they should evaluate and I think they should use the non-ionizing approach unless there are gross differences in cost or effectiveness. As either device has not caught a bomber so far, these machines may be "security theater" and the safer machine is roughly equally scary in appearance. I think the real hope is that the stress triggers behavioral clues in real security threats, not that the machine catches them. ________________________________ From: James R. Frysinger <[email protected]> To: U.S. Metric Association <[email protected]> Sent: Mon, December 6, 2010 10:50:02 PM Subject: [USMA:49047] Backscatter X-ray personnel screening Here's another reference on using backscatter x-ray technology for screening passengers. http://www.rapiscansystems.com/sec1000faqs.html This one claims typical doses per passenger-screening amount to 3 µrem. The TSA site claimed 5 µrem to 10 µrem. There is even more information on this page regarding dosages. For example, frequently scanned personnel (such as flight crew members) can safely receive at least 5000 scans per year. It's obvious to me that they pick up much, much more radiation from flying at altitude than they do from these scans. Nicely, this source cites ANSI N43.17 "Radiation Safety for Personnel Security Screening Systems Using X-rays". If you have access to that standard, it should be able to tell you more. What frustrates me is that these devices are called "low energy" x-ray sources, but the plate voltage in use are not given. The phrase "low energy" can refer to the flux level ("brightness") of the device, but more often it refers to the energy of the highest energy photon generated, which is determined by plate voltage (and to a small extent, material). Yet another source of information is at http://www.public.asu.edu/~atppr/images/RPD-Manuscript.pdf This is the pre-print for the article published by Oxford Journals at http://rpd.oxfordjournals.org/content/early/2010/11/09/rpd.ncq358.abstract but that site charges $32 for the download. The paper cites an ANSI 43.17 limit of 0.25 µSv [25 µrem] per screening. It states that the equipment manufacturers are claiming doses per scan of less than 0.1 µSv. For comparison, it provides the figure of 6.2 mSv [6200 µSv] that members of the US population receive every year from all sources [which are mostly natural] of radiation. I would like to point out that the cosmic radiation we all receive naturally includes particles of vastly higher energy than those from x-ray sources, especially those with low plate voltages. That makes cosmic radiation particles much more likely to cause ionization damage. According to this second source, typical scanning machines use plate voltages in the order of 100 kV. Thus, such a machine generates x-ray photons (particles) with energies of 100 kV or less. (Backscattering machines for vehicles use higher voltages than do those for personnel, such as 200 kV.) In comparison, cosmic radiation particles are frequently in the megaelectronvolt region and go even into the gigaelectronvolt region. Note that this second source concludes that frequently scanned personnel could reach their annual limits in fewer scans than the first source above claims. The second paper used "ab initio" calculations to reach that conclusion. It also uses the NRC's annual limit of 0.25 mSv for the general public. Jim -- James R. Frysinger 632 Stony Point Mountain Road Doyle, TN 38559-3030 (C) 931.212.0267 (H) 931.657.3107 (F) 931.657.3108
