Radiation risks: Are airport body x-ray scanners ‘a great public health experiment’?

This was a recent discussion amongst myself and some colleagues regarding the new x-ray backscatter imaging machines being used by the TSA at airports. Names have been obscured to protect the innocent.

 

Astrophysicist #1 - I was looking for a spectrum of the X-rays these machines use - this is the only place I've found so far.

http://www.diagnosticimaging.com/safety/content/article/113619/1521147

Astrophysicist #2 - Interesting. I had understood that it was skin-only from the descriptions. A whole-body dose may be safer than a concentrated skin dose.

Astrophysicist #1 - The Rapiscan website claims it only penetrates 1/10 inch which I found hard to believe since it has to go through several feet of atmos.

Astrophysicist #3 - I haven't read the article yet, but from particle & radiation physics, I remember something about a stopping power parameter for materials. I forget what it was called, though. 

I'm curious what the equivalent thickness of atmosphere is for 1/10" of skin.

Webmaster #1 - I still think it's funny/disturbing that the scanners are made by "Rapiscan". :-/

Astrophysicist #1 - [Webmaster #1], the other company which makes these scanners is AS&E, who are familiar because it was Riccardo Giacconi's employer when he founded X-ray astronomy.

Optical Physics - The attenuation length (the distance into a material when the probability has dropped to 1/e that a particle has not been absorbed) is approximately linear vs energy for water (i.e., us) at 1mm/10 keV. (I used http://henke.lbl.gov/optical_constants/atten2.html) So, for 8-9 keV photons, (the tungsten L lines) 2/3 of them that reach your skin will penetrate at least 1 mm. For the far more powerful 60 keV photons from the tungsten K line, we're talking at least 6 mm. Now, a quarter inch might not sound like much, but there are plenty of genetically sensitive parts that are within 1/4" of one's skin, as well as other organs that are more sensitive than skin to radiation damage like one's eyes. Oh, yeah, and that whole skin cancer thing.

Now, the dose is supposed to be very small, according to other articles I have read, but cancer is a statistical problem, not a maximum limit problem. 

As a bunch of people in the x-ray field, I would recommend that everyone share this article, as it points out things like uninformed consent of the masses, who don't know anything about this technology.

Astrophysicist #4 - Stopping power is density dependent, and the density of air at 1 atm pressure and ~20K is 1.2 kg/m^3, about a thousand times less than that for human skin (a couple times denser than water at 1000 kg m^3). So if the X-rays only penetrate 1/10" in human skin, the penetration depth in air would be right around 10 ft.

Astrophysicist #5 - I heard somebody claim that the dosage is equivalent to a brief period of time flying at 30k feet. Can anybody comment?

Optical Physics - [@Astrophysicist #4] - that is accurate for photons in the 20 keV range, which may be about the average for the AS&E machines. For the Rapiscan, the average will be much higher, fit to a large component being the tungsten emission lines at 60 keV. 

[@Astrophysicist #5]- I have read a similar thing. But, the points are
1) this is an unnecessary dose, and when it comes to x-rays, any dose that can be avoided should be. 
2) acceptable dose is a bitch to calculate, since it depends on what organs you are considering and the spectrum of x-rays reaching the organ
3) x-ray dose in an airplane is pretty much uni-directional - top down. So, most of the dose is absorbed by your skull and thighs (not particularly sensitive). These machines target everything, and I mean everything, including bone marrow in your extremities, gonads, thyroid, etc.

Astrophysicist #2 - This response by the US FDA: http://bit.ly/dyrExs has quite a bit of information about scanner dosage limits *in general*, phrased in terms of "effective dose." Their claim is that you can be scanned 1000x per year and still be under the annual dosage limit. Annual dosage limit is 0.25 mSv, which according to their own calculations would increase risk of fatal cancer by 1 in 80,000 per year if you reached the limit (footnote 8). The FDA letter also does not provide information about specific scanners, or how they were calculated (Rapiscan claims 0.00005 mSv dosage per scan).

Astrophysicist #4 - [@Optical Physics], you have a point, and that is where the maximum fluence is (actually, it looks like a second line is present as well). Although the general principle (i.e., how stopping power varies with density) is still valid.

What I would like to see, personally, is a study on absorption for both machines, as a function of depth. They can easily build a 'meat man' where they put dosimeters in leftover meat that is shaped like a human. Not that it would be exact, but it would give much more useful info than what they have now. Also I would like to see a level of training for the attendants, similar to what they give radiologists who do mammograms.

Astrophysicist #1 - [@Astrophysicist #4], I was wondering about radiation exposure for TSA employees and how much training they get on the dangers. It would be an interesting conversation to have should the opportunity arise. I wonder whether they carry monitors to track their total exposure.

Astrophysicist #2 - The answer is, no, they are not allowed to wear a radiation monitor badge. (at least anecdotally) http://www.sciencefriday.com/blog/2010/09/airport-screeners-denied-radiation-badges/

Optical Physics - [Astrophysicist #2], you are right. Here's is some Congressional testimony from the AFL-CIO on what TSA employees have requested (and been denied) regarding dosimeters. http://bit.ly/djdhZB

Astrophysicist #4 - Fascinating stuff indeed. It's a shame that the TSA staff are being denied dosimeters ... although, given that the booths are circular and the source is only emitting when it's closed, they do have at least a few inches of glass between them and the X-ray source. If a tenth of an inch is tau=2 (i.e., 5% of the softer X-rays get through) they'd get very little. But for the 60 and 70 keV lines I'm not sure what tau=1 or 2 translates to in terms of column length.