More on whole body scanners

| Comments (1) | Security: Airport
You may have heard that the TSA is moving to ubiquitous use of whole body scanners for security screening. They won't actually make you go through the scanner, but as Jeff Goldberg reports, patdown they're offering isn't a lot of fun:

At BWI, I told the officer who directed me to the back-scatter that I preferred a pat-down. I did this in order to see how effective the manual search would be. When I made this request, a number of TSA officers, to my surprise, began laughing. I asked why. One of them -- the one who would eventually conduct my pat-down -- said that the rules were changing shortly, and that I would soon understand why the back-scatter was preferable to the manual search. I asked him if the new guidelines included a cavity search. "No way. You think Congress would allow that?"

I answered, "If you're a terrorist, you're going to hide your weapons n your anus or your vagina." He blushed when I said "vagina."

"Yes, but starting tomorrow, we're going to start searching your crotchal area" -- this is the word he used, "crotchal" -- and you're not going to like it."

"What am I not going to like?" I asked.

"We have to search up your thighs and between your legs until we meet resistance," he explained.

"Resistance?" I asked.

"Your testicles," he explained.

'That's funny," I said, "because 'The Resistance' is the actual name I've given to my testicles."

One gets the impression from his report that it's being made less fun than strictly necessary. After all, once you've paid a zillion dollars for a bunch of gee whiz technology you want to use it.

I was planning to opt for the patdown next time I went through security anyway, but then I read this letter from UCSFSF Citizen). You should read the whole thing, but this is the really scary part:

Unlike other scanners, these new devices operate at relatively low beam energies (28keV). The majority of their energy is delivered to the skin and the underlying tissue. Thus, while the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high.

The X-ray dose from these devices has often been compared in the media to the cosmic ray exposure inherent to airplane travel or that of a chest X-ray. However, this comparison is very misleading: both the air travel cosmic ray exposure and chest X- rays have much higher X-ray energies and the health consequences are appropriately understood in terms of the whole body volume dose. In contrast, these new airport scanners are largely depositing their energy into the skin and immediately adjacent tissue, and since this is such a small fraction of body weight/vol, possibly by one to two orders of magnitude, the real dose to the skin is now high.

In addition, it appears that real independent safety data do not exist. A search, ultimately finding top FDA radiation physics staff, suggests that the relevant radiation quantity, the Flux [photons per unit area and time (because this is a scanning device)] has not been characterized. Instead an indirect test (Air Kerma) was made that emphasized the whole body exposure value, and thus it appears that the danger is low when compared to cosmic rays during airplane travel and a chest X-ray dose.

In summary, if the key data (flux-integrated photons per unit values) were available, it would be straightforward to accurately model the dose being deposited in the skin and adjacent tissues using available computer codes, which would resolve the potential concerns over radiation damage.

That's sure encouraging. And of course that's just assuming that the machines are functioning as designed. The authors of the letter go on:

Moreover, there are a number of 'red flags' related to the hardware itself. Because this device can scan a human in a few seconds, the X-ray beam is very intense. Any glitch in power at any point in the hardware (or more importantly in software) that stops the device could cause an intense radiation dose to a single spot on the skin. Who will oversee problems with overall dose after repair or software problems?

Surely that could never happen.

1 Comments

The return of Therac 25?

A chest X-ray isn't something to sneeze at. Getting them once or twice isn't an issue, but some people travel dozens of times a year.

Even scanning 1000 people with the radiation in 1 chest X-ray will have bad effects. Wikipedia says that each X-ray adds one in a million to your cancer risk. If there are 200 installed, and each scans 200 people per day (one every 3 minutes for 10 hours), they are stochastically giving someone cancer every 25 days.

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