TSA Back-scatter (X-RAY) Machines: What you need to know.
A while back (April 6, 2010) there was a letter released by a group of scientists at the University of California San Francisco to John Holdren. I urge you to read this letter. The researchers expressed concern about the lack of third party independent testing available on these back-scatter machines. Specifically, they expressed concern that since these x-ray’s were going to be absorbed entirely by the first few layers of skin it could not be compared to the radiation received in flight nor the amount of radiation in a banana. These radiation levels are calculated on full body absorption, not the first few layers of skin which receives the entire dose with these back-scatter machines. Quotes from the first letter:
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 Xrays
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.
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? The TSA is
already complaining about resolution limitations; who will keep the manufacturers
and/or TSA from just raising the dose, an easy way to improve signal-to-noise and get
higher resolution? Lastly, given the recent incident (on December 25th), how do we
know whether the manufacturer or TSA, seeking higher resolution, will scan the groin
area more slowly leading to a much higher total dose?
On April 28, 2011 a new letter was released from the group that composed the first letter. In this letter they updated their concerns. One of the most interesting elements is that the machine that was used for the governments study is a mock-up prototype and not a production model. Independent testing has still not been allowed and the UCSF group states that the data that has been released was suspect and without signatories so there is no-one to accountable. Some quotes from the second letter:
In summary, the independent testing of the safety of these specific scanners has
not been rigorous nor has it been held to the standards usually associated with new
devices before approval for utilization in the public sector. Usually the exact
technology, as installed, is sent to a university, national laboratory or other outside
facility that has the expertise to test, for an extended period of time to enable an indepth
study–usually by several independent groups. Different test equipment, optimal
for this configuration, can be used at a site that specializes in the potential problems of
this technology. The hardware and software is tested in all aspects, finally arriving at a
place where the true capabilities of this system are totally known, similar to testing of
new aircraft, spacecraft and other technology that impacts on a national level.
We call attention to the whole issue of ‘effective dose’. Although effective dose
is widely used in conjunction with the Linear No Threshold (LNT) model to predict
carcinogenesis and mortality, it has serious shortcomings. The entrance skin exposure
and the related entrance skin dose are quantities (see Fig. 1) that can be measured and
are always higher than the “effective dose”. The effective dose is an average where the
dose in different organs is weighted according to factors published by the International
Commission on Radiation Protection (ICRP). The conversion from entrance skin
exposure to effective dose relies on modeling of X-ray interactions with mathematical
representations of the human body (phantoms). The energy of X-rays for the widely
deployed Rapiscan “Single Pose” system spans 0-50 keV with the average being
28keV. The dose follows an exponential decay and reaches half the value of the skin
dose at a depth of 4 cm. Since organs near the center of the body are more strongly
weighted the effective dose is a factor of 6 less than the entrance skin dose for an
average male. However for small children, these internal organs receive a much higher proportion of entrance skin dose, and the effective dose is much higher. The NIST
report, using the crude Cristy and Eckerman phantoms, shows that the effective dose
for children’s organs is 1.5 times higher than the effective dose for adults. Examination
of the figure below indicates that it might be higher. Moreover, radiation effects are
more serious for children.
The casual nature for maintenance of these devices is alarming to us. These
machines are built with components from clinical X-ray machines and are capable of
delivering large X-ray doses. The actual doses are undefined by any objective tests
disclosed to us or to the public. Large doses also pertain if there are errors or
maintenance problems. Hospitals usually check for problems on X-ray machines daily,
but we understand that TSA will only check once a year, at best, in spite of the fact that
these machines are being used 24 hours a day, 7 days a week.
I urge each and every one of you readers to talk to your family, friends and neighbors and spread these reports. There are very few members of congress working on this problem. One I do know of that has entered legislation, HR 6416 The American Traveler Dignity Act, is Congressman Ron Paul. I am sure there are others. Perhaps, if you fax these to your locals congress critter we can have some movement. I will not travel with my children until this is resolved! How about you?