- In the recent weeks TSA started to aggressively steer
people towards the whole-body scanners, which are capable of producing
"naked" images of people. This policy was introduced without
much public debate, raising numerous concerns about privacy, legality,
civil rights, etc. In this article we'll concentrate on safety of these
screening devices.
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- TSA and a number of officials from FDA have issued assurances
that these scanners are safe, claiming that a number of experts have reviewed
the radiation exposure data and agrees that the doses of radiation travelers
get from being scanned are well within exposure limits established as safe.
However, the technical specifications, details of operation and construction,
and other data necessary for an independent review of safety of these devices
are not published.
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- What is known is that there are two different types of
scanners one uses scattering of "soft" X-rays, and another
uses the terahertz (millimeter) microwaves to form an image. We will discuss
the X-ray scanners first.
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- The general public is well aware of danger of exposure
to X-rays; this danger is being forcefully underlined by the usual safety
procedures of clinical radiologists donning lead vests and retreating before
turning the X-ray machines on. Unlike high-intensity radiation, low-intensity
X-rays do not kill or burn cells outright, but the energetic photons can
and do damage DNA in the cells. Most DNA damage is not critical, and is
repaired by the cells; however some damage remains unrepaired crippling
cellular mechanisms, or disabling them completely when enough damage is
accumulated. One of these mechanisms, called apoptosis, or programmed cellular
death, prevents cells in our bodies from multiplying uncontrollably. Once
this mechanism is disabled, the cell becomes cancerous. (Radiation exposure
was also recently found to increase mortality from cardiovascular diseases).
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- It is important to understand that because of this cumulative
nature of damage caused by radiation, the effects of being exposed to additional
radiation do not usually appear immediately. It often takes 10-15 years
from the initial exposure for the irradiated tissue to become cancerous
by which time it would be impossible to say what exactly caused the
cancer. The dangers are better understood in terms of number of unnecessary
deaths the scanners would likely cause given the millions of travelers
and crew members being scanned every year.
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- The commonly used estimate is that relative risk of death
from cancer increases 3% for every 10 mSv or 1000 mrem of additional exposure
(J.P. Ashmore et al, First Analysis of Mortality and Occupational
Radiation Exposure based on the National Dose Registry of Canada, Am. J.
Epidemol. (1998) 148(6): 564574) with melanoma (an aggressive
and usually fatal form of skin cancer being the biggest contributor). For
comparison, a single cross-country flight on an airplane yields about 4
mrem of exposure, and chest X-ray is at about 10 mrem.
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- TSA claims that single whole-body backscatter scan yields
only 0.002 mrem of exposure. For now, we will accept this figure.
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- What TSA and FDA experts do not say is that X-rays used
by the scanners are very different from the X-rays used in the medicine.
Medical X-rays are "hard," and use higher frequency photons with
energies from 50KeV to 150KeV. These energetic photons are penetrating
(like background radiation from the Sun, radioactive elements in soil,
etc), and for them biological tissues are semi-transparent. The "soft"
X-rays used in backscatter machines (20KeV or less) are mostly absorbed
in the skin making them useless for medical imaging. All medical
X-ray emitters are equipped with special filters to suppress soft X-rays
both to increase clarity of images and to reduce exposure.
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- The health effects of hard X-rays are well studied due
to their wide-spread usage. Information on health effects of soft X-rays
is scarce.
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- Because of the penetrating nature of hard X-rays, the
exposure is spread all over the body (and exposure limits are calculated
per kilogram of body mass). With soft X-rays the exposure is concentrated
in the skin, which makes the effective exposure per unit of mass of tissue
100 or more times bigger than for hard X-rays.
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- Unfortunately, skin is exactly the wrong tissue to gather
the additional radiation exposure. Unlike muscles, fat, bones, or most
internal organs skin contains very large numbers of quickly dividing cells.
We are constantly growing the new skin and hairs from inside, shedding
the old dead cells on outside. The abundance of dividing cells means that
DNA damage is quickly multiplied by getting into daughter cells, and that
some mechanisms which suppress cell growth are less active in skin cells.
These are the reasons why skin cancers are the most prevalent kind of cancer
(though the mortality is usually low except for melanomas).
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- With penetrating X-rays, most absorbed X-ray photons
are absorbed in dense tissue and bones, which are much less pre-disposed
to cellular damage turning into cancer. This may multiply the biological
effects of soft X-ray irradiation by as much as an order of magnitude.
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- Another difference between soft and hard X-rays is their
absorption by the tissue. Absorption depends critically on the probability
of interaction between particles (such as photons) and atoms, a property
called "cross-section" by the physicists. You can understand
cross-section by imagining atoms to be targets and photons to be bullets
with cross-section being the size of the targets. Perhaps counter-intuitively,
cross-section is often bigger when particles have smaller energies (this
is the reason nuclear reactors employ moderators to slow down neutrons
so the cross-section of their interaction with uranium nuclei will increase).
Similarly, soft X-rays are more likely to be absorbed by the tissue (and
thus cause damage) than the hard X-rays. The mass energy absorption coefficient
in soft tissues for 10KeV X-rays is 4.6 cm2/g versus 2.5 cm2/g for 100KeV
(NISTIR 5632, Tables of X-ray Mass Attenuation Coefficients and Mass
Energy-Absorption Coefficients, J.H.Hubbell and S.M.Seltzer, 1996)
meaning that soft X-ray photons are absorbed nearly 2 times as often as
hard X-ray photons.
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- Taken together, these equivalent-dose multipliers can
amount to a factor of about 2000.
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- One more issue to consider is intensity of X-rays. Medical
imaging uses essentially point-like sources, with rays spreading out from
a bright dot on a tungsten electrode, resulting in a uniform low intensity.
Although the details of construction of the backscatter whole-body scanners
are not published, it is possible, by simply looking at the geometry of
these devices, to conclude that they use a moving spot of concentrated
X-rays scanning over the body. While the overall dose could be small, the
intensity (brightness) in the spot can be high.
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- The probability of DNA damage depends significantly on
the X-ray intensity. DNA is two-stranded, and damage to one strand is relatively
easily and reliably repaired by the cells because the other (complementary)
strand serves as a template for repair. Damage to both strands in the same
or nearby locations is much harder to repair there's no longer a
reliable second copy. The higher intensity of X-rays, the more probability
is that two photons will strike DNA molecule near each other.
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- The simplified rules radiologists use to calculate exposure
limits do not take into account this intensity dependency because most
medical X-ray sources use similar intensities, just sufficient to provide
sufficient exposure to the films or sensors. Applying these rules to estimate
safety of exposure to higher intensity X-rays is simply not valid.
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- All these effects can make biological effects of radiation
from the whole-body scanners to be up to 10000 times stronger than effects
of the same dose of X-rays from a medical X-ray source i.e. the equivalent
effective dose of radiation would be more like 1020 mrem per scan,
using the published scanner radiation exposure figure.
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- However, this starting figure, 0.002 mrem, is highly
suspicious. To create an image, a sensor has to capture sufficient energy
(i.e. to have sufficient exposure) to "heat" pixels above thermal
and other noise in the sensor (i.e. it has to have a good signal/noise
ratio). It does not matter if radiation passed through or was scattered.
Are we being led to believe that manufacturers of the backscatter X-ray
scanners have invented some revolutionary new X-ray detection technology
which is 5000-10000 times more sensitive than the sensors used in mere
medical devices? Why these fantastic new sensors are not used in medicine
then, to reduce doses to the patients and doctors? Would they care to explain
how did they manage this minor miracle?
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- Assuming that there was no such break-through in sensor
technology, the 0.002 mrem figure is simply a lie. (Alternatively, this
could be simple incompetence such as using X-ray radiometer designed
for calibration of medical X-ray sources to measure exposure in a full-body
scanner. However, the first thing a medical radiometer does is filtering
out soft X-rays and other electromagnetic radiation with irrelevant frequencies,
so that using such radiometer in a backscatter scanner will result in readings
which are way too low).
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- Given so little actual information on the construction
of these X-ray scanners, it is impossible to estimate the actual level
of danger they pose to the public; but until all the concerns above are
addressed in a credible way, we should assume that the backscatter X-ray
scanners are at least as dangerous as medical X-ray machines and
probably more.
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- Using the increased risk per dose estimate above and
multiplying that by more than 750 mil of passengers per year (assuming
that all of them are scanned once, and using the generally accepted linear
risk dependency per dose of radiation) we can estimate about 225 thousand
of additional cases of cancer per year (3%/1000 mrem * 10 mrem * 750,000,000)
in addition to 2 million cases diagnosed in US yearly (<http://www.skincancer.org/Skin-Cancer-Facts>Skin
Cancer Facts); resulting in approximately 1700 additional deaths. This
makes these machines about six times more dangerous than terrorists: the
average number of people killed in terrorist attacks is only about 286
per year for the 11 year period spanning 1995 through 2005.
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- If the manufacturers of these machines are lying about
the radiation exposure, the danger to the public can be much higher
up to 500 times more, if we assume that sensitivity of the X-ray sensors
used is in line with the common medical sensors. Deployment of these machines
today could become public health disaster ten years later.
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- The terahertz microwave scanners are a bigger unknown,
mostly because the ability to produce terahertz radiation with useful intensity
is relatively new. The actual numbers on intensity of terahertz microwaves
emitted by these scanners is not published. Only little is known about
the health effects of terahertz microwave radiation. TSA and FDA officials
mislead public by comparing this radiation to more common (and relatively
benign) gigahertz microwaves used in cell phones and wireless data networks.
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- What is different about terahertz radiation is that it
was shown that many biological molecules, including DNA, have resonant
frequencies in this range. (Resonance is a phenomenon known to anybody
who ever pushed a playground swing when you get the frequency of
pushes and pulls just right, it really starts to swing). When photons have
just the right frequency for a particular kind of molecules, these molecules
start absorbing energy from these photons. Non-resonant microwaves simply
heat the tissue; hit the resonant frequency, and they start tearing the
specific molecules apart.
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- How much this is dangerous to the live organisms for
the intensities and specific frequencies of terahertz radiation being used
is largely unknown. In fact, it won't be possible to learn about these
effects until 1020 years worth of statistics is collected. Essentially,
US travelers are being forced to become unwilling and mostly uninformed
laboratory rats. Same is true about effects of small doses of soft X-rays.
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- We should understand that claims of safety made by FDA
and TSA are not based on any kind of empirical evidence we will not
have this evidence without clinical trials taking decades; the basis for
their assurances is opinions of their "experts." We have shown
above that these opinions are not grounded in science, and are merely the
result of mechanical and scientifically invalid application of safety data
from other frequency ranges despite the obvious differences in biological
effects.
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- We should demand removal of these machines before the
actual trials establishing their safety empirically are done, and before
the details of construction of the specific models are made public and
available for independent review. There are sufficient grounds to challenge
the validity of the theoretical numbers used as a basis of claims that
these machines are safe. The valid trials cannot be completed in less than
10 years simply because it takes that long for cancers to appear
following the exposure.
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- Meanwhile, travelers would be well advised to stay clear
of the whole-body scanners.
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