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TSA and Street Scanners.

The Whole-Body Scanners — Are They Safe?

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.

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.

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.

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).

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.

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): 564–574) – 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.

TSA claims that single whole-body backscatter scan yields only 0.002 mrem of exposure. For now, we will accept this figure.

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.

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.

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

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).

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.

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.

Taken together, these equivalent-dose multipliers can amount to a factor of about 2000.

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.

Link to source: http://tiny.cc/0u9rs