Firstly, I'll say one thing: there is no such thing as a truly safe radiation dose. At a fundamental level, the main effect we need to be concerned with is high energy particles smashing into a piece of DNA and doing just the right amount of damage to cause the cell concerned to start doing something it shouldn't. If we get lucky, the DNA molecule is damaged enough that it's no longer viable, making the cell concerned no longer viable. No big deal, we've all got plenty of cells. However, if the particle impact has the effect of changing the information stored in the DNA, things can get weird. The cell can start reproducing wildly, carring its radiation-affected DNA programming with it and thereby causing all its children to go similarly bananas. Hey presto, cancer, leukaemia.
So what is a radiation dose anyway?
In effect, a radiation dose is being exposed to a particular strength of a particular kind of radiation for a particular length of time. The strength of the radiation is basically a measure of how many particles per second are flying at you. The kind of radiation can vary widely, and consequentially the chances that a particle interaction will cause damage also varies. Duration of exposure is pretty self-explanatory. Putting this another way, it's like every single cell of your body all playing Russian roulette, all at once, many million times a second. Though each gun has only one bullet and a (very) large number of empty chambers, it's worth remembering that one hit that gives you cancer or leukaemia is enough to kill you. Just one. Even though most interactions won't kill you, it only takes one good one to finish you off. It is also worth remembering that the hit that kills you will not be apparent for weeks, months or years -- this is why it can be so difficult to prove a causal link between teratogenic effects and cancer or leukaemia.
The next thing that's worth talking about is that there are many kinds of radiation, and they all have wildly different properties, and thus wildly different chances of damaging you. One of the problems here is that the lay intuition here can trip you up. People, particularly in the media, talk about 'high energy' and 'low energy' radiation, with the implication that high energy radiation is more likely to kill you than low energy radiation. Makes sense, yes? Actually, this is hugely misleading. Firstly, remember that the real risks are based on how much, what kind, and for how long. Low and high energy refers only to the 'what kind' part of the equation. Worse, it is typically the case that low energy radiation is actually more dangerous -- potentially MUCH more dangerous -- than high energy radiation.
Like everyone who is wrong side of 40, I grew up during the cold war, when the association between terror and radiation was due to the threat of global thermonuclear war, rather than with X-ray scanners in airports. Though the consequences of a thermonuclear detonation is far worse (duh!) than a trip through a body scanner, there is a useful lesson here about X-rays. Here is a diagram of the classic Teller-Ulam hydrogen bomb design:
In a Teller-Ulam bomb, a fission bomb (the thing at the top of the diagram) detonates with enough energy in its own right to destroy a small city. Though a very rapid blast wave propagates outwards from the detonation, a huge pulse of soft (low energy) X-rays moving out at the speed of light hugely outruns the blast front, causing the fusion bomb (the thing at the bottom of the diagram) to implode, fuse, and thereby create a far larger secondary explosion. But, remember: soft X-rays. Low energy X-rays, yet they comprise arguably about 80% of the energy output of the fission device. Though usually omitted from public sources about H-bombs, including all of the sources I managed to find this morning with a bit of Googling, I remember hearing somewhere, years ago, that there is an extra component between the primary and the secondary that slows down hard X-rays to soft X-rays, because in the form of hard X-rays they would otherwise just go through the secondary without significantly interacting with it. Though, obviously, the radiation level inside a detonating H-bomb is many orders of magnitude greater than what we're dealing with in medical imaging or TSA-style backscatter imaging, the critical take-away point here is that higher energy particles tend to go straight through and out the other side, whereas lower energy particles have a greater tendency to be absorbed. Consequentially, it's just these lower energy particles that are more concerning from the point of view of radiation safety.
Let's start doing some numbers to see what's really going on here.
Rapiscan themselves are a little wooly about detail, so I'm pulling information from an article on the diagnosticimaging.com web site (http://www.diagnosticimaging.com/safet
In the above graph, the curves represent X-ray intensity over depth within the human body. I'm arbitrarily deciding that we're all 20cm thick, here, just to make things simple. The upper curve represents the effects of the AS&E scanner, the bottom line represents the Rapiscan. The difference here is that the AS&E device uses higher energy X-rays, which penetrate further. Clearly, with the AS&E device, a bit more than half of the radiation goes all the way through the body and out the other side, whereas with the Rapiscan, nearly all of it is absorbed, but (more scarily) much of that is concentrated in the first few centimeters. Though the X-ray energies are lower than a medical X-ray machine, they aren't strictly speaking 'soft' X-ray sources -- actually they class as intermediate, being in the range used for mammography and CT scans.
Let's remind ourselves what the TSA had to say about this on their blog (http://blog.tsa.gov/2009/11/response-t
The Transportation Security Administration (TSA) has assessed multiple types of AIT systems including X-ray backscatter and millimeter wave. Both offer safe and effective whole body screening for weapons and explosives concealed on a person’s body. Backscatter X-ray technology uses X-rays that penetrate clothing, but not skin, to create an image.
This is clearly false, as demonstrated not only by the physics, but also by the TSA's own published images.
Here's one from a Rapiscan 1000:
Um... I'm sorry, but those things in the legs. You know, between the knees and the feet? I'm thinking that they look awfully like bones to me. In that part of the body, as with the face, the bones are very close to the surface, so they are showing up clearly. I'm pretty sure I'm not kidding myself that I can also see the prefrontal sinus, the brain, and on the other image, the spine. On these images, brightness represents reflected X-rays, so anything appearing dark is a good indication that X-rays have been absorbed. The brain looks nice and dark on there. Here's another one, this time from an AS&E scanner:
Bones, lots of bones there too. Even clearer this time, which is what the graph above would lead me to expect. And what looks suspiciously like lungs, an oesophagus, a stomach, and some other junk.
So what is the actual dose? And what is the risk?
Unfortunately, this is a lot harder to figure out from published sources. The Rapiscan 1000 specfication claims a scan time of 7 seconds, with a total claimed dose of 10 microrem. The same TSA blog post quoted above claims:
... the X - ray dose received from the backscatter system is equivalent to the radiation received in two minutes of airplane flight at altitude (0.003 millirem by backscatter (2 scans) compared to .0552 millirem for two minutes of flight). Newer technologies require less scanning time, reducing individual X - ray exposure to .002 millirem for the entire process.
The Rapiscan specification itself claims 10 microrems (0.010 millirems) per scan, though the TSA blog is claiming 0.003 millirems for two scans, so the Rapiscan is giving six times the dose that is being claimed. AS&E also claim a 10 microrem limit.
According to the NOAA web site (http://www.swpc.noaa.gov/info/RadHaz.h
So what is a (milli)rem anyway?
The unit 'rem' is actually an acronym: Roentgen Equivalent Man, and is the result of multiplying the radiation dose in rads (more on those later) with a weighting factor, WR, representing the effectiveness for the radiation in causing biological damage to mammalian cells. The underlying unit, the rad, is defined as the dose causing 100 ergs of energy to be absorbed by 1 gram of matter -- equivalently, it may also be defined as the dose causing 0.01 joule of energy to be absorbed by 1kg of matter. So, the figures should at least in principle be comparable. However, the kinds of radiation are extremely different, so reducing the comparison to a single number is a dangerous oversimplification. Where an X-ray dose is analogous to an extremely large number of relatively low energy particles all at once, a cosmic ray dose is equivalent to a relatively small number of extremely high energy particles far less often. Though there is a considerable amount of variability within the particle energy levels seen, the most extreme examples will commonly penetrate several metres of lead. The more commonly seen particles are less extreme than that, but practically speaking, adding a third line to the graph:
effectively gives you a nearly straight line. Therefore, the dose is effectively distributed evenly throughout the body, whereas the kinds of X-rays used in the scanners have a tendency to concentrate their dose toward the surface, or in the skin. It is also notable that, on attempting to research this, that whilst there is quite a lot of material available on medical X-ray imaging and its health consequences, there is little or nothing to be found that is related to more frequent lower dose exposures, or to the effects of these longer wavelengths over longer periods. Surprisingly, there is also relatively little to be found about cosmic ray exposure effects. Though the rem unit corrects for different kinds of radiation, this is only as accurate as the research on which the weighting factors themselves were based.
So, to sum up, it seems that the TSA don't really understand what they are using. Their X-ray transparent pants are on fire, frankly, with regard to their claims that backscatter X-ray technology does not penetrate skin. What is harder to establish is the health issues. Whilst this little from-basic-principles study doesn't show unequivocally that scanners are definitely a very significant radiation risk, it doesn't show that they are safe, either. Looking at the curves, it really wouldn't surprise me at all to find, several years hence, correlation between repeated exposure and elevated risks of skin and eye cancers, and possibly also brain cancers. I'm not alone in these worries, however -- http://www.whitehouse.gov/sites/def
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[ETA: A friend (who really does play a physicist on TV!) pointed out that whilst the radiation levels travelers see (which he points out are actually just 10 times background) are probably going to be fine, those machines have essentially no shielding for their operators, who will get many thousands of times the dose that travelers get just by standing next to the machines all day every day. Whilst I don't approve of the groping, I would not wish to see TSA people dying from cancer in 20 years either. I'm inclined to agree, though the jury is definitely still out.]