It’s no secret that security in our airports has changed dramatically in the last decade and, whether we think these changes are effective (or needed) or not many of them are here to stay. One of the technologies that has come into vogue recently uses what are called backscatter x-rays to help airport security to see if a traveler is hiding weapons or explosives beneath his or her clothes without the time and indignity of a strip search. But these machines have become highly controversial, partly for reasons of personal privacy (they can produce images of the body that are a bit more detailed than many people feel comfortable with) and also because the exposed passengers to radiation. It was this latter aspect that was the subject of a story on National Public Radio’s “Talk of the Nation – Science Friday” show on November 19, 2010 .
Before getting into the show it might be useful to understand a little bit about how the machines work. Backscatter x-rays (unlike normal x-rays, which pass through an object) are reflected off an object and back into the scanner – this happens because they have too little energy to pass through an object. In practical terms this means that the skin is exposed to the highest dose of radiation and the internal organs receive very little. How much dose? I measured the output of a backscatter x-ray machine and the radiation exposure was on the order of a few hundred micro-rem. To put this in perspective, we are all exposed to natural radiation every day of our lives, to the tune of about 20 micro-rem (give or take a little) every hour – a single backscatter x-ray image, then, exposes us to the equivalent of perhaps a half-day or so of natural background radiation. The flight also exposes us to radiation – on a recent trip to Japan I picked up 6000 micro-rem (or 6 milli-rem (mrem) on each flight. So a frequent flyer might pick up several tens of mrem in a year from backscatter x-ray examinations. The question is whether or not this poses an unacceptable health risk to frequent flyers – this is the topic being debated.
We are normally exposed to about 300 mrem a year from natural sources, but there is tremendous variability in this dose – when I was in the Iranian spa city of Ramsar in the year 2000 I measured radiation dose rates of up to 2500 micro-rem (2.5 mrem) per hour and some of the residents receive as much as 100 times the background radiation exposure that we see on average here in the United States. In spite of this the residents of Ramsar do not seem to develop cancer more readily than do residents of nearby areas . Similarly, residents of high-background radiation areas in Kerala, India and Guapari, Brazil seem to have no greater incidence of cancer than do the residents of nearby areas with normal radiation levels. In fact, in the United States, the Rocky Mountain states (that have the highest natural radiation levels) experience lower rates of cancer than do the lower-dose rate Gulf Coast states. What all of this is getting at is that our experience with variations in natural radiation seems to suggest that elevated – but low – levels of radiation do not seem to affect the health of those in those areas. Having said that, there is a huge debate among the world’s radiation scientists as to whether or not low levels of radiation – such as those to which we might be exposed during backscatter (or normal medical) x-ray examinations. Which brings us back to the November 19th Science Friday….
One of the show’s guests, Columbia University radiation biologist Dr. David Brenner, made the point that, although every individual dose is low, exposing hundreds of millions of air travelers to this exposure every year is likely to cause some of them to get cancer. If, say, 100 million travelers are exposed to 100 micro-rem from each of these scans then the total radiation exposure to these people will come out to 10,000 person-rem of exposure among the traveling population – enough to induce a fatal cancer in five people. This is a concept that is called “collective dose.” The problem is that Brenner’s conception of the risks from collective dose runs afoul of the recommendations of major national and international advisory bodies – the Health Physics Society and the International Council on Radiation Protection (ICRP).
Let’s start with the Health Physics Society’s 2010 position paper, Radiation Risk in Perspective. According to this paper, “In accordance with current knowledge of radiation health risks, the Health Physics Society recommends against quantitative estimation of health risks below an individual dose of 5 rem in one year or a lifetime dose of 10 rem above that received from natural sources.” Radiation exposure from backscatter x-ray machines clearly falls into this category for each traveler. According to the Health Physics Society – one of the world’s pre-eminent radiation safety organizations – it is simply not appropriate to calculate risks and possible cancer deaths from this level of exposure because the dose is so low that “risks of health effects are either too small to be observed or are nonexistent.”
While the Health Physics Society has looked at the effects of exposure to relatively low doses of radiation, the ICRP has looked at the impact of collective dose to large numbers of people. In a 1999 paper published in the Journal of Radiological Protection Roger Clarke (then-chairman of the ICRP) noted that individual radiation doses on the order of a few mrem (in SI units, a few tens of micro-Sieverts) pose a “trivial” risk to the individual. Clarke goes on to suggest that doses of this magnitude are “so low as to be beneath regulatory concern” and that “if the most exposed representative individual is sufficiently protected from a given source, then everyone else is also sufficiently protected from that source.” Clarke also makes the point in this paper that “If the risk of harm to the health of the most exposed individual is trivial, then the total risk is trivial – irrespective of how many people are exposed.” An analogy might be in order – if I throw a 1-gram rock at each person in New York City the cumulative weight is about 10 tons – enough to crush a lot of people. Brenner would have us looking for bodies; Clarke’s position is that nobody will be crushed because even 10 million small rocks – each tossed at a different person – aren’t going to hurt anyone. Similarly, Clarke’s position is that it is highly likely that even exposing a billion travelers to a vanishingly small dose of radiation is not going to hurt anyone because every individual exposure is “trivial.”
But (I hear you ask!) what about the cumulative dose – if a person is exposed to a few hundred of these backscatter x-rays every year, then the dose over the course of the year exceeds what Clarke (and the ICRP) consider to be a trivial dose. After all, if we pile 10 million small rocks on top of a single person it will likely do some damage – perhaps x-rays work the same way. That might be true if the x-rays come in quick succession but, in practice, most of us pass through security one day and back through a few days (or weeks) later. Since our bodies produce DNA repair proteins that ramp up following identification of genetic damage (an effect called adaptive response) a single backscatter x-ray might cause minor amount of damage that will be fairly quickly repaired. A frequent traveler is looking at, say, a hundred or so individual trivial exposures in a year rather than the equivalent of a single higher exposure. So, the Health Physics Society tells us that it is scientifically inappropriate to calculate the risks from any exposures of less than 10 rem and Clarke (and the ICRP) suggest that a large number of “trivial” exposures do not add up to a significant population dose. Both of these positions are contrary to the suggestion that backscatter x-rays might cause a public health hazard.
Brenner is a distinguished and respected radiation biologist, and it is not easy to disagree with him on this point. An eminent health physicist, Argentina’s Daniel Beninson, commented in his 1996 Sievert Award lecture that the inability to “see” a health effect does not mean that it does not exist. But science is a field that makes predictions that can be tested and falsified – any putative risk from exposure to 100 mrem is far too small to be detected via epidemiological studies of the affected populations and the hypothesis that this harm is taking place cannot be tested and cannot be falsified. As such, until our epidemiological tools improve, such speculations are intriguing but may not be scientific because of this lack of falsifiability – they represent a belief or even a philosophy rather than a scientific position. We must keep this in mind when we are comparing the risks (privacy and health) of undergoing backscatter x-ray exposure against the risks these examinations are intended to avert.
Dr Y is a certified health physicist, trained in nuclear power plant design and operations, with experience in nuclear power, environmental science, and planning for radiological and nuclear emergencies. He has 30 years of experience in the areas of nuclear and radiation safety.