Backscatter x‐ray machines at airports are safe (2012)

Both backscatter x ray and millimeter-wave-based whole-body scanners are used at security check points at airports. Because the former involves ionizing radiation, it has been suggested that their use should be discontinued because of the potential biological hazards of x ray exposures. Others argue, however, that these hazards are negligible at the low exposure levels used, and that backscatter x ray body scanners are safe. This is the premise debated in this month's Point/Counterpoint. Wide consensus exists that the effective dose of radiation delivered by airport backscatter x ray scanners is exceedingly low. Direct measurements from several independent sources, including the U.S. Food and Drug Administration,1 Johns Hopkins University,2 and the U.S. Army,3 showed that a single scan is associated with a range of roughly 0.015–0.1 μSv. To put this in context, this dose is similar to that absorbed during 1–2 min of flight. Some have argued that the dose to the skin may be “dangerously high,” even if effective doses are low. Skin (and lens) doses are, however, only two to four times higher than effective doses3 and a traveler would have to be screened several hundred thousand times per year before reaching the dose limits. Radiation-induced cancers have only been demonstrated for doses exceeding 100 000 μSv. No epidemiological study will, therefore, ever be able to prove a carcinogenic effect for the range of doses delivered by backscatter scanners. Nevertheless, very precise estimates of the number of cancers induced by backscatter scanners are sometimes publicly given, leading to a false impression of mathematical certainty. The crucial information often not presented to the public is that these cancers are purely hypothetical, and would never even have been hypothesized without the linear no-threshold (LNT) model. The LNT model postulates that every dose of radiation, no matter how small, increases the probability of getting cancer. This highly speculative hypothesis was introduced on the basis of flimsy scientific evidence more than 50 years ago, at a time when cellular biology was a largely unexplored field. Over the past decades, an ever-increasing number of scientific studies have consistently shown that the LNT model is incompatible with radiobiological and experimental data, especially for very low doses.4–6 The LNT model was mainly intended as a tool to facilitate radioprotection regulations and, despite its biological implausibility, this may remain its raison d’être. However, the LNT model is now used in a misguided way. Investigators multiply infinitesimal doses by huge numbers of individuals in order to obtain the total number of hypothetical cancers induced in a population. This practice is explicitly condemned as “incorrect” and “not reasonable” by the International Commission on Radiological Protection,7 among others. Airport backscatter x ray scanners are safe. One reason to discontinue their use would be if they were ineffective against competing techniques. They should not be dismissed solely on the basis of an old speculative hypothesis that has been both severely questioned by modern scientific findings and used erroneously. What do we mean by safe? The most direct interpretation of “safe” refers to an exposed individual. One may ask what the best estimate of the lifetime cancer risk is to an individual receiving one or more x ray backscatter scans. Using the standard “5%/Sv” cancer mortality risk formula, this would result in an estimated lifetime cancer mortality risk estimate of about 10−7 for two 1-μSv screening scans.8 “Safe” by almost any standard! Of course this individual risk estimate is exceedingly uncertain. Some have argued that the risk at very low doses is zero. Others have argued that phenomena such as tissue/organ microenvironment effects, bystander effects, and “sneaking-through” immune surveillance, imply that low-dose radiation risks could be higher than anticipated. The bottom line is that individual risk estimates at very low doses are extremely uncertain. But when extremely large populations are involved, with up to 109 scans per year in this case,9 risk should also be viewed from the perspective of the entire exposed population. Population risk quantifies the number of adverse events expected in the exposed population as a result of a proposed practice,10 and so depends on both the individual risk and on the number of people exposed. Population risk is described by ICRP as “one input to … a broad judgment of what is reasonable,”11 and by NCRP as “one of the means for assessing the acceptability of a facility or practice.”12 Population risk is considered in many other policy areas where large populations are exposed to very small risks, such as nuclear waste disposal or vaccination.13 It has been claimed that moving from individual risk to population risk is “bad science.” In fact there is no science at all here. An individual cancer risk of 1 in X is just another way of saying that if X people are exposed to that risk, the expected number of induced cancers is 1; and if (say) 100 times X people are exposed to that risk, it is essentially certain that there will be some induced cancers. One can argue what the individual risk actually is, but one cannot argue about the average population consequences of a given individual risk–-it is simply what we mean when we talk of individual risk. So x ray backscatter scanners are associated with very small but very uncertain individual risks. This uncertainty is irrelevant for an individual–-whether the individual risk is 0, or 10−8 or 10−6, these are all small enough risks for any individual not to be concerned. But if 109 scans per year are performed,9 the uncertainties in individual risk mean that we have little idea whether the population consequences of this activity will be 0 or 10 or 1000 cancers per year. If there were no practical alternatives, it could be argued that any such population risks would be more than balanced by the benefits of reducing the risk of a terrorist event. Millimeter-wave-based whole-body scanners,14 however, currently function equally well in airports, do not involve ionizing radiation, and are associated with essentially no mechanistic or experimental evidence of biological risks.9,15 X ray backscatter scanners are probably “safe” from an individual perspective, but their population safety, if they are to be used up to a billion times annually, is unknown. Given available practical alternatives, it would be prudent to curtail their use. I am pleased that Dr. Brenner agrees that backscatter scanners are safe on an individual basis. However, he then goes on to compute the effect of trivial doses delivered to huge populations to obtain a population risk, an approach I strongly dispute. In support of my position, such calculations have been unequivocally condemned by the major scientific associations as “incorrect,”7 illegitimate,”16 “without any scientific validity,”17 known to produce a “distorted image of risk, completely out of perspective with risks accepted every day,”12 and “should not be used for the purpose of estimating population health risks,”18 and “predictions of hypothetical cancer incidence and deaths in patient populations exposed to such low doses are highly speculative and should be discouraged.19 Generally, the misuse of the LNT model may create in the public unfounded fears of small radiation doses, which often lead to nonsensical individual choices, such as avoiding necessary medical examinations. The carcinogenic effects of very low doses of radiation are obviously uncertain. They are not even amenable to scientific assessment. The LNT model postulates that risk increases linearly with dose. However, defense mechanisms are qualitatively dose-dependent and are highly effective in the range of natural background dose levels,4 which is not surprising from an evolutionary perspective. Notably, no cancer excess has been observed in areas with naturally high radiation levels, where individuals absorb the equivalent of a screening dose every minute or less. The screening dose of 1 μSv (or, more precisely, 0.88 μSv) cited by Dr. Brenner was indirectly obtained in one study by extrapolation from image characteristics.8 However, several other studies that used direct measurements consistently showed the dose to be about 0.05 μSv or less.1–3 I wish to make it clear that I am not endorsing the use of backscatter scanners; I do not know how truly useful they are in the context of airport safety, or whether better alternatives exist, though at least some reports suggest that millimeter-wave scanners are less reliable.20 My point is made from a purely medical perspective. A machine that delivers a dose equivalent to 1–2 min of flight is not a public health hazard. Contrarily, anxiety and fear engendered by irrational health scares can certainly take a toll on public mental health. In the absence of direct data on radiation carcinogenesis at very low doses, it is hard to see how Dr. Hindié can be so certain that there are zero population risks associated with performing a billion x ray backscatter scans. Lacking epidemiological data, Dr. Hindié cherry-picks some references to suggest “ever-increasing” numbers of laboratory studies questioning the LNT extrapolation of risks to very low doses. But one can equally well cite laboratory studies suggesting that LNT may underestimate risks at very low doses.21–23 In truth, we do not have credible laboratory models of radiation-induced cancer, and it follows that we do not have any hard evidence, one way or the other, as to the status of LNT at very low radiation doses. Surprisingly, Dr. Hindié implies that, even if there is indeed a very small individual risk from x ray backscatter scans, the overall population risk is still zero. This suggests a misrepresentation of what individual risk actually means. An individual cancer risk of, say, one in a million, is just another way of saying that if one million people are exposed, we expect about one cancer to be induced. Here there is nothing to debate. So we return to the question: In the complete absence of relevant data, how can one be so certain what the long-term population risks actually are? In the presence of such uncertainty, and given the availability of equally efficient whole-body screening tools which do not use x rays (millimeter-wave scanners), is it wise to base policy solely on wishful thinking?