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Michael C. Dorf

How to Think About the Tiny Cancer Risk Posed by Airport Scanners


Wednesday, January 13, 2010

In the wake of the foiled Christmas Day bombing, officials responsible for airport security in the United States and abroad are considering implementing new, more thorough screening methods for detecting weapons and explosives. One possibility under consideration would be the increased use of full-body "backscatter" X-ray scans. The health risks to any individual passenger from a backscatter X-ray are minuscule. As reported recently in the New York Times, a single backscatter X-ray delivers no more than one one-hundredth of the radiation associated with a dental X-ray, itself extremely low. Nonetheless, when aggregated over billions of passengers, even tiny numbers can add up to a handful of excess cancer deaths.

Do the benefits of full-body scans justify the cost? In strictly utilitarian terms, the answer depends (in part) on whether X-ray scans would save more lives—by detecting or deterring would-be terrorists—than they would sacrifice—by generating fatal cancers. Yet the number of terrorists likely to be detected or deterred is unknown and arguably unknowable. How, then, should policymakers think about whether or not to implement
X-ray body scans?

In this column, I identify the most relevant factors to consider in making this decision. I also try to quantify the costs and benefits. Although there are no easy answers, clear thinking about these issues can help our leaders formulate sensible policy.

The Radiation Risk

At the outset, it would be useful to be able to quantify the costs and benefits in terms of lives saved or lost. Although both sides of the equation present difficulties, the risks of X-ray exposure are somewhat easier to calculate.

Studies of patients who had dental X-rays in the 1950s showed increased risks of cancer. Since then, new technology has enabled dentists to perform X-rays that deliver much smaller doses of radiation, so much so that the cancer risk of common dental X-rays is virtually impossible to detect against the background risk from ambient atmospheric radiation.

Nonetheless, undetectable risk is not non-existent risk. Although it is conceivable that the cancer-causing effects of radiation do not occur below a threshold of exposure, given what we know about the mechanisms of radiation-induced cell mutation, it is quite possible that the effects rise with exposure in an at least a roughly linear way. Or, to put that in something closer to plain English, the increased cancer risk is roughly proportional to the dose, even at very small doses.

A typical dental X-ray exposes the patient to about 2 millirems of radiation. According to one widely cited estimate, exposing each of 10,000 people to one rem (that is, 1,000 millirems) of radiation will likely lead to 8 excess cancer deaths. Using our assumption of linearity, that means that exposure to the 2 millirems of a typical dental X-ray would lead an individual to have an increased risk of dying from cancer of 16 hundred-thousandths of one percent. Given that very small risk, it is easy to see why most rational people would choose to undergo dental X-rays every few years to protect their teeth.

More importantly for our purposes, assuming that the radiation in a backscatter X-ray is about a hundredth the dose of a dental X-ray, we find that a backscatter X-ray increases the odds of dying from cancer by about 16 ten millionths of one percent. That suggests that for every billion passengers screened with backscatter radiation, about 16 will die from cancer as a result.

Understood in individual terms, it is difficult to imagine any rational passenger being concerned about that risk. According to one expert quoted in the New York Times story, a high-altitude flight exposes an airline passenger to as much added ionizing cosmic radiation in four minutes as the backscatter X-ray delivers. (High-altitude flights increase radiation exposure because the Earth's atmosphere, which absorbs radiation, is thinner at high altitudes.) Someone who is willing to accept the very small risk from radiation exposure through high altitude flight is thus highly unlikely to be concerned about the very much smaller incremental exposure due to X-ray backscatter scanning.

Nonetheless, the issue looks somewhat different when the likely consequences of the radiation exposure are considered in the aggregate. Globally, about 2 billion passengers fly each year, so screening all passengers with backscatter X-ray scans could reasonably be expected to result in about 32 excess cancer deaths per year. If all of the foregoing assumptions are correct, that represents a real cost, and thus encourages us to look carefully at the likely benefits of full-body backscatter X-ray screening, to examine the cost-benefit tradeoff.

How Many Terrorists Would Be Detected or Deterred By Backscatter X-ray Screening?

Quantifying the benefits from backscatter X-ray screening, in turn, requires considerable guesswork. But even if it detects or deters only one otherwise successful terrorist (or team of terrorists) per decade, such screening would appear to be cost-justified in terms of lives saved versus lives lost. Such screening would save the lives of passengers and crew potentially numbering in the hundreds and of other potential victims who could number in the thousands (in the event of a 9/11-style attack in which airplanes are used as missiles). To put it somewhat crudely, the 320 excess cancer deaths caused by the backscatter X-ray scans of twenty billion people over the course of a decade would be the "price" of saving at least one jumbo jet worth of passengers, crew, and possibly others as well.

Would such scans in fact detect or deter at least one otherwise successful terrorist or team of terrorists per decade? Since 9/11, no Americans have fallen victim to airline terrorists, but that hardly shows that airport screening, in its current form, is foolproof. Both Richard Reid (the "shoe bomber") and Umar Farouk Abdulmutallab (the "Christmas Day bomber") were foiled only by the combination of their own ineptitude and alert fellow passengers. In addition, spot checks by undercover personnel have revealed security breaches that could have been exploited with horrific consequences.

The likely efficacy of X-ray backscatter scans depends in part on the alternative. American airports already make selective use of "millimeter wave" full-body scans, which do not carry even the minuscule cancer risk potentially associated with backscatter X-rays. However, millimeter wave scans do not produce images that are as clear as those produced by backscatter X-rays. So here too, we would want to know whether, roughly once per decade, the increased clarity of backscatter images is likely to detect or deter one terrorist who would have avoided detection and succeeded in his nefarious plans if only millimeter wave scanning or some other technology had been used. If the answer is yes, then the aggregate increase in cancer risk would be offset by the benefits of backscatter X-ray imaging.

Privacy, Time, and Money: Other Costs That Inform the Analysis

The foregoing lives-to-lives comparison is fuzzy at best, but it is still incomplete because it does not take account of other factors, such as privacy, time, and money.

X-ray backscatter full-body scans work much like Superman's X-ray vision, and therefore, as in the fantasies of countless adolescents, permits the body-scan operator to see beneath the clothing of airline passengers. Indeed, that is the whole point of the scans—to detect materials, such as liquid explosives concealed inside clothing, that do not set off the metal detector. Accordingly, subjecting every passenger to such a scan means exposing a relatively clear image of every passenger's naked body to the scrutiny of a security screener (of the same sex as the passenger). For many passengers, exposing body parts and matters such as colostomy bags or adult diapers would be, as the ACLU put it, a humiliating "virtual strip search."

Routine full-body scans would also add to the time that it takes passengers to get to the gate. As Steven Levitt and Stephen Dubner explain in their recent book, Super Freakonomics, even terrorists who fail can "succeed" by generating added security measures. They calculate that the time lost by Americans each year in removing and then putting back on their shoes—a measure inspired by unsuccessful shoe bomber Reid—adds up to the equivalent of 14 lives per year. To be sure, spreading the delay in one-minute increments among half a billion passengers makes this lost time much easier to swallow than actually killing 14 people, but the aggregate time lost is substantial.

The financial cost of X-ray backscatter screening would also be substantial. The machines alone cost over $100,000 per unit, to which would have to be added the cost of maintaining and operating them. One might be tempted to say that it is wrong to put a dollar value on human lives, but we invariably do so. Cars could be made marginally safer for more dollars, for example.

More directly, the money saved by not implementing X-ray backscatter scans could be directed to saving lives. That money could be spent on the prevention and treatment of conditions such as diarrhea, which kills over a million children worldwide per year. Or, if that seems like an apples-to-oranges comparison, one would at least want to know whether other air-security measures—such as a greater investment in information technology to fix the flaws in the coordination of intelligence, or a decision to fund the hiring of more sky marshals—would produce equal or better results than X-ray backscatter scans.

The Ingenuity and Flexibility of Terrorists

Moreover, there is always a danger that any increased security measures will simply constitute a reaction to the most recent attack, without taking adequate account of the flexibility of terrorists. Richard Reid attempted to create an explosion with his shoe, so the U.S. responded by X-raying shoes. Umar Farouk Abdulmutallab hid his explosives beneath his pants, so we are now contemplating machinery that can look through clothing. Even if such measures prove perfectly effective at detecting shoe-borne and body-borne explosives, a determined terrorist can take other measures. (I have thought of a few possibilities for evading shoe removal and body scans but I am not including them here because of the admittedly remote possibility that this would be useful to terrorists.)

Nor should we think of the issue simply in terms of air travel. Thus far, I have discussed the costs and benefits of X-ray backscatter scanning by considering terrorists who would be detected or deterred by the technology's implementation. However, a terrorist who would be deterred from trying to bring down an airplane might simply choose a different target. Inducing suicide bombers to switch from targeting airplanes to targeting public buildings might save some lives, but it would hardly count as a victory. And needless to say, it would not be feasible or desirable to require X-ray backscatter scans for access to all public places.

In the end, the best argument for implementing X-ray backscatter scans at airports may appeal more to the emotions than to hardheaded calculations of costs and benefits. Despite statistics showing that air travel is substantially safer than automobile travel, many people are jittery about flying, even without the worry of potential terrorism. Full-body scans would be a way to give millions of travelers some added peace of mind. That is not quite the same thing as making air travel safer, but it is a real benefit nonetheless.

Michael C. Dorf, a FindLaw columnist is the Robert S. Stevens Professor of Law at Cornell University. He is the author of No Litmus Test: Law Versus Politics in the Twenty-First Century and he blogs at

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