Airport Body Scanners: Not Much Risk, Even Less Benefit?

Are the new backscatter x-ray airport body scanners going to give you skin cancer? Probably not. And when I say probably, I mean really really really unlikely. The risk posed by these scanners is well within the “if you are worried about this, best not ever leave your house or turn on the light switch” type parameters we consider “safe” for all our other modern “conveniences”. For an excellent discussion of their safety, go read Mike’s post “Airport Body Scanners Won’t Give You Cancer” now. I’ll wait for you to come back. . .

. . .ok, welcome back. That was pretty solid, eh? Mike does good work, no?

What is curious about this debate is its absolutist nature.

The scanners will give you cancer.
– or-
The scanners are the only way to stop the terrorists.

In the rest of our lives, we regularly balance probabilistic risks with probabilistic benefits all the time, and reject absolutist thinking as childish.

For example, in my extensive rugby injury career, doctors have had occasion to send yours truly for diagnostic x-rays “just to be sure” – shorthand identification of recommendation based on a biomedical version of cost-benefit analysis. In their expert opinion, the potential risks due to x-ray exposure were out-weighed by the potential benefit of identifying unpleasant modifications to my physiology, like displaced sterno-clavicular joints or a fracture ribs and preventing further damage, like a clavicle entering the chest cavity or a pierced lung.

When it comes to the new airport security procedures, my question is whether the benefit (prevention of terrorist attacks) outweighs the costs, such as a marginal, but medically acceptable increase in skin cancer risk, the expense of installation/maintenance/operation of scanners, the time and expense of training security agents, and the social costs of decreasing cooperation with security procedures[1].

The risk of terrorist attacks is very difficult to quantify and the modification of that risk by different security procedures is even more difficult. The extremely low rate of attempted attacks per passenger screened makes an absence of attacks very difficult to interpret in a rigorous manner. The efficacy of the new security procedures for preventing “successful” terrorist attacks[2] has not been demonstrated to the public, fueling criticism this policy is more a product of lobbying than research. For similar reasons the relative efficacy of previous security measures have rarely been demonstrated.

The mathematics, however, are relatively simple. In the absence of demonstrable benefit to an intervention, any risk, however marginal, becomes unacceptable. It may not be the way the writers of the Fourth Amendment to the US Constitution intended the word to be used, but a benefit-cost ratio of zero[3] is the definition of unreasonable.


  1. While I remain unconvinced that Israeli style security can be easily and effectively implemented in US airports, they fundamental premise of the successful Israeli style security does suggest that implementing policies that cause more people to behave erratically reduces security.
  2. In my opinion, what we consider a “successful” attack must bear in mind the goals of attackers. It is unlikely that the true goal of relatively small terror networks is victory by attrition (i.e., killing their opponents into submission). A more likely goal is to negatively affect the policy/lifestyle of more powerful entities. From that perspective, attacks that we consider “failures”, may be “successes” in the eyes of the attackers because these “failures” still generate policy changes, such as the new security procedures for which a “failed” attack was a strong impetus.
  3. I have a nerdy issue with benefit-cost ratio (BCR) formulation, especially when applied to health/safety considerations. By putting the benefit in the numerator, the potential benefit is not bounded. As cost approaches zero, the BCR approaches infinity (unbounded); but as the benefit approaches zero, the BCR approaches the hard limit of zero (bounded), and its apparent approach to zero slows as it gets closer and closer. This may give undue weight to the “benefit” of a proposal, especially if costs are low or underestimated, or if benefits are overestimated. For many procedures, avoiding undue cost perhaps should be the primary concern, leading to a cost-benefit ratio (CBR, which I have used before). The BCR, however, is a more intuitive way for humans to think about this kind of issue, even if it statistically does not align well with our actual interests.

Author: Josh Witten

3 thoughts on “Airport Body Scanners: Not Much Risk, Even Less Benefit?”

  1. I have never seen a better explanation of the 4th amendment.

    And thanks for the link kickback…. I’m sure nobody will ever suspect we’re in cahoots.

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