What follows is a post of mine originally featured on smallerquestions.org.
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I think everyone has seen them by now - the new scanners that the Transport Safety Administration has installed at airports all over the US. As the “radiation person” in my group of friends, I get a lot of people asking about them. “How much radiation do they use?” “Are they safe?” “Should I opt out of the scan and get a pat down?” The answers to the second two questions are subjective, but several studies have been done to evaluate #1. The first studies, done by the TSA itself, met with a lot of criticism from various groups. The study made the assumption that the radiation distributed itself uniformly across the body, which turns out not to be the case. Several members of the UCSF faculty wrote a letter of concern (pdf link), asserting that the risk was underestimated because most of the radiation from low-energy x-rays is deposited in the skin. And since there are nearly 1 billion airline passengers in the US each year, any small increase in cancer risk could add up to significant harm to the overall population.
First, we need to make a distinction. There are two types of these new scanners: “millimeter wave” and “backscatter.” The two airports I use the most, Atlanta and Denver, both have millimeter wave scanners. These use the same radiation that your cell phone does, which don’t have enough energy to break chemical bonds in your cells. The backscatter scanner uses so-called “ionizing radiation”, which is the same x-rays used for dental scans or CT imaging. These photons have enough energy to break chemical bonds, and can cause DNA mutations that lead to cancer (more info in Sarah’s post about EM spectrum). From here on I use “radiation” to mean “ionizing radiation” for the sake of brevity.
The radiation dose delivered during one of these backscatter scans has been evaluated in several venues, but the most recent is a paper by Hoppe and Schmidt out of Marquette University. Although the abstract reads like an episode of Ghost Hunters (“Voxelized phantoms of male and female adults and children were used with the GEANT4 toolkit to simulate a backscatter security scan.”), in my opinion these types of papers tend to be very dry (a voxelized phantom is just a computer model of an object receiving radiation). However, the conclusions can be profound – “For a full screen, all phantoms’ total effective doses were below the established 0.25 μSv standard, with an estimated maximum total effective dose of 0.07 μSv for full screen of a male child.” (!)
