The recent deployment of backscatter scanning devices meant for airline passengers has caused controversies focused on both the privacy issues of the scans and the safety of the devices themselves (not to mention the unpleasant alternative of an aggressive frisking). The discussion of safety issues has been clouded by two competing narratives. On one side, there's radiation exposure that's comically low compared to what comes from simply boarding the aircraft and being lifted above a lot of the Earth's atmosphere. On the other, there are arguments that the sort of exposure generated by backscatter devices is somehow different.

To provide a better perspective on matters, we'll explain why both of these arguments are right.

What is backscattering?

In a traditional medical X-ray, the imaging is based on sending a lot of high-energy X-rays at an object of interest, such as a sore ankle. The film or sensor that creates the images is placed on the opposite side of the object from the radiation source. If the object has some materials that aren't very dense—like, say, a liver—the X-rays will pass straight through and expose the film. Denser objects, like bones, absorb more of the X-rays, and so leave an area of film underexposed compared to the surroundings. That's why bones appear as white on a traditional black-and-white film X-ray.

The key thing to note here is that the image is created by the X-rays themselves. The photon source is what exposes the film (or sensor).

That's not how backscattering works. For a backscatter system, the X-ray source and the sensor are placed on the same side of the object being imaged. The X-rays themselves are lower energy, and nearly all of them are absorbed within the first few millimeters of the object; effectively, none of them will pass through all but the thinnest of objects.

Instead, backscatter systems take advantage of what happens when the X-rays are absorbed by the object. When they strike an atom and get absorbed, some of that energy gets used to kick an electron off the atom, creating an ion. Some of the remainder gets bounced back out as a lower-energy photon. The sensor in a backscatter device picks up these photons as they come off the object that was exposed to the X-rays, and can use these photons to create an image of the object in question.

There are some good and bad things about this process. For starters, the energy of the re-emitted photons depends in part on the atom doing the absorbing. As a result, the spectrum of the light coming back off the item being scanned provides some rough information—metal vs. organic material—about what's doing the scattering. From a security standpoint, this can be very helpful.

The other thing is that the process requires that the X-rays be absorbed efficiently. This is in contrast with a standard medical X-ray, which requires the use of high-energy photons that are more likely to penetrate the thing being imaged and reach the film or sensor on the other side. The wavelengths used for the security scanners are lower energy, and they're efficiently absorbed within the fist few millimeters of the object's surface. Really, they have to be in order to ensure there are enough backscattered photons to create an image.

That's the good stuff.

The primary downside of this technology is that it's ionizing radiation by definition—it wouldn't work if electrons weren't popped free from atoms. And that poses a health risk, since the resulting ions can be rather energetic, and undergo chemical reactions, either internally within an ionized molecule or with another substance in their environment. Damage to DNA from ionizing radiation is a known cancer risk, and steps are generally taken to minimize this.

But we're all exposed to ionizing radiation every day, since there's a constant flux that originates in everything from small quantities of radioisotopes in every-day items to cosmic rays arriving from space (both of which act much like medical X-rays in terms of their ability to pass through objects). The body scanners have been measured as producing a three microREM dose of radiation; a typical cross-country flight will produce exposures over a thousand times higher, and a chest X-ray is 10 times higher again than the flight. But, because of the differences between those sorts of exposures and the backscatter dose, it's not clear whether the same risk analysis applies to both, as we'll discuss in the next section.