This part of their letter doesn't make sense: Unlike other scanners, these new devices operate at relatively low beam energies (28keV). The majority of their e

This part of their letter doesn't make sense: Unlike other scanners, these new devices operate at relatively low beam energies (28keV). The majority of their energy is delivered to the skin and the underlying tissue. Thus, while the dose would be safe if it were distributed throughout the volume of the entire body, the dose to the skin may be dangerously high. The main issue is that the limit for a smaller volume of the body is actually greater than the body as a whole. Radiation dose is somewhat similar to a density, in that it's measured in energy deposited per mass. Your body is better at dealing with a high dose in a small volume, than it is with a low dose in a large volume. ...these new airport scanners are largely depositing their energy into the skin and immediately adjacent tissue, and since this is such a small fraction of body weight/vol, possibly by one to two orders of magnitude, the real dose to the skin is now high. This statement doesn't make sense either. Dose is J/kg, so the fact that the volume (read mass) is small is taken into account. This part sounds the most interesting: In addition, it appears that real independent safety data do not exist. A search, ultimately finding top FDA radiation physics staff, suggests that the relevant radiation quantity, the Flux [photons per unit area and time (because this is a scanning device)] has not been characterized. Instead an indirect test (Air Kerma) was made that emphasized the whole body exposure value, and thus it appears that the danger is low when compared to cosmic rays during airplane travel and a chest X-ray dose. The problem with this statement is that Air Kerma is the quantity that you'd use to calculate the dose. It is only measured indirectly in the sense that you can set the detector to have the conversion coefficients built in (you actually measure the quantity Exposure). It seems that the letter was probably written by people with concern, but not expertise. I just know enough to see some problems.