Unlike a medical X-ray, the TSA X-ray machines are a sci-fi fan's dream: they are lower-energy beams that can only penetrate clothing and the topmost layers of skin. This provides TSA agents with a view that would expose any explosives concealed by clothing. But according to the UCSF professors, the low-energy rays do a "Compton scatter" off tissue layers just under the skin, rather than the bone, possibly exposing some vital areas and leaving the tissues at risk of mutation.
When an X-ray Compton scatters, it doesn't shift an electron to a higher energy level; instead, it hits the electron hard enough to dislodge it from its atom. The authors note that this process is "likely breaking bonds," which could cause mutations in cells and raise the risk of cancer.
Because the X-rays only make it just under the skin's surface, the total volume of tissue responsible for absorbing the radiation is fairly small. The professors point out that many body parts that are particularly susceptible to cancer are just under the surface, such as breast tissue and testicles. They are also concerned with those over 65, as well as children, being exposed to the X-rays.